Y. Tomii

Progress in potential formation and findings in the associated radially sheared electric-field effects on suppressing intermittent turbulent vortex-like fluctuations and reducing transverse losses

Following the 19th IAEA Fusion Energy Conference (Lyon, 2002), (1) three-time progress in the formation of ion-confining potential heights c including a record of 2.1 kV in comparison to those attained 1992–2002 is achieved for tandem-mirror plasmas in the hot-ion mode with ion temperatures of several kiloelectronvolts. (2) The advance in the potential formation gives the bases for finding the remarkable effects of radially produced shear of electric fields Er, or non-uniform sheared plasma rotation on the suppression of intermittent vortex-like turbulent fluctuations. (i) Such a shear effect is visually highlighted by x-ray tomography diagnostics; that is, spatially and temporally intermittent vortex-like fluctuated structures are clearly observed as two-dimensionally reconstructed visual structures for the first time in kiloelectronvolt order ion-cyclotron heated plasmas having a weak shear in GAMMA 10. (ii) However, during the application of plug electron-cyclotron heatings (ECH), the associated potential rise produces a stronger shear (dEr/dr = several 10 kV m−2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement. X-ray observations also show elongation of a vortex structure from a circular into an ellipsoidal shape, as depicted in H-mode theories, with an outward shift. (3) For the physics interpretations and control of such potential and the associated shear formation, the validity of our proposed theory of the potential formation is extensionally tested under the conditions with auxiliary heating. The data described above fit well to the extended surfaces calculated from our proposed consolidated theory of the strong ECH theory (plateau formation) with Pastukhovs theory on energy confinement.

Recent Progress in the GAMMA 10 Tandem Mirror

Following the 2002 Conference on Open Magnetic Systems for Plasma Confinement, a three times progress in the formation of ion-confining potential height is achieved in the hot-ion mode. The advance in the potential formation leads to a finding of remarkable effects of radially produced shear of electric fields dEr/dr on the suppression of not only coherent drift waves but turbulence-like fluctuations for the first time in GAMMA 10. Also, the progress in the potential formation is made in line with the extension of our proposed physics scaling of potential formation covering over representative tandem-mirror operational modes, characterized in terms of (a) a high-potential mode having kV-order plasmaconfining potentials and (b) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures (Ti).

X-ray tomography systems for observations of the effects of radially sheared electric fields on fluctuations in plasmas

Experimental verification of the effects of radially sheared electric-field (or potential) formation in plasmas is one of the most critical issues to understand the physics basis for plasma confinement improvements. In the GAMMA 10 tandem mirror, recent experimental results show shear formation effects on the suppression of not only coherent drift waves but turbulence-like fluctuations without any coherent phasing relation during the ion-confining potential formation period. Contours of the central-cell soft x-ray brightness show spatially and temporally fluctuated structures during a weak sheared period by the use of the 50 channel microchannel plate system. A new x-ray tomography system is developed for analyzing temporally and spatially resolved plasma behavior in the presence or absence of these shear formation effects in GAMMA 10. The system consists of two 48-channel silicon semiconductor detector arrays with different viewing angles. X-ray energy responses of the new detector arrays along with resp...

Novel compact electrostatic ion-current detector using a self-collection method for secondary-electron suppression

For the purpose of observations of the absolute values of ion-end-loss currents for plasma confinement studies in open-field plasma devices, a compact-sized electrostatic ion-current detector is proposed on the basis of a self-collection principle for suppressing the effects of secondary-electron emission from a metal collector. For employing this specific method, it is worth noting that no further additional magnetic systems except ambient open-ended magnetic fields are required for the proposed detector structure. This characteristic property provides a compactness of the total detector system and availability for its use in a plasma confinement device without disturbing plasma-confining magnetic fields. The detector consists of a set of parallel metal plates with respect to lines of ambient magnetic forces of a plasma device for analyzing incident ion currents along with a grid for shielding the collector against strays due to the metal-plate biasing. Availability of the detector is clearly demonstrated in the GAMMA 10 plasma experiments.

Comparison of the Radially Produced Electric-Field Shear Effects Analyzed from End-Loss Current and Central-Cell Soft X-Ray Data

Significant effects of sheared transverse electric fields in plasmas on both turbulent fluctuations and drift waves are experimentally demonstrated with improvement in plasma confinement for the first time in the tandem mirror GAMMA 10. Here, electron-cyclotron heatings (ECH) for ion-confining potential formation are applied in association with a significant rise in the absolute value of the central-cell potential and the resulting formation of a strong shear of electric fields of the order of 10 kV/m2 in the radial direction of the plasma column (dEr/dr). The central-cell line density increases during ECH in association with decreasing fluctuations. Various fluctuation diagnostics, in particular, the frequency analyses of end-loss ion currents and central soft x-ray brightness, show the consistent features. This encourages the usefulness of potentials and radial electric-field shear for confinement improvements.

