{"history":[],"article":{"id":"d46c2a45-4d52-45f8-bb74-3ea62dd1d766","title":"Madison Symmetric Torus","slug":"madison-symmetric-torus","content":":::summary\n![MST](https://upload.wikimedia.org/wikipedia/commons/2/26/Mst///picture.png)\n**Name:** MST \n**Fullname:** Madison Symmetric Torus \n**Type:** [Reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) \n**City:** [Madison, Wisconsin](https://bliptext.com/articles/madison-wisconsin) \n**Country:** [US](https://bliptext.com/articles/united-states) \n**Affiliation:** [University of Wisconsin–Madison](https://bliptext.com/articles/university-of-wisconsin-madison) \n**Website:** http://plasma.physics.wisc.edu/viewpage.php?id=mst \n:::\n\n# \nThe **Madison Symmetric Torus** (**MST**) is a [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) (RFP) [physics](https://bliptext.com/articles/physics) experiment with applications to both [fusion energy](https://bliptext.com/articles/fusion-power) research and [astrophysical plasmas](https://bliptext.com/articles/astrophysical-plasma).\nMST is located at the Center for Magnetic Self Organization (CMSO) at the [University of Wisconsin-Madison](https://bliptext.com/articles/university-of-wisconsin-madison).\nRFPs are significantly different from [tokamaks](https://bliptext.com/articles/tokamak) (the most popular [magnetic confinement](https://bliptext.com/articles/magnetic-confinement-fusion) scheme) in that they tend to have a higher power density and better confinement characteristics for a given average [magnetic field](https://bliptext.com/articles/magnetic-field).\nRFPs also tend to be dominated by non-ideal phenomena and turbulent effects.\n\n# Classification\nAs in most such experiments, the MST [plasma](https://bliptext.com/articles/plasma-physics) is a [toroidal](https://bliptext.com/articles/toroid) [pinch](https://bliptext.com/articles/pinch-plasma-physics), which means the [plasma](https://bliptext.com/articles/plasma-physics) is shaped like a donut and confined by a [magnetic field](https://bliptext.com/articles/magnetic-field) generated by a large current flowing through it.\nMST falls into an unconventional class of machine called a [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) (RFP.) The RFP is so named because the toroidal magnetic field that permeates the plasma spontaneously reverses direction near the edge.\nA [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) is formed similarly to other toroidal pinch devices, by driving current through the [plasma](https://bliptext.com/articles/plasma-physics) from an associated [capacitor bank](https://bliptext.com/articles/capacitor-bank) or other high-current power sources.\nIn a [tokamak](https://bliptext.com/articles/tokamak) the toroidal field is much stronger than the poloidal field, but in an RFP it's just the opposite.\nIn fact, in an RFP the externally applied toroidal field is switched off shortly after startup.\nThe plasma in an RFP is also much closer to the wall than in a tokamak.\nThis permits a peculiar arrangement of the magnetic field lines, which will 'relax' into a new state such that the total [magnetic energy](https://bliptext.com/articles/magnetic-energy) in the plasma is minimized and the total [magnetic helicity](https://bliptext.com/articles/magnetic-helicity) is conserved.\nThe relaxed state called a [Taylor state](https://bliptext.com/articles/taylor-state) is marked by a peculiar arrangement of magnetic field lines where the toroidal magnetic field at the edge spontaneously reverses direction.\n# [The geometry inside the Madison Symmetric Torus. | upload.wikimedia.org/wikipedia/commons/5/5c////Madison///Symmetric///Torus.png]\n\n# Ongoing experiments in the MST program\n\n## Oscillating field current drive\nLike most toroidal confinement schemes, the RFP relies on a transient burst of [current](https://bliptext.com/articles/electric-current) to create the plasma and the [magnetic](https://bliptext.com/articles/magnetism) fields that confine it.\nBut for the RFP to be a viable [fusion energy](https://bliptext.com/articles/fusion-power) candidate the plasma must be sustained by a steady state current source.\nOFCD is a scheme for driving a steady current in a relaxed plasma by adding sizable [oscillating](https://bliptext.com/articles/oscillation) perturbations to the toroidal and [poloidal](https://bliptext.com/articles/poloidal) fields injecting both power and helicity into the plasma.\nA similar approach was patented and suggested for the [Lockheed-Martin Compact Fusion Reactor](https://bliptext.com/articles/lockheed-martin-compact-fusion-reactor).\nA nonlinear reaction in the plasma combines the two oscillations in such a way that, on average, a steady current is maintained.\n\n## Pellet injection\nOne of the challenges facing the RFP is fueling the hot core of the plasma directly, rather than relying on the [deuterium gas](https://bliptext.com/articles/deuterium) to seep in slowly from the edge.\nThe Pellet Injector fires a frozen pellet of deuterium into the plasma using a blast of gas or a mechanical punch.\nThe pellet is vaporized and [ionized](https://bliptext.com/articles/ionization) as it travels into the core of the plasma.\n\n## Pulsed poloidal current drive\nEvery [gradient](https://bliptext.com/articles/gradient) is a source of free energy, especially if it's across a magnetic field.\nIn MST the current is stronger in the core than at the edge.\nThis peaked current profile serves as a source of free energy for magnetic fluctuations culminating in violent events in the plasma called [sawteeth](https://bliptext.com/articles/sawtooth-wave).\nPPCD alleviates this effect by driving a current at the edge of the plasma, flattening the current profile.\nSmall pulses are added to the power supply currents that drive the toroidal field.\nThe resultant pulsed toroidal magnetic field, with the aid of [Faraday's law](https://bliptext.com/articles/faraday-s-law-of-induction), creates a [poloidal](https://bliptext.com/articles/toroidal-and-poloidal) electric field and hence a [poloidal](https://bliptext.com/articles/toroidal-and-poloidal) current.\nA great deal of research on MST is devoted to the study of this effect and its application for enhanced confinement.\n\n## Neutral beam injection\nIn order to initiate a sustained [fusion reaction](https://bliptext.com/articles/fusion-reaction), it is usually necessary to use many methods to heat the plasma.