Draft:Compact Toroidal Hybrid

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The Compact Toroidal Hybrid (CTH)[1], is an experimental device at Auburn University that utilizes magnetic fields to confine high-temperature plasmas. CTH is a torsatron type of stellarator with an external, continuously wound helical coil which generates the bulk of the magnetic field for containing a plasma.

The Compact Toroidal Hybrid (CTH) device at Auburn University

Background[edit]

Toroidal magnetic confinement fusion devices create magnetic fields that lie in a torus. These magnetic fields consist of two components, one component points in the direction that goes the long way around the torus (the toroidal direction), while the other component points in the direction that is the short way around the torus (the poloidal direction). The combination of the two components creates a helically shaped field. (You might imagine taking a flexible stick of candy cane and connecting the two ends.) Stellarator type devices generate all required magnetic fields with external magnetic coils. This is different from tokamak devices where the toroidal magnetic field is generated by external coils and the polidal magnetic field is produced by an electrical current flowing through the plasma.

History - Previous Magnetic Confinement Devices at Auburn University[edit]

CTH is the third torsatron device to be built at Auburn University. The two previous devices were:

  • The Auburn Torsatron - 1983-1990
The Auburn Torsatron - the first torsatron built at Auburn University.
 This Auburn Torsatron had an l=2, m=10 coil helical coil.  The vacuum vessel major radius was Ro = 0.58m, the minor radius was av=0.14m, and the field strength was |B| ≤ 0.2T.  It was used to study basic plasma physics and diagnostics, and magnetic surface mapping techniques[2][3]
  • The Compact Auburn Torsatron[4] - 1990-2000
The Compact Auburn Torsatron (CAT) - The second toroidal plasma confinement device at Auburn University.
 This Compact Auburn Torsatron had two helical coils, an l=1,m=5 and an l=2,m=5.  Varying the relative currents between the coils modified the rotational transform. The vacuum vessel major radius was Ro = 0.58m, the minor radius was av=0.14m, and the field strength was |B| ≤ 0.2T  It was used to study magnetic islands[5], magnetic island minimization[6], and driven plasma rotations[7]

The CTH Device[edit]

The main magnetic field in CTH is generated by a continuously wound helical coil. An auxiliary set of ten coils produces a toroidal field much like that of a tokamak. This toroidal field is used to vary the rotational transform of the confining magnetic field structure. CTH typically operates at a magnetic field of 0.5 to 0.6 Tesla at the center of the plasma. CTH can be operated as a pure stellarator, but also has ohmic heating transformer system to drive electrical current in the plasma. This current produces a poloidal magnetic field that, in addition to heating the plasma, changes the rotational transform of the magnetic field. CTH researchers study how well the plasma is confined while they vary the source of rotational transform from external coils to plasma current.

A drawing showing the CTH vacuum vessel (shown in grey) and magnetic field coils.HF(red) - Helical Field,TF - Toroidal Field,OH1,2,3 - Ohmic Transformer Coils, MVF - Main Vertical Field, TVF - Trim Vertical Field, SVF - Shaping Vertical Field, RF - Radial Field, EF, Equilibrium Field, ECC - Error Correction Coil

The CTH vacuum vessel is made of Inconel625®, which has a higher electrical resistance and lower magnetic permeability than stainless steel. Plasma formation and heating is achieved using 14 GHz, 10 kW electron cyclotron resonance heating (ECRH). A 200 kW gyrotron has recently been installed on CTH. Ohmic heating on CTH has an input power of 100 kW.

Operations[edit]

  • Plasmas temperatures have been measured up to 400 electronvolts with electron densities up to 5x1019m-3.
  • Plasmas last between 60ms and 100ms
  • It takes 6min-7min to store enough energy to power the magnet coils

Subsystems[edit]

The following gives a list of subsystems needed for CTH operation.

  • a set of 10 GE752 motors with attached 1-ton flywheels to store energy and produce currents for magnetic field generation
  • two 18GHz klystrons for Electron cyclotron resonance heating
  • gyrotron for 2nd harmonic Electron cyclotron resonance heating
  • a 2kV, 50uF capacitor bank and a 1kV, 3F capacitor bank to power the ohmic system
  • a 900 channel data acquisition system

diagnostics[edit]

The CTH has a large set of diagnostics to measure properties of plasma and magnetic fields. The following gives a list of major diagnostics.

V3FIT[edit]

The CTH group uses the V3FIT[9] and VMEC[10] codes for equilibrium reconstruction

Goals and major achievements[edit]

CTH has made and continues to make fundamental contributions to the physics of current carrying stellarators.[11][12][13] CTH researchers have studied disruption limits and characterizations as a function of the externally applied rotational transform (due to external magnet coils) for:

  • Low-q (safety factor) tokamak-like disruption avoidance[14]
  • vertical displacement events (VDEs)[15]

Ongoing experiments[edit]

CTH students and staff work on a number of experimental and computational research projects. Some of these are solely in house while others are in collaboration with other universities and national laboratories in the United States and abroad. Current research projects include:

  • Density limit studies as a function of the vacuum rotational transform
  • Using spectroscopic techniques to measure tungsten erosion with the DIII-D group
  • Measuring plasma flows with a Coherence Imaging system on CTH and on the W-7X stellarator
  • Heavy ion transport studies on the W-7X stellarator
  • Studying transition regions between fully ionized and neutrally dominated plasmas
  • Implementation of a 4th channel for the interferometer system
  • 2nd harmonic electron cyclotron resonance heating with a gyrotron