Measurements of the relaxation of ion anisotropic distribution functions in tandem-mirror plasmas

Our proposed “matrix-type” semiconductor detectors are applied for studying the relaxation of ion anisotropic distribution functions having several keV in the central cell of the tandem mirror GAMMA 10. The matrix-type detector array consists of compactly produced six “rows” having different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channel units (“columns”) for measuring radiation profiles in the radial direction of plasmas. These various SiO2 layers are, thus, employed as “unbreakable ultrathin radiation–absorption filters” having various thicknesses to analyze x-ray energies and distinguish x rays from neutral particles simultaneously. Development of these detectors along with high power gyrotrons allow us to investigate energy transport analyses between ion-cyclotron heated hot ions and electron-cyclotron heated electrons. An application of the matrix-type detectors reveals detailed behavior of electron and ion relaxation from the perpendicular to parallel ion energies ...

Investigation of Electron Distribution Functions in the Plug Region of the GAMMA 10 Tandem Mirror

Generalized scaling laws for the formation of plasma confining potentials are investigated to find the physics essentials common to representative tandem-mirror operational modes in GAMMA 10. These modes are characterized in terms of (i) a high-potential mode and (ii) a hot-ion mode. The potential-formation scalings in these modes are consolidated and generalized on the basis of the consistency with finding of the wider validity of Cohen’s strong electron-cyclotron heating (ECH) theory covering over both modes. A plateau-shaped electron distribution function is observed when a plug electron-confining potential is formed in the hot ion mode of GAMMA 10, as predicted in terms of the strong ECH theory.

X-ray diagnostics for investigating electron distribution functions in the central cell of the GAMMA 10 tandem mirror

The quantum efficiency of an ultralow-energy-sensitive pure-Ge (ULE Ge) detector is investigated using synchrotron radiation from the storage ring at AIST especially for x-ray pulse-height analyses (PHAs), down to a few hundred eV. Several types of x-ray diagnostics such as x-ray PHA, x-ray absorption methods, and x-ray tomography using the ULE Ge detector, a NaI(Tl) detector, as well as a microchannel-plate tomography system are employed for investigating electron distribution functions and electron temperature profiles with preliminary central electron-cyclotron heating in the central cell of the GAMMA 10 tandem mirror. These measurements play an important role in studying an essential physics scaling of the electron temperature as a function of electron confining potential in tandem mirror plasmas.

Experimental estimation of the local energy balance of the potential-confining electrons in tandem-mirror plasmas

Our proposed “matrix-type” semiconductor detectors are applied for studying the local energy balance of bulk electrons in the tandem-mirror GAMMA 10. The matrix-type detector array consists of compactly produced six “rows” having different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channel units (“columns”) for measuring radiation profiles in the radial direction of plasmas. These various SiO2 layers are, thus, employed as “unbreakable ultrathin radiation-absorption filters” having various thicknesses to distinguish x rays from charge-exchange neutral particles and analyze the radial profiles of both plasma ion and electron temperatures simultaneously. The radial profiles of the energy confinement time and the thermal diffusivity obtained from the local energy balance analysis imply that the improvement of the plasma confinement is associated with the strong shear of radial electric fields due to a high plasma confining potentials.

X-Ray Energy Responses of Silicon Tomography Detectors Irradiated with Fusion Produced Neutrons

In order to clarify the effects of fusion-produced neutron irradiation on silicon semiconductor x-ray detectors, the x-ray energy responses of both n- and p-type silicon tomography detectors used in the Joint European Torus (JET) tokamak (n-type) and the GAMMA 10 tandem mirror (p-type) are studied using synchrotron radiation at the Photon Factory of the National Laboratory for High Energy Accelerator Research Organization (KEK). The fusion neutronics source (FNS) of Japan Atomic Energy Research Institute (JAERI) is employed as well-calibrated D-T neutron source with fluences from 1013 to 1015 neutrons/cm2 onto these semiconductor detectors. Different fluence dependence is found between these two types of detectors; that is, (i) for the n-type detector, the recovery of the degraded response is found after the neutron exposure beyond around 1013 neutrons/cm2 onto the detector. A further finding is followed as a “re-degradation” by a neutron irradiation level over about 1014 neutrons/cm2. On the other hand, (ii) the energy response of the p-type detector shows only a gradual decrease with increasing neutron fluences. These properties are interpreted by our proposed theory on semiconductor x-ray responses in terms of the effects of neutrons on the effective doping concentration and the diffusion length of a semiconductor detector.