The effects of externally imposed magnetic perturbations on plasmas in TEXT
The Texas Experimental Tokamak (TEXT) has a set of eight perturbing coils which imposes a spatially resonant radial magnetic field structure on a tokamak plasma. These coils were designed to produce a magnetic perturbation with an m = 7, n = 2 mode structure. This perturbation produces magnetic island chains on the rational surfaces in the plasma. The perturbing coils are driven with alternating current in a 2-8 kHz range of frequencies, thus oscillating the magnetic islands. These frequencies are of the same order as the natural MHD oscillations in TEXT. When used in this manner, the imposed magnetic perturbation is termed the Resonant Helical Oscillator (RHO). A theoretical model developed by Dr. Alan Boozer predicts that currents will flow on the perturbed rational surfaces so as to oppose the applied magnetic perturbation. The plasma parameters of density, current, toroidal field, and working gas, and the RHO settings of frequency and current are varied to characterize the plasma response under several conditions. The plasma response to the RHO is detected by a special set of sensing coils and compared to the predictions of the model. The experimental data from these coils supports the theory that small toroidal currents are being driven by the RHO on the island chains. For normal TEXT plasmas exhibiting low levels of MHD activity, these currents oppose the applied perturbations as predicted. The magnitude of the plasma response increases with perturbation frequency and strength as expected. In addition, the plasma response from the perturbed resonant surfaces appears to exhibit the predicted inverse relationship to the local parallel resistivity. This type of measurement can thus be used to measure changes in Z[sub eff] on the resonant rational surfaces. The phase shift of the plasma response is also examined and its dependences are consistent with a model of the plasma as an inductive element.
- Research Organization:
- Texas Univ., Austin, TX (United States)
- OSTI ID:
- 6170954
- Resource Relation:
- Other Information: Ph.D. Thesis
- Country of Publication:
- United States
- Language:
- English
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MAGNETIC ISLANDS
SYNTHESIS
MODE RATIONAL SURFACES
ELECTRIC CURRENTS
TEXT DEVICES
MAGNETOHYDRODYNAMICS
KHZ RANGE 01-100
MAGNETIC FIELD CONFIGURATIONS
MAGNETIC FIELDS
PLASMA DENSITY
TOKAMAK DEVICES
CLOSED PLASMA DEVICES
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FREQUENCY RANGE
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700340* - Plasma Waves
Oscillations
& Instabilities- (1992-)