Frequency Broadening of High Harmonic Fast Wave

T.Taniguchi, Y.Takase, A.Ejiri, S.Shiraiwa, M.Ushigome, Y.Nagashima, H.Kasahara, T.Yamada, M.Aramasu, S.Ohara, K.Yamagishi

University of Tokyo, Bunkyo-ku, Tokyo, Japan

In a spherical tokamak, the high harmonic fast wave ( HHFW ) is a candidate for noninductive current drive because of good accessivility in high β plasmas [1]. HHFW excitation and wave propagation experiments have been performed in TST-2 [2]. Frequency broadening of HHFW has been observed. This phenomenon is likely caused by scattering by plasma density fluctuations [3], and it can affect absorption by the plasma. Therefore, understanding of the interaction between the incident wave and the density fluctuations is important.
The frequency spectrum of the HHFW measured by a probe about 60 degrees separated from the antenna toroidally was found to be broadened by abount 30kHz (full width at -10dB from the peak). A correlation between the core plasma density and the width of the spectrum was observed. A ray tracing simulation reproduces the same tendency, suggesting that a change of ray trajectory is playing a role. At higher density, the wave spends a longer time to cross the edge region, and suffers more scattering, leading to a more broadened spectrum.
The relationship between the HHFW and the density fluctuation was modeled in a cold plasma approximation and the time evolution of the HHFW spectrum was formulated without invoking the electrostatic approximation. The relationship is expressed in the form of the Pauli equation and the probability of scattering is proportional to the square of the fluctuation level. The validity of the cold plasma model is examined by comparing the hot and cold dielectric tensors for the HHFW. The difference is of order ο(b), where b= ( kvT/ Ω ) 2 and is only a few percent.
A comparison between the experimantally obtained spectrum and the calculated spectrum in a circular cross section plasma model was made and the power deposition profile was also calculated. Electron Landau damping was found to be dominant and a 5% change in ktoloidal leads to about 5% change in &rho, the peak damping location. This can affect the heating profile and heating efficiency. A lower density at the edge and a higher density at the center would give a more favorable condition.

References

[1]M.Ono, Phys.Plasmas 2 (1995) 4075
[2]Y.Takase et al., Nuclear Fusion 41 (2001) 1543
[3]R.Cesario et al., Nuclear Fusion 32 (1992) 2127


This study is sponsored by University of Tokyo