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Fusion Plasma Research


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Aug - Nov 1999


(1) High performance plasmas
Full CD reversed shear plasma with high bootstrap fraction (about 80%) was demonstrated for 2.6 s at Ip = 0.8 MA, τE = 0.4-0.5 s, HITER-89L ~ 3.6, βp= 2.6-3.0, and βN = 1.9-2.2. The plasma has an internal transport barrier (ITB) which helped with causing the bootstrap current to flow at r/a = 0.3-0.7 and maintaining reversed shear. The discharge was fulfilled by highly plasma shaping (the triangularity (δ) of 0.37, and the ellipticity of 1.44) and newly developed stored-energy feedback control. This achievement will be a great first step towards steady-state operation on the basis of reversed shear discharges. It should be also stressed that HH = 2.0-2.2 was obtained at high density ne/nGW ~ 0.65.
The stored energy feedback was applied to high βP ELMy H-mode plasmas (Ip = 1 MA, Bt = 2.1 T) with high δ (~ 0.4) for the first time, which led to a sustainment of high βN (=2.8-2.9) for 4.5 s at q95 ~ 3.4. This is the longest sustainment of such high βN on JT-60U.

(2) Current Drive
Current drive experiment with N-NBI was performed to reach a reactor-relevant current drive efficiency ηCD (> 1.3x1019 A/W/m2). N-NBI was injected into an optimized high βP H-mode plasma with high Te (Te(0) = 9.5 keV). ηCD in the shot is still in analysis, but it seems to surpass the record set in last April (about 1.3x1019 A/W/m2).

(3) Confinement Physics
Neutral beam power (PNB) for ITB formation in reversed shear discharges was investigated. By scanning PNB at the Ip ramp-up phase, it was found that the power needed to form the ITB does not depend on Bt. Dependence on other parameters such as density remains to be investigated.
Confinement improvement was investigated against Te/Ti in LHCD-sustained reversed shear with ECH. In the range of Te/Ti = 1-2, the temperature ratio does not affect confinement improvement so much.

(4) MHD
Wall stabilization effect on the beta limit in reversed shear plasma with ITB was examined. One of this series of discharges reached the highest βN (= 2.76) in reversed shear plasmas. Whether or not this series of discharges exceed the ideal MHD stability limit without wall is a future issue to be resolved.
ECRF was applied to a plasma with continuous m = 2 oscillations to investigate the effects of ECRF on MHD modes. During ECRF injection at 0.75 MW, a little reduction in the amplitude of the oscillations was observed although it was not identified whether the oscillations were NTM or not.

(5) ECRF experiments
Dependence of Te rise on the polarization of ECRF waves was experimented. For O-mode ECRF heating with the purity of 0.95-1, the highest Te rise of 4.4 keV was obtained at Pinj ~ 0.75 MW and the ECRF pulse of 0.3 s.
When ECRF waves were injected during LHCD, the current profile was found to be changed, suggesting the possibility of current profile control with LHCD plus EC. This is probably because ECRF accelerates fast electrons produced by LHW.
ECRF was also injected to study sawtooth stabilization. The stabilization was very sensitive to the injection angle: When the waves were injected to heat around q ~ 1, the sawtooth-free period was stretched 9-10 times long.
Plasma startup with ECRF was tested for the first time in JT-60U. The initial result indicated plasma startup at Vl = 4 V, corresponding to the toroidal electric field of 0.19 V/m.

(6) Energetic Particles
In order to setup a plasma which will help energetic particle research, D-3He experiment in reversed shear produced the fusion output exceeding 100 kW. The spectrum of γ-rays showed two peaks originating from D-3He reactions, well separated from the background noise by 56Fe(n, γ). The estimated fusion output is 107 kW, a little smaller than the output produced in JET ICRF experiment (140 kW).
In terms of Alfvén eignmode (AE mode) experiment with N-NBI, the regime where AE modes are unstable was exploired by extending the experimental conditions towards higher vb/vA up to ~1.2. Interestingly, chirping modes and bursting modes were readily destabilized when vb/vA ~ 1, rather than higher vb/vA. Another experiment indicated that AE modes were destabilized in low βh regime (~0.05%, comparable with AE modes in ICRF heating).
Sawtooth stabilization experiment by N-NBI started to confirm whether passing fast ions can stabilize sawtooth oscillation. The experiment was carried out with low power N-NBI (~3 MW), extending the sawtooth-free period to 240 ms from 105 ms.

