T. Obiki

In this article we describe a diamagnetic double-loop method for measurement of the energy stored in a magnetized plasma. Two concentric diamagnetic loops enclose a plasma column. The advantage of this method is the capability to eliminate to a high extent the background magnetic field and the effect of eddy currents in the measurement of relatively small diamagnetic signals. This method was applied to initial plasmas produced in the helical axis stellarator Heliotron J. A stored energy of 0.7 kJ was measured with a resolution of ±0.02 kJ at rippling magnetic field of 1 T in a plasma heated by rf discharges up to 400 kW. This diamagnetic double-loop method improved the signal to noise ratio of the conventional single loop method sixfold. The magnetic diagnostics hardware, experimental results, and an error estimate are presented.

The effects of the plasma profile on the global energy confinement have been studied in Heliotron E with special regard to differences between heating methods (ECH, NBI and NBI + ECH). With high power NBI, peaked T_i and peaked n_e profiles (T_i(0)/T_i 2.7, n_e(0)/n_e 4.5) were simultaneously achieved under low recycling conditions. A peaked n_e profile (n_e(0)/n_e 2.5 ) could lead to the high T_i mode where the ion heat transport in the central region is substantially reduced. By changing the ECH launching condition (on-axis, off-axis and toroidally oblique injection), the peakedness of the T_e profile could be controlled in the range 1.3 T_e(0)/T_e 4.5. A peaked T_e profile and a flat n_e profile (3.5 T_e(0)/T_e, n_e(0)/n_e 1.8) were brought about by the well focused on-axis ECH. The ECH plasma with a peaked T_e profile has higher stored energy than that with a moderately peaked T_e profile for the same injected ECH power and the same density region. The global energy confinement time normalized by the LHD scaling, τ_E ^G/τ_E ^LHD, showed n_e(0)/n_e dependence for the low T_i mode NBI plasmas. For the high T_i mode, the n_e(0)/n_e dependence of τ_E ^G/τ_E ^LHD was weak. These findings suggest that the LHD scaling should be modified to scale the global energy confinement of the helical plasmas in a wide range of n_e(0)/n_e.

Third harmonic electron cyclotron heating has been experimentally studied in Heliotron DR (R = 90 cm, ā 7 cm) by using a 28 GHz, 200 kW gyrotron. When the line averaged electron density _e exceeds a certain critical value, n_e,crit ~ 0.5 × 10^l3/cm³, the absorption power increases rapidly and plasmas are substantially heated as long as the third harmonic resonant layer is located within the last closed magnetic surface. On the other hand, heating of the bulk plasma is not effective at lower density, _ee,crit although strong hard X-rays are emitted, which suggests the generation of high energy electrons. In the high density regime, the central plasma region is heated even when the electron density is much higher than the cut-off value of the O-mode (overdense plasma), almost independent of the location of resonant layers. It is found that overdense plasmas produced by third harmonic heating have basically the same characteristics as plasmas produced by second harmonic heating in Heliotron DR. The absorption of the electron Bernstein wave generated via a mode conversion process is considered to be the heating mechanism of overdense plasmas at the third harmonic resonance.

A kinetic analysis of the global energy confinement time for neutral beam heated Heliotron E plasmas has been performed with a 1-D, time independent transport analysis code, PROCTR-Mod. From a regression analysis of a representative sample selection of the presented data sets, the global energy confinement time, , is found to scale as , where α = 0.53 ± 0.10, β = −0.71 ± 0.09, and γ = 0.35 ± 0.14; the error bar indicates the 95% confidence region deduced from the limited data points; is the line average electron density; P_heat[MW] is the neutral beam heating power; and B[T] is the vacuum magnetic field at the magnetic axis. The data analysis shows that the favourable density dependence partially offsets the unfavourable power dependence and that anomalous electron transport loss becomes dominant in the overall energy balance as the beam power and the plasma density are increased. An alternative scaling law is also presented, which is to fit by an 'offset linear' law, , where ζ = 1.1 ± 0.33 and η = 15.0 ± 1.9; n_e[10¹⁴ cm⁻³] is the volume average electron density. The latter scaling is found to provide a better fit to the presented data sets, in spite of its simple form. The parametric scaling of the local electron thermal diffusivity, χ_e is also discussed on the basis of the kinetic analysis.

Diagnostic applications of visible and VUV spectroscopic techniques, as applied to the currentless Heliotron E plasma device, are described. Visible spectroscopy has been used to measure (i) ion temperature, (ii) proton‐to‐electron density ratio, (iii) Z_eff by charge exchange recombination from an intense neutral beam, (iv) radial electric field by poloidal rotation velocity measurement, and (v) electron density around an ablating pellet by a Stark profile. VUV spectroscopy has been used to investigate emission spectra due to multiply ionized impurity species. This information is used to measure the densities of these species, and to learn about the transport of these particles. Recently, a flat‐field survey spectrometer has been constructed and used to study the emission spectra due to metallic impurities in ICRF‐heated plasmas.

Impurity transport in currentless Heliotron E plasmas is examined. The main aim of the experiment is to clarify the density dependence of impurity transport both in electron cyclotron heated (ECH) and neutral beam injected (NBI) plasmas. In experiments with a stationary electron density, the confinement time of impurities is determined as a function of the line averaged electron density. The inherent difference between ECH and NBI plasmas, observed in an earlier measurement, seems to have been masked by the operating conditions. From a direct comparison between NBI plasmas and simultaneous ECH and NBI heated plasmas of the same density, a pumping-out effect of impurities by the ECH pulse is established. From numerical analyses, the unexpected discrepancy between observed and calculated VUV spectral data can be explained by a charge exchange process. The observed impurity transport is found to be well described by a model which incorporates the effects of radial diffusion and inward convection. The inward flow does not seem to be due to the plasma potential because the flow is also observed in ECH plasmas where the heating power is expected to contribute to a reversed potential. The flow velocity is of the order of the classical friction between background ions and seems to play only a minor role in various aspects of impurity transport in NBI plasmas, such as impurity accumulation. The variety in impurity transport in Heliotron E plasmas is best understood by changes in the diffusion coefficient.

A currentless plasma with a volume-averaged beta value of 2% has been produced with neutral beam heating. Target plasmas were created by second harmonic resonance heating with electron cyclotron waves (150–350 kW and 53.2 GHz) at a magnetic field strength of 0.94 T. Neutral beam injection (23–30 keV and 1.3−2.6 MW) was used to heat the plasma further. MHD stable and unstable high-beta plasmas were observed. The Q-mode plasmas were produced with the help of intense neutral gas puffing. Properties of the MHD activity and confinement of high-beta plasmas are discussed and compared with theoretical studies.