\n[Neutral Beam Injection](https://bliptext.com/articles/neutral-beam-injection) (NBI) involves injecting a high energy beam of [neutral atoms](https://bliptext.com/articles/energetic-neutral-atom), typically hydrogen or deuterium, into the core of the plasma.\nThese energetic atoms transfer their energy to the plasma, raising the overall temperature.\nThe neutral atoms injected don't remain neutral.\nAs the beam passes through the plasma, the atoms are ionized as they bounce off the ions in the plasma.\nBecause the magnetic field inside the torus is bent into a circle, the fast ions are hoped to be confined in the background plasma.\nThe confined fast ions are slowed down by the background plasma, the same way air resistance slows down a baseball.\nThe energy transfer from the fast ions to the plasma increases the plasma temperature.\nThe actual injector can be seen from the observation window.\nIt looks like a long silver cylinder laying on its side but tilted slightly downward against the torus near the back of the machine.\nWhen the injector is pulsed, 20,000 volts accelerates the beam to about 30 amperes of current for about 1.5 milliseconds.\nProblems would occur if the fast ions aren't confined within the plasma long enough for them to deposit their energy.\nMagnetic fluctuations bedevil [plasma confinement](https://bliptext.com/articles/plasma-containment) in this type of device by scrambling what we hoped were well behaved magnetic fields.\nIf the fast [ions](https://bliptext.com/articles/ion) are susceptible to this type of behavior, they can escape very quickly.\nHowever, there is evidence to suggest that they aren't.\n\n## Electron Bernstein wave current drive\nEBW is an acronym for Electron Bernstein Wave and is named after the plasma physicist, [Ira Bernstein](https://bliptext.com/articles/ira-b-bernstein).\nBernstein Wave Mode relates to a method of injecting ion or electron energy (IBW or EBW) into a plasma to increase its temperature in an attempt to reach fusion conditions.\nA plasma is a phase of matter which occurs naturally during lightning and [electrical discharges](https://bliptext.com/articles/electric-discharge) and which is created artificially in fusion reactors to produce extremely high temperatures.\nThis is an experiment on the MST to heat the plasma and to drive electric current inside the plasma.\nThere is a large electric current in the plasma inside this machine; it is responsible for creating the necessary magnetic fields to make the reversed field pinch configuration.\nIt also heats the plasma very quickly — the same way wires inside your toaster get hot.\nYour toaster probably uses about 10 ampere of current, while the plasma in MST is heated by up to 600,000 amperes.\nBut even though the plasma reaches over 10,000,000 degrees Fahrenheit, it is not hot enough for practical fusion energy and we need to find other ways to deposit energy into the plasma.\nThe EBW is a way to inject [microwave power](https://bliptext.com/articles/microwave-power) to further heat the plasma.\nThe standard microwave oven produces around 1 kW of power at a frequency of 2.45 GHz; the EBW experiment is currently producing 150 kW at 3.6 GHz, and it is a goal of the team to upgrade to over 2 MW.\nTo generate this type of power (on a low budget), decommissioned military radar equipment and home-made voltage power supplies are used.\nThe second (and perhaps more scientifically important) goal of the EBW experiment is to drive electric current in a prescribed place within the plasma.\nThe main plasma current distributes itself naturally, and the plasma tends to concentrate current into the center, leaving less current near the edge.\nThis can lead to instability of the plasma.\nIt has been shown (both theoretically and by experiments in the Madison Symmetric Torus) that driving current in the edge makes the plasma more stable to fluctuations in the magnetic field, resulting in better confinement of the hot plasma and leading to much higher temperature.\nUsing the EBW to drive this stabilizing current would be a very important scientific result.\nThe ability to deposit very specifically the auxiliary current gives us the opportunity to optimize our current drive schemes.\nThe heating is also very localized, allowing us to study how hot (at least locally) the plasma can become within this magnetic confinement scheme — in plasma physics terms, this is called finding the beta limit.\nThis is an unanswered question for the RFP and will give insight on whether or not this type of machine could be scaled up to a cost-effective, efficient fusion reactor.\n\n## The heavy ion beam probe\nThe Heavy Ion Beam Probe (HIBP) fires [potassium ions](https://bliptext.com/articles/potassium) into the plasma.\nBy measuring their [trajectory](https://bliptext.com/articles/trajectory) we get a profile of several key properties inside the plasma.\nThis versatile diagnostics tool has been used in magnetic confinement fusion experiments to determine the [electric potential](https://bliptext.com/articles/electric-potential), electron density, electron temperature, and [magnetic vector potential](https://bliptext.com/articles/magnetic-vector-potential) of the plasma.\nA stream of [sodium ions](https://bliptext.com/articles/sodium) (the primary beam) is injected from the [ion gun](https://bliptext.com/articles/ion-gun) across the magnetic field into the plasma.\nAs the singly charged particles pass through the plasma, they are further ionized creating the doubly charged secondary beam.\nThe secondaries are then detected and analyzed outside the plasma.\nBy curving the trajectories, the magnetic field separates secondary ions from primary ions.\nBecause of this, only secondaries ionized at a given plasma position reach a given detector location.\nThis allows the HIBP to make measurements localized to the ionization position.\nThe secondary current is related to local [electron density](https://bliptext.com/articles/electron-density) and the ionization cross-section of the primary ions, which is itself a function of the electron temperature.\nThe electric potential can be obtained from the energy difference between primary and secondary ion beams.\nThe energy of the secondary beam can be determined from the angle at which it enters the energy analyzer.