(7) Runaway Electrons
Behavior of runaway electrons in disruption was investigated. In the experiment, runaway electron tail with the current of ~0.9 MA was intentionally generated, and then a value of qs (the surface q) at which the current tail disappeared was examined by reducing qs in time. The experiment indicated that qs ~ 3 is a hard value where the current tail diminishes in spite of high one-turn voltage (150-200 V).


(1) Impurity seed in reversed shear and ELMy H-mode plasmas
Argon and neon seed experiments in reversed shear (RS) plasmas with the internal transport barrier (ITB) were conducted in August and September. The confinement performance of HITER-89L = 2 - 2.5 at ne/nGr = 0.75 was achieved in the Ar and Ne seed RS plasmas at 0.9 MA and Bt = 3.5 T. In these discharges, the fraction of total radiative loss to the absorbed heating power was estimated to be 60 - 70%. However, the removal of Ar and Ne in RS plasmas was difficult and Ar and Ne accumulation in the core plasma was observed. Then the feedback control of RS discharges to keep constant radiation was also difficult. As a result, the effective ionic charge was estimated to be Zeff = 4.5 for Ne seed and Zeff = 5 for Ar seed. The high Zeff > 5 in Ar seed RS experiment was problem to increase radiative loss. This result indicates that neon seed in combination with D2 gas puff in RS plasmas seem to be better than Ar seed in order to keep low Zeff.
Kripton and argon seed experiments in ELMy H-mode hydrogen plasmas were carried out to produce radiative mantle in collaboration with PPPL. In the case of mixture gas puff of Kr : H2 = 1 : 1, the confinement performance of HITER-89L = 1.4 - 1.25 at ne/nGr = 0.4 and the radiative loss fraction of 50% was achieved at 1.2 MA , Bt = 2.5 T and PNB=12 MW. In these discharges, the electron density and the radiative loss fraction were increased up to ne/nGr = 0.85 and 100%, respectively, without disruption by using the radiation feedback control.

(2) Helium Exhaust
Helium exhaust characteristics of reversed shear plasmas has been studied by using a He-beam injection. The He residence time inside the ITB was 1.5 times as long as that outside the ITB in the reversed shear plasma (IP = 1.7 MA, Bt = 3.7 T and ne = 3x1019 m-3). This result indicates that it is difficult to remove helium particles inside the ITB as compared with those outside the ITB as predicted from the previous result. This is due to the improvement of He particle confinement inside the ITB. If the residence time of the He density inside the ITB is assumed as the local τ*He, the ratio of τ*HeE = 11 was achieved with He pumping, within the range generally considered necessary for successful operation of future fusion reactors. In reversed shear plasmas, efficient He exhaust was realized at the first time.
After the W-shaped divertor modification from inner-leg pumping to both-leg pumping, the pumping rate strongly depends on the gap_in and gap_out (i.e. distances between the inner/outer separatrix and the inner/outer slots) in L- and H-mode plasmas. Efficient He exhaust was realized in a divertor-closure configuration with the both-leg pumping. In steady state, good He exhaust capability was successfully demonstrated in attached ELMy H-mode plasmas. A global particle confinement time of τ*He = 0.4 s and τ*HeE = 3 was achieved in attached plasmas. The He exhaust efficiency with the both-leg pumping was enhanced by 40% as compared to one with the inner-leg pumping. Good He exhaust capability was also obtained in a detached ELMy H-mode plasma (ne/nGr = 0.9). In detached divertor plasma, however, the He exhaust efficiency of τ*He = 0.6 s was degraded as compared with one in attached plasma.
In-out asymmetry with He flux in the divertor during ELMy H-mode was successfully controlled by changing neutral beam (NB) power and plasma current. The in-out asymmetry with He and deuterium flux profiles in the divertor during high βp ELMy H-mode strongly affected the He exhaust capability. Selective He exhaust was studied in combination with the outboard enhanced He flux profile, the inboard enhanced deuterium flux profile and the outer-leg pumping in high triangularity ELMy H-mode. The ratio of the He neutral pressure to the deuterium neutral pressure in the divertor decreased with increasing electron density in the main plasma. The selective He exhaust in the low density operation is possible to enhance the enrichment factor of He, η=[PHe/2PD2]div/[nHe/ne]main.