K. Ida

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.

Impurity behaviour in PBX L- and H-mode plasmas

Intrinsic impurity behaviour and transport properties in neutral beam heated L- and H-mode PBX tokamak plasmas were studied with a variety of impurity diagnostics. Central impurity accumulation was most often observed in H-mode discharges; sometimes it resulted in a thermal collapse due to high central metallic radiation (~1.5 W • cm−3). The impurity accumulation was evident from peaked Zeff and radiated power profiles and was further substantiated by specific vacuum ultraviolet and X-ray spectroscopy measurements. It is shown that impurity accumulation was neither unique nor inevitable in H-mode discharges and it could be suppressed by sufficient gas puffing. Central impurity accumulation was also seen in L-mode plasmas even with co-injected neutral beams. This usually occurred at high beam power and relatively low density. While there was no significant difference in the degree of accumulation between L-mode and H-mode discharges, the Zeff profile itself was more peaked in the H-mode because the electron density profiles are flatter in H-mode plasmas than in L-mode plasmas. The degree of accumulation increased as Zeff(0) itself increased, which suggests that neoclassical convection and diffusion driven by both impurity-impurity and impurity-plasma ion collisions contribute to the central plasma particle transport.

Topology bifurcation of a magnetic flux surface in magnetized plasmas

Bifurcation phenomena in the radial propagation of a heat pulse driven by modulated electron cyclotron heating are observed in the Large Helical Device. There are two patterns of heat pulse propagation observed in the flat temperature region. One is a bi-directional slow heat pulse propagation observed in the fast magnetic shear drop and the other is a fast heat pulse propagation observed in the slow magnetic shear drop. This bifurcation in the heat pulse propagation suggests the topology bifurcation of a magnetic flux surface in the plasma.

Impurity pellet injection into current driven plasmas of the JIPP T-IIU tokamak

For interaction studies, impurity pellets of stainless steel and plastic carbon with a diameter of 0.5 mm and a velocity of 400 ± 100 m·s−1 have been injected into plasmas driven by fast wave current, with a sustained plasma current of 35-50 kA and an electron density of (2-5) × 1012 cm−3. The density rise is (6-8) × 1012 cm−3 for stainless steel pellets and 4 × 1012 cm−3 for plastic carbon pellets. At pellet injection, the current driven plasmas show no disruption, whereas all of the Ohmic discharges are disruptive. These phenomena are interpreted by a difference in the collision time with ablated pellets between thermal and non-thermal electrons. From measurements of the temporal evolution of the soft X-ray emission, the decay time of the injected impurity is estimated to be 25 ms. The effective charge states of the material of the injected pellets are calculated from the density rise and it is found that they are in the range of 0.8-1.5.

Zeff measurements and low-Z impurity transport for NBI and ICRF heated plasmas in JIPP T-IIU

A visible bremsstrahlung detector array system for Zeff measurements and a charge exchange recombination spectroscopy system for fully ionized impurity profile measurements have been installed on JIPP T-IIU to study impurity transport in neutral beam injection (NBI) and ion cyclotron resonance frequency (ICRF) heated plasmas. More impurities are sputtered during ICRF heating than during NBI and/or Ohmic heating. The contribution of carbon to Zeff is 80-90% for NBI heated plasmas and 60% for NBI + ICRF heated plasmas. With carbon coating of the vacuum vessel, the contribution of carbon to Zeff becomes 80-90% also for NBI + ICRF heated plasmas. From the radial profile and the time evolution of fully ionized carbon after the ICRF pulse is turned on, a diffusion coefficient Da of 1.0 m2·s−1 and a convective velocity va(a) of 13 m·s−1 for the carbon impurity at the plasma edge are obtained.

1st Asia-Pacific Transport Working Group (APTWG) Meeting

This conference report summarizes the contributions to, and discussions at, the 1st Asia-Pacific Transport Working Group Meeting held in Toki, Japan, on 14–17 June 2011. The topics of the meeting were organized under four main headings: momentum transport, non-locality in transport, edge turbulence and L–H transition and 3D effects on transport physics. The events which initiated this meeting are also described in this report.

Topology bifurcation of a magnetic flux surface in toroidal plasmas

The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

New concepts of transport physics in toroidal plasmas

New concepts of transport physics in toroidal plasmas is reviewed. The transport characteristics of non-diffusion and non-locality of transport has been recognized to be important in determining radial structures of density, rotation, and temperature, as well as non-linearity in the flux-gradient relation. The non-diffusive term of momentum transport appears as a ?spontaneous rotation and intrinsic torque?, while the non-diffusive term of particle transport appears as a ?particle pinch and particle exhaust?. The sign and magnitude of these non-diffusive terms have been found to be sensitive to the turbulence state, which causes reversal phenomena. In the momentum transport, the spontaneous flow reversal from the co- (parallel to plasma current) direction to the counter- (anti-parallel to plasma current) direction and vice versa have been commonly observed even when the plasma parameter changes slightly. In the particle transport, especially in the impurity transport, reversals of convective radial flux from inward to outward are also observed in toroidal plasmas as phenomena of density peaking/flattening and impurity accumulation/exhaust. Non-locality of transport has been observed in the response to perturbations, such as a core temperature rise associated with the cooling at edge by pellet injection, strong coupling of transport at different radii as seen in the curvature transition of ITB, and spatial propagation of ITB regions. The turbulence with meso-scale and long correlation, which are strong candidates for causing the non-locality of the transport, have been confirmed experimentally and the coupling between the different turbulence scales has also been identified in various devices.