\nThe MST-HIBP system consists of:\n\n## Far infrared polarimetry-interferometry system\nFIR, or [far infrared](https://bliptext.com/articles/far-infrared), refers to light with wavelengths between 1 and 10 mm. The FIR system in MST is based on the FIR lasers enclosed in the beige-colored laser safety room to the right of the picture shown, in the second floor hallway.\nThere are four FIR lasers in the system.\nOne is a [CO2 laser](https://bliptext.com/articles/co2-laser) which produces a continuous power of about 120 W. This beam is then split in three.\nEach beam optically pumps a [formic acid](https://bliptext.com/articles/formic-acid) vapor laser operating at a wavelength of 432.6 mm, and a power of about 20 mW.\nThe FIR system has 2 modes of operation: interferometry and polarimetry.\n**What does FIR diagnostic system measure?**\nThe electron density, plasma current density, and magnetic field are three important plasma parameters of MST.\nThe FIR system is used to measure their spatial and temporal distributions.\n**How does FIR interferometry work?**\nLike glass, a plasma has a [refractive index](https://bliptext.com/articles/refractive-index) different from that of vacuum (or air) that depends on plasma electron density.\nWe send one [laser beam](https://bliptext.com/articles/laser) through the plasma (the probe beam), one through the air (the reference beam), and measure the phase difference between them.\nThis experimental configuration is called a Mach-Zehnder interferometer.\nThe measured phase is proportional to the average plasma electron density along the beam path.\nIn MST, we send multiple probe beams (blue lines in the figure) through the plasma at different radii.\nWe then apply the Abel inversion technique to obtain a profile of the plasma electron density.\n**How does FIR polarimetry work?**\nA plasma is also an optically active media, meaning when a linearly polarized [electromagnetic wave](https://bliptext.com/articles/electromagnetic-wave) is propagating parallel (or anti-parallel) to the magnetic field, the [polarization](https://bliptext.com/articles/polarization-waves) of the wave exiting the plasma will rotate a small angle.\nThis is called Faraday rotation, and the angle is called the Faraday rotation angle.\nThe FIR system measures the Faraday rotation, which is proportional to the line average of the electron [density](https://bliptext.com/articles/density) times the magnetic field component parallel to the beam path.\nThe reason for Faraday rotation is as follows: When a linearly polarized wave is propagating along a [magnetic field line](https://bliptext.com/articles/magnetic-field), it is de-composed into left-hand and right-hand circularly polarized components.\nThe phase difference between them as they exit the plasma causes the recombined linearly polarized wave to rotate its polarization direction.\nIn MST, we launch two co-propagating, counter-rotating waves to probe the plasma.\nWe then measure the phase difference between these two beams, which will be twice the Faraday rotation angle.\nIn the figure, each of the 11 blue probe beams is a combination of two counter-rotating, circularly polarized beams, measuring the Faraday rotation angles along the same chords as the [interferometer](https://bliptext.com/articles/interferometry) does.\nThe combined interferometer phases and Faraday rotation angles can then be combined to determine the poloidal magnetic field distribution.\nUsing [Ampere's law](https://bliptext.com/articles/ampere-s-law), the toroidal plasma current can be determined as well.\n**How well does the FIR diagnostic system work?**\nThe FIR system for MST is very precise.\nThe Faraday rotation angle for MST plasmas is typically within 5 degrees.\nTo measure such small signal, we have achieved an accuracy of 0.06 degree.\nThe temporal resolution is less than 1 microsecond.\n**What are some of the research topics related to FIR?**\nFIR is an essential tool for most of the research topics in MST since it provides information about the basic plasma parameters.\nThe system measures electron density, toroidal current, poloidal magnetic field, and the spatial profiles of each.\nCurrently, we are exploring the possibility of measuring toroidal magnetic field and poloidal plasma current by using the plasma bi-refringence effect, or the [Cotton-Mouton effect.](https://bliptext.com/articles/cotton-mouton-effect) When a linearly polarized EM wave is propagating perpendicular to the magnetic field, the refractive index depends on whether the wave polarization is parallel or perpendicular to the magnetic field direction.\n**Why choose FIR lasers?**\nFor plasma [polarimetry](https://bliptext.com/articles/polarimetry)-interferometry, the [wavelength](https://bliptext.com/articles/wavelength) we chose is sufficiently long to provide measurable plasma induced phase changes, but sufficiently short to avoid\ncomplicated [plasma-wave](https://bliptext.com/articles/waves-in-plasmas) interactions, including the bending of the beam.\nThere are many high power molecular laser lines available in this wavelength range, and many commercially available detectors.\n\n## Thomson scattering\n**What is Thomson Scattering?**\n[Thomson scattering](https://bliptext.com/articles/thomson-scattering) is the result of a collision between a photon (an electromagnetic wave) and a charged particle, such as an electron.\nWhen an electron and [photon](https://bliptext.com/articles/photon) \"collide\" the electron feels a [Lorentz force](https://bliptext.com/articles/lorentz-force) from the oscillating electric and magnetic fields of the [photon](https://bliptext.com/articles/photon) and is accelerated.\nThis acceleration causes the electron to emit a different photon in a different direction.\nThis emitted photon has a wavelength shifted from that of the incident photon by an amount dependent on the electron energy.\nAnother way of looking at this is that the electron absorbs the energy of the photon and re emits the energy in the form of a different electromagnetic wave.\nThis scattering of a photon by an electron is called Thomson Scattering.\n**How is Thomson Scattering useful to plasma physicists?**\nSince the wavelength of the scattered photon depends on the energy of the scattering electron, Thomson scattering is good way to measure the energy of an electron.