Mime-Version: 1.0 Date: Mon, 1 May 2000 20:17:45 +0900 To: Arthur Peeters E4 <email@example.com> From: Takeshi Fukuda <firstname.lastname@example.org> Subject: Re: TTF Varenna Cc: email@example.com TRANSPORT BARRIER PHYSICS IN JT-60U STEADY-STATE IMPROVED CONFINEMENT PLASMAS T. Fukuda and the JT-60 Team Naka Fusion Research Establishment, Japan Atomic Energy Research Institute Naka-machi, Naka-gun, Ibaraki-ken 311-0193, Japan Recent achievements in the advanced tokamak development at JT-60U are presented, which are highlighted by the sustainment of full CD strong negative and weak shear plasmas with both internal and edge TBs. In regard to the negative shear discharges, improved confinement in terms of H89P equal to 3.6 and normalized beta of 2.0 were simultaneously sustained for 2.7 s with the bootstrap fraction of 80% peaked at the location of ITB, whereas 25% of the total plasma current was carried by NBCD. Here, the location of ITB was expanded beyond 0.6 of the normalized minor radius, which was indeed effective to maintain the stationary current density profile, as well as to broaden the pressure profile and improve the stability together with the H mode edge. In addition, the reduced plasma current of 0.8 MA and high triangularity of 0.37 contributed much to enhance the poloidal beta up to 3.0 and to achieve reasonably high normalized density of 0.65 times Greenwald limit, respectively. It is noteworthy to mention that similar operation scheme is considered in the steady-state operation of ITER-FEAT. The termination of the steady-state period is often ascribed to the NB faults in JT- 60U. In particular, in case one of the critical co-tangential NB unit encounters a breakdown, the ITB quality is degraded, which consequently enhance the current penetration and gradual reduction of the ITB radius, and the plasma is occasionally terminated with hard collapses. Further optimization of negative shear plasmas at higher triangularity of 0.45 and higher current of 1.5 MA is scheduled this year, utilizing various TB control schemes, the result of which will be reported at the meeting. As to the weak shear discharges, H89P of 2.6 and normalized beta of around 2.4 were sustained with a large non-inductive CD fraction of 92% with NNB at 1.5 MA and high triangularity of 0.35, whereas H89P of 2.2 and normalized beta of around 2.7 were sustained for 2.8 s in a similar discharge. The point of emphasis is, however, in high triangularity discharges with relatively high edge safety factor of larger than 5 to 6, the edge pedestal width is substantially broadened in the second stability regime with Type II Grassy ELMs, which remarkably reduces the divertor heat load. In addition, reduction of both the ion and electron thermal diffusivities is observed at the ITB often located near the q = 3 surface, in spite of the monotonic magnetic shear. Other characteristic features of weak shear plasmas with ITBs are such that higher normalized beta is obtained, owing to larger internal inductance, and less vulnerable to disruptive instabilities. Although we apparently employ two different approaches, optimization of the magnetic shear is the focus of the intensive experimental campaign for this year, considering its role on the ITB physics besides the E X B shear stabilization, which is an issue of controversy to be addressed at the meeting. It has been found that the necessary heating power for the ITB formation is substantially lower in the negative shear configuration and the density dependence of the threshold power is also much weaker in the negative shear case, although the magnetic field dependence of the threshold power is not obvious in both cases. In addition, (1) the location as well as the spatial extension of ITB are intimately related to the magnetic shear profile and its width is reduced as much as the ion poloidal gyroradius for strong barriers in reversed shear plasmas, (2) the changes in the toroidal flow velocity profile can modify the ITB quality in terms of the ion thermal diffusivity, and it can thus be applied to enhance the loss power to induce the L-H transition and (3) reduction of the radial correlation length at the ITB has been documented in the density fluctuation measurement, which is consistent with the E X B shear stabilization model. In addition, the ITB database has been compiled, in order to assure the confinement margin in the future steady-state fusion experimental reactor. Although the IPB98 (y, 2) scaling derived from the ELMy H-mode database is presently adopted to assess the Q>5 steady-state operation capability in ITER-FEAT. Provisional result of the analysis indicates that the total stored energy is strongly related to the poloidal magnetic field at ITBs.