Tsuneo Amano

Compact Helical System physics and engineering design

This paper reports on the Compact Helical System designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable effort was devoted to reducing error fields from current feeds and crossovers. The final machine parameters are as follows: major radius of 1 m; minor radius of the helical field coil of 0.313 m; plasma aspect ratio A{sub p} = 5; pole number and toroidal period number of the helical field coil of l = 2 and m = 8, respectively; and helical pitch modulation of {alpha}{sup *} = 0.3.

Parametric Effects of an Alternating Field on Inhomogeneous Plasmas

Parametric effects of an alternating electric field on an inhomogeneous plasma in a constant magnetic field are studied by taking into account of density gradients. A general dispersion relation governing the four-mode coupling of electrostatic waves is obtained in terms of linear susceptibilities of the plasma. The dispersion relation is applied to the analysis of the parametric excitation of ion-acoustic and Langmuir waves. It is shown that the parametric coupling via density gradient may have important effects in the experimental condition of Stern and Tzoar. It is also shown that if the pumping frequency is about twice the Langmuir (or upper hybrid) frequency, the Langmuir (or upper hybrid) mode can be excited by the inhomogeneity of the plasma. The threshold intensity is sufficiently low to be attainable by microwave irradiations.

Ballooning beta limits of dee- and bean-shaped tokamaks

Numerical studies of the beta limit in the first region of stability for n = ∞ ballooning modes in advanced-shaped tokamaks are presented. A higher beta value than corresponds to the present conventional beta scaling is expected for advanced-shaped tokamaks with sufficient triangularity/indentation. Extremely elongated configurations without appropriate triangularity do not lead to an increase in critical beta. Dee or bean shapes with sharply tipped cross-sections are more favourable for achieving high beta values than those with round-tip cross-sections having nearly the same safety factor profile. A new beta scaling for elongated ellipse, Dee- and bean-shaped tokamaks is derived.

Effects of a High-Frequency Electric Field on Micro-Instabilities Driven by Density Gradient

Drift, drift-dissipative, and drift-cyclotron instabilities in an inhomogeneous plasma are considered for the case when a uniform high-frequency electric field is applied along the magnetic field. For the applied field, the dipole approximation is made and the skin effect is neglected. Without averaging the distribution function over the period of the applied electric field, the dispersion relations are derived for both the collisionless and collisional regimes. It is found that the high-frequency field whose frequency is within some ranges increasing the frequencies of perturbations can stabilize drift instabilities and also drift-cyclotron instabilities with finite k z of the fluid-type, where k z is the component of the wave vector along the magnetic field. It is found, however, that when the unstable drift-dissipative mode is stabilized the stable mode becomes unstable.

Partially Relaxed Minimum Energy States of Reversed-Field-Pinch Plasma

An energy principle is formulated for construction of toroidal equilibria in partially relaxed minimum energy states. It is shown that additional global constraint yields an additional condition on the force-free current term of the MHD equilibrium equation. It is clarified that the partially relaxed state model (PRSM) for toroidal plasmas, such as the tokamak plasma and the reversed-field-pinch plasma (RFP plasma), represents the minimum-energy states with finite β-values under global invariants. Comparisons of numerical results by the PRSM for the RFP plasma with two typical experimental data indicate that RFP configurations close to the PRSM are realized in the two RFP experiments. The most advanced partially relaxed state with finite β-values is also discussed.

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

A 3-D Resistive MHD Simulation Analysis of Field Reversal Control in the Reversed Field Pinch

A three-dimensional (3-D) resistive MHD simulation code under the compressibility condition has been applied to analysis of the field reversal control in the reversed field pinch (RFP). Simulation results have shown that a loss of the field reversal at the wall leads to flattening of the toroidal field profile, and also to the rapid increase of the fluctuating magnetic energy due to nonlinear tearing modes with m =1. Moreover, the fluctuating magnetic energy spectrum for the m =1 and m =2 modes rapidly broadens toward high and low n , which is caused by the ( m =0; n =-1) mode. By deepening the field reversal at the wall, the reversal surface keeps away from the plasma surface, and safety factor becomes a narrow profile. As a result, a rapid growth and saturation of these tearing modes is suppressed by the field reversal control.

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.

Slow-wave antenna coupling to ion Bernstein waves for plasma heating in ICRF

The coupling of ICRF power from a slow-wave antenna to a plasma with finite temperature is examined. A heuristic model, allowing explicit representations of ion Bernstein waves, fast waves and slow waves, is used to clarify how the antenna power is partitioned into the various wave energy fluxes. This model is complemented quantitatively by a more elaborate and realistic computer model. It is shown that such antennas can be highly efficient in transferring most of the antenna power directly to ion Bernstein waves, with only a very small fraction going into fast waves. The potentiality of this coupling scheme for plasma heating in ICRF is briefly discussed.