Observation of asymmetric power spectra of density fluctuations in the JIPP T-IIU plasma by far-infrared laser scattering

Abstract

Asymmetric power spectra of the density fluctuations in the JIPP T-IIU plasma have been observed by submillimetre wave scattering using a newly developed dual homodyne detection system. A linear dispersion relation between the frequency at the peak of the broad spectra of the density fluctuations and the poloidal wave number k e has been obtained. The observed frequencies are about three times higher than the theoretical values for electron diamagnetic drift waves. It is shown that this discrepancy can be accounted for quantitatively by the Doppler effect due to the 2 X B fluid rotation of the plasma. Low frequency line spectra are also observed, exclusively for low k e ; they are identified with MHD waves. Finally, it is pointed out that the extended scattering region could have significant effects on the apparent spectral distribution of the density fluctuations.

Evaluation of spectral broadening due to an extended non-uniform scattering region in microwave scattering experiments

“…This interesting feature is thought to be mainly brought about by the decrease of the scattering volume V s with increasing k$. It is interesting to compare the calculated S(kfl) spectrum with the experimental one which varies approximately as ke~a with a = 2.0 [11]. These results suggest that the extended scattering volume has a significant effect on the evaluation of the dynamic form factor S(k#).…”

“…( 24) under the same conditions, is shown in Fig. 4 for the two cases, together with the corresponding experimental results of JIPP T-IIU [11]. When compared with the experimental results, the spectrum for case I with m = 0.8 rather than that for case II appears to account for the spectral broadening of the scattered waves.…”

“…The local density fluctuations n ^r ^t ' ) can approximately be expressed as an integral of the z'-dependent Fourier components n e (kj_; z',t') with the wavenumber k x perpendicular to the z-axis, because the scattering region extends mainly along the z-axis, its extension perpendicular to the z-axis being much less than the characteristic length of non-uniformity of the plasma parameters. Thus, we have (11) As mentioned above, the value of o> (=adrift + W D ) * S uniquely determined for given k, and z. Hence, the Fourier component n e (k x ; z',t') can be expressed as n e (ic x ; z', t') = n e (ic x , z')expUco(ic x ,z')t']…”

“…Recently, we have carried out FIR laser scattering experiments in the JIPP T-IIU plasma, using a 370 mW, 337 /zm HCN laser and a dual homodyne detection system [11]. We obtained the usual broad spectra with a clear frequency shift towards one side, corresponding to the electron diamagnetic drift frequency.…”