\nThis is done by creating a photon of known wavelength and measuring the wavelength of the scattered photon.\nThe Thomson Scattering configuration at MST uses a 1064 nm [YAG laser](https://bliptext.com/articles/yag-laser) system, which produces the best time-resolution electron temperature readings in the world.\nWe create our photons with high power lasers that we shine into a window on the top of the MST, and collect scattered photons with a large collection lens on the side of the MST.\nThe wavelength distribution of the scattered photons tells us the energy distribution of the electrons in the plasma, giving us a direct unobtrusive way of getting the temperature of the electrons.\nThe amount of photons we actually collect can also tell us something about the density of the electrons in the plasma.\n\n## Charge exchange recombination spectroscopy and ion Doppler spectroscopy\nFusion plasmas are typically generated from [ionization](https://bliptext.com/articles/ionization) of a neutral gas.\nIn most cases, [deuterium](https://bliptext.com/articles/deuterium) is used as the plasma fuel.\nThese plasmas are therefore primarily made up of deuterium ions (plus electrons), and it is necessary to diagnose the behavior of these ions if the relevant plasma physics is to be understood.\nHowever, in any fusion device, other types of ions (\"impurities\") are also present.\nThese exist naturally due to the inability to achieve a perfect vacuum in a fusion reactor before fueling.\nThus, materials such as [water vapor](https://bliptext.com/articles/water-vapor), nitrogen, and [carbon](https://bliptext.com/articles/carbon) will be found in small amounts in typical plasma discharges.\nImpurities may also be generated during plasma discharges due to plasma-wall interactions.\nThese interactions primarily cause material from the wall to be ejected into the plasma through sputtering.\nIn the Madison Symmetric Torus (MST), properties of the impurity ions (e.g. carbon, [oxygen](https://bliptext.com/articles/oxygen), etc.) are closely linked to properties of the deuterium ions as a result of strong interaction between the ion species.\nThus, impurity ion measurements can, in principle, provide direct information about the deuterium ions.\nMeasurements of the impurity ion temperature (T_i) and flow velocity (v_i) are obtained on MST using Charge Exchange Recombination Spectroscopy, or CHERS.\nThe CHERS process can be broken down into two separate steps: Charge Exchange and Radiative Decay.\nIn the first stage, an electron is transferred from a neutral atom (e.g. deuterium) to an impurity ion that has no electrons (e.g. C⁺⁶).\nDuring this transfer, the electron typically winds up in an [excited state](https://bliptext.com/articles/excited-state) (high energy level) of the impurity ion.\nAs the electron decays down to the [ground state](https://bliptext.com/articles/ground-state) (minimum energy level), energy conservation requires radiation to be emitted by the impurity ion.\nThis emission has discrete values of energy, or wavelength, which correspond to the energy differences between the initial and final atomic levels of a particular electron transition.\nFor example, consider charge exchange between a deuterium atom and a C⁺⁶ ion: if the electron is transferred to the n=7 energy level of the carbon ion, then the ion will emit radiation at discrete energies given by the difference in energy between the n=7 and n=6 levels, the n=6 and n=5 levels, the n=5 and n=4 levels, and so on (down to n=1).\nThis line emission is Doppler-broadened as a result of ion thermal motion, and Doppler-shifted as a result of ion flow.\nThe [Doppler shift](https://bliptext.com/articles/doppler-shift) causes the emission to be blue-shifted (towards shorter wavelength/higher frequency) if the ions are moving towards the point of observation, or red-shifted (towards longer wavelength/lower frequency) if the flow is away from the point of observation.\nMeasurements of the carbon emission line shape are therefore used to extract values for the impurity ion temperature and velocity.\nCharge Exchange: H + C⁺⁶ →\nH⁺¹ + C⁺⁵ (n=7, l=6)\nRadiative decay: C⁺⁵ (n=7, l=6) →\nC⁺⁵ (n=6, l=5) + h (photon)\nIn a typical fusion device the neutral atom density is small.\nTherefore, the amount of radiated emission that results from charge exchange between impurity ions and neutrals is also small.\nOn MST, the neutral density is enhanced by injection of fast hydrogen atoms via a diagnostic neutral beam (DNB).\nAs a result, the radiated emission is greatly increased, though primarily along the beam injection path (the DNB is located below the deck, and cannot be seen from here; the injection path is from right to left across the plasma).\nPerpendicular to the beam path, there exist a number of optical ports for viewing the plasma at different radial positions.\nFor a given plasma discharge, a fiber bundle system is placed on one of these ports, and is used to collect emission along its line-of-sight (black tubes on top of the machine contain light collection optics; fibers are placed in the long, curved white tube when not in use).\nThis emission is sent to a [spectrometer](https://bliptext.com/articles/spectrometer) where it is dispersed over a finite wavelength range — which is centered on the emission line of interest — by a pair of optical gratings.\nHowever, because the collected emission is dominated by radiation from along the beam path, the measurements are effectively localized to the intersection volume between the fiber view and the beam.\nOn MST, this intersection volume is small (~ 2 cm³) compared to the plasma volume, allowing spatially resolved measurements of T_i and v_i to be obtained.\nData collected from a number of plasma discharges — for which the location of the fiber bundle system is varied — are used to construct radial profiles of the impurity ion temperature and velocity, providing important information for understanding the physics of plasmas in MST.\nTypical ion temperatures measured by CHERS on MST are in the range of 100 to 800 eV (2 million to 17 million degrees Fahrenheit), depending on position in the plasma and type of discharge.