Studies of short-waveturbulence in ECR heated plasma of the L-2M stellarator

In an attempt to explain anomalous transport in toroidal confinement systems, recent attention has been focused on short-wave turbulence associated with ETG (electron-temperature gradient driven drift waves) instability. Shortwave turbulence was observed in the L-2M and LHD stellarator by using scattering of gyrotron radiation at the operating frequency and at its second harmonics. The excitation of short-wave turbulence (kv
_Te
/omega
_e
~0.3-0.4) in plasma of the FT-2 tokamak was studied by using enhanced scattering diagnostic. The report presents results of studies of characteristics of short-wave turbulence measured from the scattering spectra of gyrotron radiation at the second harmonic of operating frequency (according to the Bragg relation, we have kap40 cm
^-1
; kv
_Te
/omega
_e
ap0.2), whereas long-wave plasma fluctuations measured from small-angle scattering of gyrotron radiation at the operating frequency (kap1-1.5 cm
^-1
, kv
_Ti
/omega
_i
ap0.1). Measurements were carried out at ECRH powers of 100 and 200 kW. The Fourier spectra of scattered radiation in both cases are continuous spectra with exponentially decreasing intensity in the frequency range from 5 kHz to 1 MHz and with feebly marked wide bands. As the ECRH power was doubled, the noise spectral density increased by a factor of 1.5-2 throughout the measured spectra. Under these heating conditions, the electron temperature in the core plasma increased by a factor of 1.5. When the sector limiter was introduced into the plasma to a depth of 2 cm, the electron temperature was lower by a factor of about 1.5; at the same time, the noise spectral density increased by a factor of 1.5-2 in the frequency range 10-150 kHz for both kap40 cm
^-1
and kap1-1.5 cm
^-1
. In the scattering spectra of fluctuations with kap40 cm
^-1
in the region r/a = 0.3-0.4, an increase in the intensities of spectral bands standing out against a background of continuum was more in evidence than in the spectra measured in the region r/a = 0.5-0.6. Estimates show that the conditions for an onset of ETG instability are satisfied in L- 2M plasma at r/a= 0.3-0.6. It may be assumed that a correlation between values of the electron temperature and spectral intensity of short-wave turbulence, observed under different discharge conditions (ECRH power, plasma-wall interaction), is some evidence of interrelation between transport processes and short-wave turbulence.