Tomohiro Morisaki

Overview of steady state tokamak plasma experiments in TRIAM-1M

An overview of steady state tokamak studies in TRIAM-1M (R0 = 0.8 m, a × b = 0.12 m × 0.18 m and B = 8 T) is presented. The current ramp-up scenario without using centre solenoid coils is reinvestigated with respect to controllability of the current ramp-up rate at the medium density region of (1–2) × 1019 m−3. The plasma is initiated by ECH (fundamental o-mode at 170 GHz with 200 kW) at B = 6.7 T, and the ramp-up rate below the technical limit of 150 kA s−1 for ITER can be achieved by keeping the LH power less than 100 kW during the current ramp-up phase. The physics understanding of the enhanced current drive (ECD) mode around the threshold power level has progressed from a viewpoint of transition probability. A transition frequency, ftrans, for the ECD transition is determined as a function of PCD. At ~70 kW no transition occurs for an ftrans value of ~0.017 Hz, meaning almost zero transition probability. With increasing PCD > Pth, ftrans increases up to 10 Hz, and the transition tends to occur with high probability. The record value of the discharge duration is updated to 3 h 10 min in a low and low power (<10 kW) discharge. The global particle balance in long duration discharges is investigated, and the temporal change in wall pumping rate is determined. Although the density was low, the gas supply had to be stopped at 30 min after the plasma initiation to maintain the density constant. After that the density was sustained by the recycling flux alone until the end of the discharges. In addition to the recycling problem, in the high power and high density experiments, the localized PWI affects the SSO of the tokamak plasma. The effects of enhanced influx of metal impurities (Fe, Cr, Ni, Mo) on sustainment of the high performance ECD plasma are investigated. In order to evaluate the helium bombarding effects on the plasma facing component and hydrogen recycling in the future burning plasma, microscopic damage of metals exposed to long duration helium discharges was studied. The total exposure time was 128 s. From thermal desorption experiments for the specimens the amount of retained helium was evaluated as 3.9 × 1020 He m−2 and the scale length to be ~1 mm in the SOL.

MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L?H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ? 0.75?T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the rational surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L?H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfv?n eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ?b?, and the ratio of energetic ion velocity to the Alfv?n velocity, Vb?/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

A study on plasma edge boundary in ergodic layer of LHD based on radial profile measurement of impurity line emissions

Vertical profiles of edge impurity emissions have been measured in upper half region of elliptical plasmas at horizontally elongated plasma cross section in large helical device (LHD). The vertical profiles near upper O-point located just below helical coil are analyzed to study the plasma edge boundary of the ergodic layer consisting of stochastic magnetic field lines with connection lengths of 30u2009≤u2009Lcu2009≤u20092000 m. As a result, C3+ ion emitting CIV spectrum is identified as the ion existing in the farthest edge of the ergodic layer. The peak position of CIV (312.4 A: 1s23p 2P1/2,3/2-1s22s 2S1/2) vertical profile does not change at all in a wide temperature range of 150u2009≤u2009Te(ρu2009=u20091)u2009≤u2009400 eV, whereas it moves inside the ergodic layer when Te(ρu2009=u20091) is reduced below a threshold temperature, e.g., 130 eV at Raxu2009=u20093.75 m configuration. It is found that the C3+ ion exists at the boundary between ergodic layer and open magnetic filed layer at which the Lc distributes in lengths of 5 to 30 m. The result indicates t...

Controlling the cross-field flux of cold α-particles with resonant magnetic perturbations in a helical fusion plasma device

A new scenario to control transport of cold ?-particles in a helical fusion device by changing poloidal field coil current during plasma discharge is proposed here. A way to enhance radial transport of ?-particles in the intermediate energy range is considered which relies on specific features of helical magnetic field. To remove helically trapped, cold ?-particles, a ?-induced change in the B/B0 modulation along the particle trajectory is employed. To remove passing particles resonant effects connected with the poloidal field coil currents are used.

High-impedance wire grid method to study spatiotemporal behavior of hot electron clump generated in a plasma

High-impedance Wire Grid (HIWG) detector has been developed to study spatiotemporal behavior of a hot electron clump generated in an electron cyclotron resonance (ECR) plasma. By measuring the floating potentials of the wire electrodes, and generating structure matrix made of geometrical means of the floating potentials, the HIWG detector reconstructs the spatial distribution of high-temperature electron clump at an arbitrary instant of time. Time slices of the spike event in floating potential revealed the growth and decay process of a hot spot occurs in an ECR plasma.

LHD diagnostics toward steady-state operation

The large helical device (LHD) is the world largest helical system having all superconducting coils. After completion of LHD in 1998, six experimental campaigns have been carried out successfully. The maximum stored energy, central electron temperature, and volume averaged beta value are 1.16 MJ, 10 keV, and 3.2%, respectively. The confinement time of the LHD plasma appears to be equivalent to that of tokamaks. One of the most important missions for LHD is to prove steady-state operation, which is also significant to international thermonuclear experimental reactor (ITER) and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 150 s. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady-state operation is whether divertor function to control particle and heat flux is effective enough. Relevant to this, LHD diagnostics should be consistent with the following: 1) continuous operation of main diagnostics during long-pulse operation for feedback control and physics understanding; 2) measurement of fraction of H, He, and impurities in the plasma; 3) heat removal and measure against possible damage or surface erosion of diagnostic components inside of the vacuum chamber; 4) data acquisition system for handling real time data display and a huge amount of data. Although there are already some achievements on the above subjects, there remain still several issues to be resolved. On the other hand, the long-pulse operation of the plasma gives benefits to the diagnostics. For example, the polarizing angle of ECE emission can be changed during the discharge, and the intensity dependence on the polarizing angle has been obtained. The spatial scanning of the neutral particle analyzer and the spectrometer can supply the spatial profiles of the fast neutral particle flux and the specific impurity lines. In this paper, the present status of these issues and future plans are described.