\nLikewise, measured equilibrium ion velocities are on the order of 1,000 to 10,000 meters per second.","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T21:48:20.785Z","updated_at":"2025-01-25T21:48:20.785Z","search_vector":"'madison':1 'symmetr':2 'torus':3","search_content":" \n \n**Name:** MST \n**Fullname:** Madison Symmetric Torus \n**Type:** [Reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) \n**City:** [Madison, Wisconsin](https://bliptext.com/articles/madison-wisconsin) \n**Country:** [US](https://bliptext.com/articles/united-states) \n**Affiliation:** [University of Wisconsin–Madison](https://bliptext.com/articles/university-of-wisconsin-madison) \n**Website:** http://plasma.physics.wisc.edu/viewpage.php?id=mst \n \n\n# \nThe **Madison Symmetric Torus** (**MST**) is a [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) (RFP) [physics](https://bliptext.com/articles/physics) experiment with applications to both [fusion energy](https://bliptext.com/articles/fusion-power) research and [astrophysical plasmas](https://bliptext.com/articles/astrophysical-plasma).\nMST is located at the Center for Magnetic Self Organization (CMSO) at the [University of Wisconsin-Madison](https://bliptext.com/articles/university-of-wisconsin-madison).\nRFPs are significantly different from [tokamaks](https://bliptext.com/articles/tokamak) (the most popular [magnetic confinement](https://bliptext.com/articles/magnetic-confinement-fusion) scheme) in that they tend to have a higher power density and better confinement characteristics for a given average [magnetic field](https://bliptext.com/articles/magnetic-field).\nRFPs also tend to be dominated by non-ideal phenomena and turbulent effects.\n\n# Classification\nAs in most such experiments, the MST [plasma](https://bliptext.com/articles/plasma-physics) is a [toroidal](https://bliptext.com/articles/toroid) [pinch](https://bliptext.com/articles/pinch-plasma-physics), which means the [plasma](https://bliptext.com/articles/plasma-physics) is shaped like a donut and confined by a [magnetic field](https://bliptext.com/articles/magnetic-field) generated by a large current flowing through it.\nMST falls into an unconventional class of machine called a [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) (RFP.) The RFP is so named because the toroidal magnetic field that permeates the plasma spontaneously reverses direction near the edge.\nA [reversed field pinch](https://bliptext.com/articles/reversed-field-pinch) is formed similarly to other toroidal pinch devices, by driving current through the [plasma](https://bliptext.com/articles/plasma-physics) from an associated [capacitor bank](https://bliptext.com/articles/capacitor-bank) or other high-current power sources.\nIn a [tokamak](https://bliptext.com/articles/tokamak) the toroidal field is much stronger than the poloidal field, but in an RFP it's just the opposite.\nIn fact, in an RFP the externally applied toroidal field is switched off shortly after startup.\nThe plasma in an RFP is also much closer to the wall than in a tokamak.\nThis permits a peculiar arrangement of the magnetic field lines, which will 'relax' into a new state such that the total [magnetic energy](https://bliptext.com/articles/magnetic-energy) in the plasma is minimized and the total [magnetic helicity](https://bliptext.com/articles/magnetic-helicity) is conserved.\nThe relaxed state called a [Taylor state](https://bliptext.com/articles/taylor-state) is marked by a peculiar arrangement of magnetic field lines where the toroidal magnetic field at the edge spontaneously reverses direction.\n# [The geometry inside the Madison Symmetric Torus. | upload.wikimedia.org/wikipedia/commons/5/5c////Madison///Symmetric///Torus.png]\n\n# Ongoing experiments in the MST program\n\n## Oscillating field current drive\nLike most toroidal confinement schemes, the RFP relies on a transient burst of [current](https://bliptext.com/articles/electric-current) to create the plasma and the [magnetic](https://bliptext.com/articles/magnetism) fields that confine it.\nBut for the RFP to be a viable [fusion energy](https://bliptext.com/articles/fusion-power) candidate the plasma must be sustained by a steady state current source.\nOFCD is a scheme for driving a steady current in a relaxed plasma by adding sizable [oscillating](https://bliptext.com/articles/oscillation) perturbations to the toroidal and [poloidal](https://bliptext.com/articles/poloidal) fields injecting both power and helicity into the plasma.\nA similar approach was patented and suggested for the [Lockheed-Martin Compact Fusion Reactor](https://bliptext.com/articles/lockheed-martin-compact-fusion-reactor).\nA nonlinear reaction in the plasma combines the two oscillations in such a way that, on average, a steady current is maintained.\n\n## Pellet injection\nOne of the challenges facing the RFP is fueling the hot core of the plasma directly, rather than relying on the [deuterium gas](https://bliptext.com/articles/deuterium) to seep in slowly from the edge.\nThe Pellet Injector fires a frozen pellet of deuterium into the plasma using a blast of gas or a mechanical punch.\nThe pellet is vaporized and [ionized](https://bliptext.com/articles/ionization) as it travels into the core of the plasma.\n\n## Pulsed poloidal current drive\nEvery [gradient](https://bliptext.com/articles/gradient) is a source of free energy, especially if it's across a magnetic field.\nIn MST the current is stronger in the core than at the edge.\nThis peaked current profile serves as a source of free energy for magnetic fluctuations culminating in violent events in the plasma called [sawteeth](https://bliptext.com/articles/sawtooth-wave).\nPPCD alleviates this effect by driving a current at the edge of the plasma, flattening the current profile.\nSmall pulses are added to the power supply currents that drive the toroidal field.\nThe resultant pulsed toroidal magnetic field, with the aid of [Faraday's law](https://bliptext.com/articles/faraday-s-law-of-induction), creates a [poloidal](https://bliptext.com/articles/toroidal-and-poloidal) electric field and hence a [poloidal](https://bliptext.com/articles/toroidal-and-poloidal) current.\nA great deal of research on MST is devoted to the study of this effect and its application for enhanced confinement.\n\n## Neutral beam injection\nIn order to initiate a sustained [fusion reaction](https://bliptext.com/articles/fusion-reaction), it is usually necessary to use many methods to heat the plasma.