Group" CHS

K. "Ohkuni

K. Toi

National Institute for Fusion Science (NIFS-Repository)

§9. Inward Turbulent Particle Transport in NBI-Heated Plasmas Ohkuni, K. (Dep. Energy Eng. Science, Nagoya Univ.), Toi, K., CHS Group Edge plasma turbulence study are carried out using a Lang-muir probe array in the CHS heliotron/torsatron. The Lang-muir probe array consists of 4 sets of a triple probe, which are separated radially[!]. Each triple probe set has 4 molyb-denum electrodes. ·Radial profiles of time-averaged plasma parameters such as electron temperature Te, electron density ne and plasma potential V, are measured by 4 set probes for one discharge. Floating potentials are measured by two elec-trodes in each probe set, which are separated by 4.5 mm in the poloidal direction. Fluctuations of poloidal electric field E0 are derived from these floating potentials. Here, fluctua-tions of electron temperature Te are ignored. The turbulent particle flux is expressed as where B,, y, a, P and f are the toroidal magnetic field, co-herence, phase, power spectrum density, and frequency, re-spectively. Therefore, the flux is reduced by reduction of fluctuation amplitude, decorrelation of ne and E0 or change of relative phase between them, but inward flux is only realized by the change in the relative phase between both fluctua-tions. In some shots, the averaged inward flux is seen around r/<a>- 0.94 during the NBI heating phase. We investigated the parameter space where the inward flux is realized, changing fuel gas, B,, NBI heating power PNB1 and ne near the edge, in the inward-shifted configuration of Rax - 0.92 m. As seen from Fig.l, the inward flux is observed in the space of Te > 15 eV and ne < 3x1018 m·3 at r/<a>- 0.94. The in-ward particle flux appeared in several fuel gases (H and He) and at B, = 0.9 - 1.4 T. This parameter space approximately corresponds to the low collisionality regime (the effective collision frequency normalized by the transit frequency of a circulating particle u* < 1) less than lower bound of Pfirsch-Schliiter regime, as seen from Fig.1. The radial electric field Er, its shear E/ and curvature E/' are thought to be the possible candidates for the reversal of fluc-tuation-induced particle flow. The inward flux obviously correlates withE/, that is, the large positive E/ is generated near the relevant region where the inward flux takes place. Moreover, Te profile in the low density discharge clearly has a plateau or slightly hollow structure, of which location ap-proximately corresponds to that of the 1/q = 1 rational sur-face r/<a>l 11q=I = 0.94- 0.97. The sign of the phase cos(~eEa) between ne and E0 in an inward flow discharge is in cos(~cEo) - -1 up to 50 kHz, in contrast to that in an outward flow dis-charge (cos(a...,Eo) > 0), while the coherence y...,Eo remains un-changed in both discharges. The correlation between the 284 flow reversal and E" E/ and E/' is further investigated for many shots in CHS. The dependence of the turbulent particle flux r, .. m on E/ is shown in Fig.2, where r,um and Er' are eval-uated at r/<a> - 0.94. From Fig.2, the critical value that in-duces the large inward turbulent flux (> 1 x 1020 m·2s-') seems to be Ere' - 1xl06 Vm·2, although the inward flux is observed even in the range of 0 < E/ < Ere'. This threshold in E/ is about twice of that in the edge ECH experiment[2]. -.:t 0) c) II 1\ ro v ""i::: "E c? E 0) 0 s Q) c 0.8 0.6 0.4 0.2 0 0 ~ {x >xx~ ~ 10 20 T (eV) at r/<a>=0.94 e 30 helical trapped Fig. 1 Data points of r,uro at r/<a> - 0.94 on the parameter space de-fined by T. and n. at rl<a> - 0.94. The solid circles, open circles, open squares and crosses respectively indicate the data with r, .. b < -3x10'\ -lx10'9 > r,urb > -3xl0'\ 0 > r,urb > -1xl0'9 and r,.rl> > 0 mls·'. Inward particle fluxes are clearly observed in the range of T. > 15 eV and n. < 3x10'8 m 3• 2 00~ - ~~ (/) C}l E 0 0 ~~ C\J 0 s -e o WC\1) o J: ...... ~ -1 O<§>ClD 0 co -2 -1 0 1 2 3 E' (1 o6vm-2) r Fig. 2 Dependence of r,uro at r/<a> - 0.94 on the radial electric field shear E,' at r/<a>- 0.94. When E,' is more than 1x1Q6 Vm2, large inward r,urb is observed. Reference [1] K. Ohkuni, et al., Rev. Sci. Instrum. 72,446 (2001) [2] M.G.Shats, K.Toi, K. Ohkuni, et al., Phys. Rev. Lett. 84, 6042 (2000) 

Inward Turbulent Particle Transport in NBI-Heated Plasmas

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)