K. Kawahata

K." Matsuoka

R. "Pavlichenko

"Pavlichenko,  R.

Kawahata,  K.

Matsuoka,  K."

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§24. Observation of the Plasma Density Fluctuation in CHS via Monostatic Antenna Pulsed Radar Reflectometer Pavlichenko, R., Kawahata, K., Matsuoka, K. Recently growing attention to the reflectometry , as the suitable diagnostic for plasma density profile recon-struction and for measurements of plasma density fluc-tuations, has been widely recognized. The routine description and schematic layout of CRS pulsed radar reflectometer are given elsewhere[1]. In the reference it was shown that due to the comparatively big reflection from the vacuum break mica window than that from the plasma use of the time-of-flight measure-ment system was not allowed. To overcome this situation special new microwave technique was used. The idea of this improvement lies on the additional 'time-gating' of the reflected signals. Various plasma experiments are made in standard op-eration mode. It is characterized by low magnetic field B t (0) (typically 0.8--0.9 T), not so high central electron density ne rv 3 x 1013cm-3 , various plasma axis posi-tion. Fundamental harmonic (53.2 GHz) of the ECR or 7.5 MHz ICRF(IBvV) microwave radiation power is employed to build up a plasma. The target plasma is heated by 1 M\i\T NBI or( and) by the ECR radiatiation power (fundamental at 53 GHz or(and) second harmonic at 106.4 GHz). To control the plasma density (ne) both gas puffing and Ti-gettering are used. In the regimes with the maximum density above the cut-off density, a pulsed radar relies on the total reflec-tion of the electromagnetic waves by the plasma. The time necessary for transmitted narrow pulse to complete a round trip to the critical layer in the plasma and back to the antenna is directly measured. In low density plas-mas, where the maximum density is below the critical density corresponding to the pro bing frequency, pulses launched into the plasma will not be reflect"ed but pass through the plasma. They will be reflected by the inner wall of vacuum vessel, returned and detected. For the time that does not overlap with ECH pulse the agree-ment is rather good. Another field, where pulsed radar can be applied, is measurement of MRD oscillations. The understanding of the nature of these phenomena gives additional in-formation for the configuration optimization , stability and transport properties of the CRS plasma. The ob-servation of the local plasma electron density fluctuation was performed during low-,B experiments. Those oscilla-tions could be originated from the magnetic islands rota-tion. As another confirmation of those idea, we present the comparison of the spectra for the magnetic probes and reflectometry data in Figure 1. Our experiments have the primary aim to check the occasion of pulsed radar operation in the helical systems. It was found that general behavior of this diagnostic tool is very close to (> -~~~~~~~~~~~~ :;; ~8 .0 ~ J'l ~~ Ii '" ., 0 .0 0 ~~ t~ 671669 E g ~6L5~~~70~~~7~5~~~BO~~~B5 time 10 -3 [sl Figure 1: Comparison of the signals from the radar and corresponding spectra during 65-85 ms and of the probes for the same time window the discharge #71669 those that operate on tokamaks. Among distinctive fea-tures for the helical systems that must be certainly men-tioned we must point out several items: (a) real three-dimensional topology of the reflected surfaces (it means small reflected power and additional problems with an-tenna arrangement, limited port access); (b) sometimes high, non-negligible shear (the deduction of the magnetic field and the effect of mode coupling becomes more dif-ficult). References [1] R.Pavlichenko, A.Ejiri, K.Kawahata, K.Matsuoka, Jour. Plasma and Fusion Res. Series,l (1998) 378. 247 

Observation of the Plasma Density Fluctuation in CHS via Monostatic Antenna Pulsed Radar Reflectometer

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