\n[Neutral Beam Injection](https://bliptext.com/articles/neutral-beam-injection) (NBI) involves injecting a high energy beam of [neutral atoms](https://bliptext.com/articles/energetic-neutral-atom), typically hydrogen or deuterium, into the core of the plasma.\nThese energetic atoms transfer their energy to the plasma, raising the overall temperature.\nThe neutral atoms injected don't remain neutral.\nAs the beam passes through the plasma, the atoms are ionized as they bounce off the ions in the plasma.\nBecause the magnetic field inside the torus is bent into a circle, the fast ions are hoped to be confined in the background plasma.\nThe confined fast ions are slowed down by the background plasma, the same way air resistance slows down a baseball.\nThe energy transfer from the fast ions to the plasma increases the plasma temperature.\nThe actual injector can be seen from the observation window.\nIt looks like a long silver cylinder laying on its side but tilted slightly downward against the torus near the back of the machine.\nWhen the injector is pulsed, 20,000 volts accelerates the beam to about 30 amperes of current for about 1.5 milliseconds.\nProblems would occur if the fast ions aren't confined within the plasma long enough for them to deposit their energy.\nMagnetic fluctuations bedevil [plasma confinement](https://bliptext.com/articles/plasma-containment) in this type of device by scrambling what we hoped were well behaved magnetic fields.\nIf the fast [ions](https://bliptext.com/articles/ion) are susceptible to this type of behavior, they can escape very quickly.\nHowever, there is evidence to suggest that they aren't.\n\n## Electron Bernstein wave current drive\nEBW is an acronym for Electron Bernstein Wave and is named after the plasma physicist, [Ira Bernstein](https://bliptext.com/articles/ira-b-bernstein).\nBernstein Wave Mode relates to a method of injecting ion or electron energy (IBW or EBW) into a plasma to increase its temperature in an attempt to reach fusion conditions.\nA plasma is a phase of matter which occurs naturally during lightning and [electrical discharges](https://bliptext.com/articles/electric-discharge) and which is created artificially in fusion reactors to produce extremely high temperatures.\nThis is an experiment on the MST to heat the plasma and to drive electric current inside the plasma.\nThere is a large electric current in the plasma inside this machine; it is responsible for creating the necessary magnetic fields to make the reversed field pinch configuration.\nIt also heats the plasma very quickly — the same way wires inside your toaster get hot.\nYour toaster probably uses about 10 ampere of current, while the plasma in MST is heated by up to 600,000 amperes.\nBut even though the plasma reaches over 10,000,000 degrees Fahrenheit, it is not hot enough for practical fusion energy and we need to find other ways to deposit energy into the plasma.\nThe EBW is a way to inject [microwave power](https://bliptext.com/articles/microwave-power) to further heat the plasma.\nThe standard microwave oven produces around 1 kW of power at a frequency of 2.45 GHz; the EBW experiment is currently producing 150 kW at 3.6 GHz, and it is a goal of the team to upgrade to over 2 MW.\nTo generate this type of power (on a low budget), decommissioned military radar equipment and home-made voltage power supplies are used.\nThe second (and perhaps more scientifically important) goal of the EBW experiment is to drive electric current in a prescribed place within the plasma.\nThe main plasma current distributes itself naturally, and the plasma tends to concentrate current into the center, leaving less current near the edge.\nThis can lead to instability of the plasma.\nIt has been shown (both theoretically and by experiments in the Madison Symmetric Torus) that driving current in the edge makes the plasma more stable to fluctuations in the magnetic field, resulting in better confinement of the hot plasma and leading to much higher temperature.\nUsing the EBW to drive this stabilizing current would be a very important scientific result.\nThe ability to deposit very specifically the auxiliary current gives us the opportunity to optimize our current drive schemes.\nThe heating is also very localized, allowing us to study how hot (at least locally) the plasma can become within this magnetic confinement scheme — in plasma physics terms, this is called finding the beta limit.\nThis is an unanswered question for the RFP and will give insight on whether or not this type of machine could be scaled up to a cost-effective, efficient fusion reactor.\n\n## The heavy ion beam probe\nThe Heavy Ion Beam Probe (HIBP) fires [potassium ions](https://bliptext.com/articles/potassium) into the plasma.\nBy measuring their [trajectory](https://bliptext.com/articles/trajectory) we get a profile of several key properties inside the plasma.\nThis versatile diagnostics tool has been used in magnetic confinement fusion experiments to determine the [electric potential](https://bliptext.com/articles/electric-potential), electron density, electron temperature, and [magnetic vector potential](https://bliptext.com/articles/magnetic-vector-potential) of the plasma.\nA stream of [sodium ions](https://bliptext.com/articles/sodium) (the primary beam) is injected from the [ion gun](https://bliptext.com/articles/ion-gun) across the magnetic field into the plasma.\nAs the singly charged particles pass through the plasma, they are further ionized creating the doubly charged secondary beam.\nThe secondaries are then detected and analyzed outside the plasma.\nBy curving the trajectories, the magnetic field separates secondary ions from primary ions.\nBecause of this, only secondaries ionized at a given plasma position reach a given detector location.\nThis allows the HIBP to make measurements localized to the ionization position.\nThe secondary current is related to local [electron density](https://bliptext.com/articles/electron-density) and the ionization cross-section of the primary ions, which is itself a function of the electron temperature.\nThe electric potential can be obtained from the energy difference between primary and secondary ion beams.\nThe energy of the secondary beam can be determined from the angle at which it enters the energy analyzer.