October 2004

  In this month, the experimental studies for (1) long pulse operation, (2) stationary sustainment of high βN plasma, (3) long-pulse high-recycling H-mode, (4) high beta full non-inductive current drive, (5) high beta in low q regime, (6) high density RS plasma, (7) beta dependence of H-mode confinement, (8) NTM stabilization with ECCD, (9) real time control of q profile, (10) CS-less operation, (11) SOL (scrape-off layer) plasma fluctuations, and (12) others have been carried out. The results are shown below.

(1) Long pulse operation: Increase in plasma current in long pulse discharges was tried and a 1.4 MA 30s H-mode discharge was obtained (previously 1.0 MA). The pulse length of LH was extended up to 23 s and the injected energy reached 16 MJ. The ECRF pulse length was also extended up to 28 s (8.4 MJ) by using 4 gyrotrons in series. The total power injected into the torus from P-NB and N-NB reached 350 MJ.
(2) Stationary sustainment of high βN plasma: The discharge scenario was further optimized to increase the heating power in a long duration (>15 s). One of the key points is to extend the duration of perpendicular NB units, which was limited by the temperature rise in armor tiles due to shine through power, by reducing the duty of injection and by increasing the plasma density. The extended pulse length of N-NB and ECRF was also beneficial to obtaining long-duration, high-heating-power phase. As a result, βN~2.3 has been sustained for 22 s with Ip = 0.9 MA, Bt = 1.6 T and q95 = 3.2.
(3) Long-pulse high-recycling H-mode: In repeated 30 s NB-heated (~10 MW) discharges (1.0 MA, 2.7 T) with ne-bar/nGW ~0.66, saturation of divertor and first-wall was realized, and an H-mode plasma, without X-point MARFE, was maintained for 6 s without wall-pumping by optimizing the strike point locations to enhance active divertor pumping. Particle balance during and between discharges was analyzed and the number of particles retained in the divertor/first-wall was estimated to be 4x1022, suggesting the first wall other than divertor targets works as a pump.
(4) High beta full non-inductive current drive: Discharge scenario optimization was performed with reduced recycling after the boronization in September. The q profile was tailored to keep q(0) >~1.5 by adjusting the start times of P-NB (for bootstrap current) and N-NB (for beam-driven current) injection. Another key point is toroidal rotation control to suppress mini-collapses, which seemed to be related to the zero toroidal rotation at the q =2 surface. As a result, high βN ~ 2.4 with good confinement H89 ~2.2 (HHy2 ~1.0) was sustained for 5.8 s (~26τE, ~2.8τR) with a non-inductive current drive fraction of >90% (bootstrap ~ 45%, NBCD ~ 50%) in a high βp ELMy H-mode plasma with a weak shear q-profile of q(0) ~1.5 and q95 ~4.5 (Ip = 1 MA, Bt = 2.4 T). No NTMs including m/n =3/2 were observed due to high q(0).
(5) High beta in low q regime: In 2003, βN=3 was maintained for 6.2 s in a low q regime (q95 = 2.2) with broad (off-axis) heating profile. The effects of low q and broad heating profile were investigated separately. It was found that (i) m/n = 3/2 NTM disappeared for q95<2.9 and (ii) sawteeth disappeared with broad heating profile. In this series of experiment, βNH89/q952 = 0.93 was achieved (βN = 3, H89 = 1.5, q95 = 2.2).
(6) High density RS plasma: Injection of seed impurity Ne together with D2 gas was attempted in high-density high-confinement reversed shear plasmas (1.0 MA, 2.9 T, q95~6.5) to enhance the divertor radiation loss. As a result, the total radiation loss was enhanced to a level greater than 90% of the net heating power with high confinement HHy2 =1.1 at a high density above the Greenwald density (ne-bar/nGW = 1.1), though the radiation power from the core plasma was still higher than that from the divertor region.
(7) Beta dependence of H-mode confinement: Dependence of BτE in ELMy H-mode plasmas on toroidal beta β was investigated. The toroidal field B was varied 2.1-2.51 T keeping other non-dimensional parameters, ρ*, ν*, q95, a/R, κ, δ fixed. It was found that B&tauE decreased with β-0.6. Non-dimensional transport analysis using the previous JT-60U database has also been done, and BτE ~ β-(0.6-0.8) has been found. These results agree well with each other.
(8) NTM stabilization with ECCD: It was found that early injection of ECCD (2.4 MW, second harmonic X-mode), before the mode onset, was effective to stabilize m/n=3/2 NTM even in a high beta regime (βN ~3, Bt = 1.7T, Ip = 0.85 MA, q95 ~3.5). Scan of ECCD location showed that the stabilization effect strongly depends on the ECCD location, as observed previously with fundamental O-mode ECCD.
(9) Real time control of q profile : Real time q-profile control system with MSE diagnostics and LH waves has been improved. The current drive (CD) location is estimated from the change of current profile evaluated in real time. The LH power is also controlled to maintain the LH-driven current constant with varying parallel refractive index N// (CD efficiency). The system worked well and q(0) increased from 1 to 1.3 in a discharge of 0.6 MA, 2.3 T, ne-bar = 0.5x1019 m-3 with LH power of ~1 MW.
(10) CS-less operation: An attempt was made to ramp up the plasma current from a low level of 0.2-0.3 MA by NB alone with fixed currents in all poloidal field coils (no flux input), and the plasma current of 0.26 MA (βN = 2, βp =4) was sustained for 1 s.
(11) SOL plasma fluctuations: Fluctuations and ELM bursts of ion saturation current were measured with 500 kHz sampling in the SOL/divertor regions of ELMy H-mode and L-mode plasmas using reciprocating probes and divertor target probes. SOL fluctuations in L-mode plasmas were also obtained for reversal of toroidal field and plasma current with ion gradient-B drift away from the X-point.
(12) Others: Other experimental studies, in connection with ITB characteristics under small central fueling, fast ion transport by RSAE (reversed shear Alfven eigenmode), and monitoring of impurity contents, have been carried out.