March 2008

  The following experiments were carried out in this month.

(1) Plasma start-up by ECRF alone
An experiment on Ip start-up by ECRF alone was carried out. Purpose of the experiment is to increase achievable Ip and study the details of plasma evolution. Achieved Ip was increased to 21 kA from 17 kA obtained in 2007. It was found that by increasing the vertical field (Bv) in time, Ip increased but started to decrease at 21 kA in most of the cases, and this did not depend on the Bv ramp-up rate. This could be related to outward plasma shift that might cause interaction with the first wall. Also it was found that fundamental O-mode was more effective than fundamental X-mode.

(2) Development of reversed shear plasmas with large bootstrap current fraction
In a previous experiment, a plasma with a large fraction of bootstrap current (~70%) was sustained for ~8 s in a reversed shear (RS) ELMy H-mode plasma (3.4 T/0.8 MA, q95~8.3). The duration was limited by the ideal beta limit without conductive wall. By utilizing large volume configuration close to the conductive wall for wall stabilization, beta limit was significantly improved. As a result, high confinement RS plasmas exceeding the no-wall beta limit with large fBS were obtained in reactor-relevant low q95 regime, where βN~2.7 and βp~2.3 were achieved in a RS plasma with the minimum q value (qmin) of ~2.4, and HH98~1.7, electron density normalized by the Greenwald density ne/nGW~0.87 and fBS~90% were obtained at q95~5. 3.

(3) Nonlinear Alfvén eigenmode activity by NNB
NNB induced non-linear behavior in the Alfvén eigenmode (AE) frequency range was previously observed as a bursting mode. To measure the redistribution of fast ions due to the bursting mode in more detail, the neutron profile monitor has been upgraded. Discharge optimization was performed for the measurement of the bursting mode with a high signal-to-noise ratio. Although the bursting mode was observed, its amplitude was less than 1/3 compared with the previous experiments due to lower energy and power of NNB. On the other hand, a new type frequency sweeping mode was observed during Ip ramp-up phase.

(4) Study of high radiation fraction and good confinement H-mode plasma
Power handling by large radiation power loss has been studied in the ELMy H-mode plasmas with argon (Ar) gas seeding. Control of the large radiation (Prad/PNB~0.7-0.9) in the good energy confinement plasma was investigated using the radiation feedback for Ar gas puff rate, for the first time, under the wall saturated condition in the long discharges (25 s). Total radiation fraction of Prad/Pabs = 0.75-0.85 was maintained during the constant Ar gas puffing (up to 8 s so far), and H89L=1.5-1.4 (HH98 = 0.9-0.78), where Type-I ELM activity was mitigated to Type-III. Feedback gains for sustaining the high edge radiation under external disturbances, such as NB break-down and increasing in recycling, have been optimized.

(5) Tungsten transport
It has been found that the tungsten accumulation in the core plasma, indicated by W44+ line intensity, tended to increase significantly with increasing the plasma toroidal rotation velocity in the opposite direction of the plasma current. The accumulation level of tungsten ions did not seem to depend on the tungsten generation flux from the tungsten coated divertor plates, suggesting that the tungsten accumulation in the core plasma is ascribed to the transport in the core and/or edge plasmas.

(6) Measurements of the SOL flow and plasma fluctuations
Fast SOL flow, fluctuation characteristics and ELM propagation were measured by three Mach probes located at the low-field-side midplane, X-point and high-field-side SOLs.
Fast SOL flow: large D2 gas puff rate up to 28Pa•m3/s was injected from the top with the divertor pumping (puff and pump) in L-mode, and plasma profiles of the high- and low-field-side SOLs were investigated. For the case of the special plasma configuration (upper SOL interacting with the first wall: rmid~2.5cm), SOL plasma density and pressure at the outer flux surfaces (rmid>2.5cm) were decreased only at the HFS. The result suggested that the fast SOL flow at the HFS was produced by in-out asymmetry in plasma pressure along the SOL field lines.
Fluctuation characteristics: simultaneous measurement of ion saturation current (Is) and floating potential (Vf) was performed at the three Mach probes. Probability distribution function (PDF) of Is at the LFS SOL was skewed positively associated with the blobby plasma transport, and the PDF at HFS SOL was close to Gaussian distribution. Conditional average of Is and correlation between Is and Vf showed that sharp peak (~ a few micro-sec) of Is and Vf changes from positive to negative values when the blob was passing the probe.
ELM propagation: Vf at the top and side of the probe (~5 mm separation) changed similarly, which suggested that radial movement may be fast, or moving in the poloidal/toroidal direction. Correlation between Is and Vf will be investigated to understand internal potential structure of ELM plasma filament.