\nThe MST-HIBP system consists of:\n\n## Far infrared polarimetry-interferometry system\nFIR, or [far infrared](https://bliptext.com/articles/far-infrared), refers to light with wavelengths between 1 and 10 mm. The FIR system in MST is based on the FIR lasers enclosed in the beige-colored laser safety room to the right of the picture shown, in the second floor hallway.\nThere are four FIR lasers in the system.\nOne is a [CO2 laser](https://bliptext.com/articles/co2-laser) which produces a continuous power of about 120 W. This beam is then split in three.\nEach beam optically pumps a [formic acid](https://bliptext.com/articles/formic-acid) vapor laser operating at a wavelength of 432.6 mm, and a power of about 20 mW.\nThe FIR system has 2 modes of operation: interferometry and polarimetry.\n**What does FIR diagnostic system measure?**\nThe electron density, plasma current density, and magnetic field are three important plasma parameters of MST.\nThe FIR system is used to measure their spatial and temporal distributions.\n**How does FIR interferometry work?**\nLike glass, a plasma has a [refractive index](https://bliptext.com/articles/refractive-index) different from that of vacuum (or air) that depends on plasma electron density.\nWe send one [laser beam](https://bliptext.com/articles/laser) through the plasma (the probe beam), one through the air (the reference beam), and measure the phase difference between them.\nThis experimental configuration is called a Mach-Zehnder interferometer.\nThe measured phase is proportional to the average plasma electron density along the beam path.\nIn MST, we send multiple probe beams (blue lines in the figure) through the plasma at different radii.\nWe then apply the Abel inversion technique to obtain a profile of the plasma electron density.\n**How does FIR polarimetry work?**\nA plasma is also an optically active media, meaning when a linearly polarized [electromagnetic wave](https://bliptext.com/articles/electromagnetic-wave) is propagating parallel (or anti-parallel) to the magnetic field, the [polarization](https://bliptext.com/articles/polarization-waves) of the wave exiting the plasma will rotate a small angle.\nThis is called Faraday rotation, and the angle is called the Faraday rotation angle.\nThe FIR system measures the Faraday rotation, which is proportional to the line average of the electron [density](https://bliptext.com/articles/density) times the magnetic field component parallel to the beam path.\nThe reason for Faraday rotation is as follows: When a linearly polarized wave is propagating along a [magnetic field line](https://bliptext.com/articles/magnetic-field), it is de-composed into left-hand and right-hand circularly polarized components.\nThe phase difference between them as they exit the plasma causes the recombined linearly polarized wave to rotate its polarization direction.\nIn MST, we launch two co-propagating, counter-rotating waves to probe the plasma.\nWe then measure the phase difference between these two beams, which will be twice the Faraday rotation angle.\nIn the figure, each of the 11 blue probe beams is a combination of two counter-rotating, circularly polarized beams, measuring the Faraday rotation angles along the same chords as the [interferometer](https://bliptext.com/articles/interferometry) does.\nThe combined interferometer phases and Faraday rotation angles can then be combined to determine the poloidal magnetic field distribution.\nUsing [Ampere's law](https://bliptext.com/articles/ampere-s-law), the toroidal plasma current can be determined as well.\n**How well does the FIR diagnostic system work?**\nThe FIR system for MST is very precise.\nThe Faraday rotation angle for MST plasmas is typically within 5 degrees.\nTo measure such small signal, we have achieved an accuracy of 0.06 degree.\nThe temporal resolution is less than 1 microsecond.\n**What are some of the research topics related to FIR?**\nFIR is an essential tool for most of the research topics in MST since it provides information about the basic plasma parameters.\nThe system measures electron density, toroidal current, poloidal magnetic field, and the spatial profiles of each.\nCurrently, we are exploring the possibility of measuring toroidal magnetic field and poloidal plasma current by using the plasma bi-refringence effect, or the [Cotton-Mouton effect.](https://bliptext.com/articles/cotton-mouton-effect) When a linearly polarized EM wave is propagating perpendicular to the magnetic field, the refractive index depends on whether the wave polarization is parallel or perpendicular to the magnetic field direction.\n**Why choose FIR lasers?**\nFor plasma [polarimetry](https://bliptext.com/articles/polarimetry)-interferometry, the [wavelength](https://bliptext.com/articles/wavelength) we chose is sufficiently long to provide measurable plasma induced phase changes, but sufficiently short to avoid\ncomplicated [plasma-wave](https://bliptext.com/articles/waves-in-plasmas) interactions, including the bending of the beam.\nThere are many high power molecular laser lines available in this wavelength range, and many commercially available detectors.\n\n## Thomson scattering\n**What is Thomson Scattering?**\n[Thomson scattering](https://bliptext.com/articles/thomson-scattering) is the result of a collision between a photon (an electromagnetic wave) and a charged particle, such as an electron.\nWhen an electron and [photon](https://bliptext.com/articles/photon) \"collide\" the electron feels a [Lorentz force](https://bliptext.com/articles/lorentz-force) from the oscillating electric and magnetic fields of the [photon](https://bliptext.com/articles/photon) and is accelerated.\nThis acceleration causes the electron to emit a different photon in a different direction.\nThis emitted photon has a wavelength shifted from that of the incident photon by an amount dependent on the electron energy.\nAnother way of looking at this is that the electron absorbs the energy of the photon and re emits the energy in the form of a different electromagnetic wave.\nThis scattering of a photon by an electron is called Thomson Scattering.\n**How is Thomson Scattering useful to plasma physicists?**\nSince the wavelength of the scattered photon depends on the energy of the scattering electron, Thomson scattering is good way to measure the energy of an electron.