Searching for Whistler Waves in the Madison Symmetric Torus (MST) Tokamak Plasma*

Experiments and simulations show Whistler waves can be driven by runaway electrons. High frequency spectra measured in MST show multiple coherent lines. These lines maybe due to discrete toroidal modes. Multipole single-turn coils are used to measure magnetic fluctuation and wave number spectra. Whistler-like magnetic fluctuation up to 400MHz have been observed. Magnetic fluctuation and x-ray intensity show a strong correlation. Both signals show alternating bands of high and low activities. The initial insertable  probe design had several resonances that caused difficulty interpreting k spectra. A new prototype high frequency probe has been designed. With improved shielding and termination, the measured frequency response is free of resonance up to 400MHz. Using two-point correlation method, k_⊥ and k_‖ are measured. Signalers are digitized at 5 GHz. The target Plasma has BT=1.38kG, I_p~64kA, 0.007x10¹³< n_e (cm^-3)< 0.4x10¹³, q(a)~1.8, and f_ce ~ 2.5. The fluctuation amplitude of these frequency lines decrease with increased density and are absent at 0.4x10¹³ cm^-3. X-ray pulses show band-like structure similar to the magnetic fluctuations. Magnetic fluctuation and x-ray spectra variation with density and magnetic field will be discussed.

*This work is support by the US DoE and WiPPL.