Other Stellarators in the US and around the world[edit]

References[edit]

  1. ^ Hartwell, G. J.; Knowlton, S. F.; Hanson, J. D.; Ennis, D. A.; Maurer, D. A. (2017). "Design, Construction, and Operation of the Compact Toroidal Hybrid". Fusion Science and Technology. 72 (1): 76.
  2. ^ Gandy, R. F.; Henderson, M. A.; Hanson, J. D.; Hartwell, G. J.; Swanson, D. G. (1987). "Magnetic Surface Mapping with an Emissive Filament Technique on the Auburn Torsatron". Review of Scientific Instruments. 58: 509.
  3. ^ Hartwell, G. J.; Gandy, R. F.; Henderson, M. A.; Hanson, J. D.; Swanson, D. G.; Bush, C.J.; Colchin, R. J.; England, A. C.; Lee, D.K. (1988). "Magnetic Surface Mapping with Highly Transparent Screens on the Auburn Torsatron". Review of Scientific Instruments. 59: 460.
  4. ^ Gandy, R.F.; Henderson, M.A.; Hanson, J.D.; Knowlton, S.F.; Schneider, T.A.; Swanson, D.G.; Gary, J.R. (1990). "Design of the Compact Auburn Torsatron". Fusion Technology. 18 (2): 281.
  5. ^ Henderson, M. A.; Gandy, R. F.; Hanson, J. D.; Knowlton, S. F.; Swanson, D. G. (1992). "Measurement of magnetic surfaces on the Compact Auburn Torsatron". Review of Scientific Instruments. 63: 5678.
  6. ^ Gandy, R. F.; Hartwell, G. J.; Hanson, J. D.; Knowlton, S. F.; Lin, H. (1994). "Magnetic island control on the Compact Auburn Torsatron". Physics of Plasmas. 1: 1567.
  7. ^ Thomas, Jr., .E; Knowlton, S. F.; Gandy, R. F.; Cooney, J.; Prichard, D.; Pruitt, T. (1998). "Driven plasma rotation in the Compact Auburn Torsatron". Physics of Plasmas. 5: 3991.
  8. ^ Herfindal, J.L.; Dawson, J.D.; Ennis, D.A.; Hartwell, G.J.; Loch, S.D.; Maurer, D.A. (2014). "Design and initial operation of a two-color soft x-ray camera system on the Compact Toroidal Hybrid experiment". Review of Scientific Instruments. 85: 11D850.
  9. ^ Hanson, J.D.; Hirshman, S.P.; Knowlton, S.F.; Lao, L.L.; Lazarus, E.A.; Shields, J.M. (2009). "V3FIT: a code for three-dimensional equilibrium reconstruction". Nuclear Fusion. 49 (7): 075031.
  10. ^ Hirshman, S.P.; Whitson, J.C. (1983). "Steepest‐descent moment method for three‐dimensional magnetohydrodynamic equilibria". Physics of Fluids. 26: 3553.
  11. ^ Ma, X.; Cianciosa, M.R.; Ennis, D.A.; Hanson, J.D.; Hartwell, G.J.; Herfindal, J.L.; Howell, E.C.; Knowlton, S.F.; Maurer, D.A.; Tranverso, P.J. (2018). "Determination of current and rotational transform profiles in a current-carrying stellarator using soft x-ray emissivity measurements". Physics of Plasmas. 25: 012516.
  12. ^ Roberds, N.A.; Guazzotto, L.; Hanson, J.D.; Herfindal, J.L.; Howell, E.C.; Maurer, D.A.; Sovinec, C.R. (2016). "Simulations of sawtoothing in a current carrying stellarator". Physics of Plasmas. 23: 092513.
  13. ^ Ma, X.; Maurer, D.A.; Knowlton, S.F.; ArchMiller, M.C.; Cianciosa, M.R.; Ennis, D.A.; Hanson, J.D.; Hartwell, G.J.; Hebert, J.D.; Herfindal, J.L.; Pandya, M.D.; Roberds, N.A.; Traverso, P.J. (2015). "Non-axisymmetric equilibrium reconstruction of a current-carrying stellarator using external magnetic and soft x-ray inversion radius measurements". Physics of Plasmas. 22: 122509.
  14. ^ Pandya, M.D.; ArchMiller, M.C.; Cianciosa, M.R.; Ennis, D.A.; Hanson, J.D.; Hartwell, G.J.; Hebert, J.D.; Herfinday, J.L.; Knowlton, S.F.; Ma, X.; Massida, S.; Maurer, D.A.; Roberds, N.A.; Traverso, P.J. (2015). "Low edge safety factor operation and passive disruption avoidance in current carrying plasmas by the addition of stellarator rotational transform". Physics of Plasmas. 22: 110702.
  15. ^ ArchMiller, M.C.; Cianciosa, M.R.; Ennis, D.A.; Hanson, J.D.; Hartwell, G.J.; Hebert, J.D; Herfindal, J.L.; Knowlton, S.F.; Ma, X.; Maurer, D.A.; Pandya, M.D.; Tranverso, P.J. (2014). "Suppression of vertical instability in elongated current-carrying plasmas by applying stellarator rotational transform". Physics of Plasmas. 21: 056113.

External links[edit]

Category:Stellarators Category:Plasma physics Category:Auburn University