\nThis is done by creating a photon of known wavelength and measuring the wavelength of the scattered photon.\nThe Thomson Scattering configuration at MST uses a 1064 nm [YAG laser](https://bliptext.com/articles/yag-laser) system, which produces the best time-resolution electron temperature readings in the world.\nWe create our photons with high power lasers that we shine into a window on the top of the MST, and collect scattered photons with a large collection lens on the side of the MST.\nThe wavelength distribution of the scattered photons tells us the energy distribution of the electrons in the plasma, giving us a direct unobtrusive way of getting the temperature of the electrons.\nThe amount of photons we actually collect can also tell us something about the density of the electrons in the plasma.\n\n## Charge exchange recombination spectroscopy and ion Doppler spectroscopy\nFusion plasmas are typically generated from [ionization](https://bliptext.com/articles/ionization) of a neutral gas.\nIn most cases, [deuterium](https://bliptext.com/articles/deuterium) is used as the plasma fuel.\nThese plasmas are therefore primarily made up of deuterium ions (plus electrons), and it is necessary to diagnose the behavior of these ions if the relevant plasma physics is to be understood.\nHowever, in any fusion device, other types of ions (\"impurities\") are also present.\nThese exist naturally due to the inability to achieve a perfect vacuum in a fusion reactor before fueling.\nThus, materials such as [water vapor](https://bliptext.com/articles/water-vapor), nitrogen, and [carbon](https://bliptext.com/articles/carbon) will be found in small amounts in typical plasma discharges.\nImpurities may also be generated during plasma discharges due to plasma-wall interactions.\nThese interactions primarily cause material from the wall to be ejected into the plasma through sputtering.\nIn the Madison Symmetric Torus (MST), properties of the impurity ions (e.g. carbon, [oxygen](https://bliptext.com/articles/oxygen), etc.) are closely linked to properties of the deuterium ions as a result of strong interaction between the ion species.\nThus, impurity ion measurements can, in principle, provide direct information about the deuterium ions.\nMeasurements of the impurity ion temperature (T i ) and flow velocity (v i ) are obtained on MST using Charge Exchange Recombination Spectroscopy, or CHERS.\nThe CHERS process can be broken down into two separate steps: Charge Exchange and Radiative Decay.\nIn the first stage, an electron is transferred from a neutral atom (e.g. deuterium) to an impurity ion that has no electrons (e.g. C +6 ).\nDuring this transfer, the electron typically winds up in an [excited state](https://bliptext.com/articles/excited-state) (high energy level) of the impurity ion.\nAs the electron decays down to the [ground state](https://bliptext.com/articles/ground-state) (minimum energy level), energy conservation requires radiation to be emitted by the impurity ion.\nThis emission has discrete values of energy, or wavelength, which correspond to the energy differences between the initial and final atomic levels of a particular electron transition.\nFor example, consider charge exchange between a deuterium atom and a C +6 ion: if the electron is transferred to the n=7 energy level of the carbon ion, then the ion will emit radiation at discrete energies given by the difference in energy between the n=7 and n=6 levels, the n=6 and n=5 levels, the n=5 and n=4 levels, and so on (down to n=1).\nThis line emission is Doppler-broadened as a result of ion thermal motion, and Doppler-shifted as a result of ion flow.\nThe [Doppler shift](https://bliptext.com/articles/doppler-shift) causes the emission to be blue-shifted (towards shorter wavelength/higher frequency) if the ions are moving towards the point of observation, or red-shifted (towards longer wavelength/lower frequency) if the flow is away from the point of observation.\nMeasurements of the carbon emission line shape are therefore used to extract values for the impurity ion temperature and velocity.\nCharge Exchange: H + C +6 →\nH +1 + C +5 (n=7, l=6)\nRadiative decay: C +5 (n=7, l=6) →\nC +5 (n=6, l=5) + h (photon)\nIn a typical fusion device the neutral atom density is small.\nTherefore, the amount of radiated emission that results from charge exchange between impurity ions and neutrals is also small.\nOn MST, the neutral density is enhanced by injection of fast hydrogen atoms via a diagnostic neutral beam (DNB).\nAs a result, the radiated emission is greatly increased, though primarily along the beam injection path (the DNB is located below the deck, and cannot be seen from here; the injection path is from right to left across the plasma).\nPerpendicular to the beam path, there exist a number of optical ports for viewing the plasma at different radial positions.\nFor a given plasma discharge, a fiber bundle system is placed on one of these ports, and is used to collect emission along its line-of-sight (black tubes on top of the machine contain light collection optics; fibers are placed in the long, curved white tube when not in use).\nThis emission is sent to a [spectrometer](https://bliptext.com/articles/spectrometer) where it is dispersed over a finite wavelength range — which is centered on the emission line of interest — by a pair of optical gratings.\nHowever, because the collected emission is dominated by radiation from along the beam path, the measurements are effectively localized to the intersection volume between the fiber view and the beam.\nOn MST, this intersection volume is small (~ 2 cm 3 ) compared to the plasma volume, allowing spatially resolved measurements of T i and v i to be obtained.\nData collected from a number of plasma discharges — for which the location of the fiber bundle system is varied — are used to construct radial profiles of the impurity ion temperature and velocity, providing important information for understanding the physics of plasmas in MST.\nTypical ion temperatures measured by CHERS on MST are in the range of 100 to 800 eV (2 million to 17 million degrees Fahrenheit), depending on position in the plasma and type of discharge.\nLikewise, measured equilibrium ion velocities are on the order of 1,000 to 10,000 meters per second.","revisionCount":0},"hasMore":false,"nextCursor":null}

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