T. Numakura

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.

Extended consolidation of scaling laws of potential formation and effects covering the representative Tandem mirror operations in GAMMA 10

Scaling laws of potential formation and associated effects along with their physical interpretations are consolidated on the basis of experimental verification using the GAMMA 10 tandem mirror. A proposal of extended consolidation and generalization of the two major theories—(i) Cohens strong electron cyclotron heating (ECH) theory for the formation physics of plasma confining potentials and (ii) the generalized Pastukhov theory for the effectiveness of the produced potentials on plasma confinement is made through the use of the energy balance equation. This proposal is then followed by verification using experimental data from two representative operational modes of GAMMA 10, characterized in terms of (i) a high-potential mode having plasma confining potentials of the order of kilovolts and (ii) a hot ion mode yielding fusion neutrons with bulk ion temperatures of 10–20 keV. The importance of the validity of the proposed physics-based scaling is highlighted by the possibility of extended capability inherent in Pastukhovs prediction of requiring an ion confining potential of ~30 kV for a fusion Q value of unity on the basis of an application of Cohens potential formation method. In addition to the above potential physics scaling, an externally controllable parameter scaling of the potential formation increasing with either plug or barrier ECH powers is summarized. The combination of (i) the physics-based scaling of the proposed consolidation of potential formation and effects with (ii) the externally controllable practical ECH power scaling provides a new direction for future tandem mirror studies.

Active control of internal transport barrier formation due to off-axis electron-cyclotron heating in GAMMA 10 experiments

The controlled formation of an internal transport barrier (ITB) is observed in GAMMA 10 [T. Cho et al., Nucl. Fusion 45, 1650 (2005)]. The barrier is localized within a layer of a strongly sheared Er×B plasma rotation (5.5<rc⩽10cm). This high-vorticity layer is formed and maintained by off-axis electron-cyclotron heating, which generates a cylindrical layer (4<rc<7cm) with a high-energy electron population that modifies the initial Gaussian radial potential profile into a nonmonotonic one with a hump structure. The local gradients of Ti and Te are appreciably enhanced in the ITB layer, similarly to those of the ITB in tokamaks and stellarators. Reductions in the effective ion and electron thermal diffusivities are obtained in the barrier layer. A reduction of the observed low-frequency turbulence in the ITB layer and a partial decoupling of the turbulent structures localized on either side of the layer are demonstrated by two-dimensional x-ray diagnostics.

EFFECTS OF NEUTRONS ON SEMICONDUCTOR X-RAY DETECTORS INCLUDING N-TYPE JOINT EUROPEAN TORUS AND P-TYPE GAMMA 10 TOMOGRAPHY DETECTORS

Characterization experiments have been carried out so as to investigate the effects of fusion-produced neutrons on the x-ray-energy responses of semiconductor detectors for x-ray tomography in the Joint European Torus (JET) tokamak (n-type silicon) and the GAMMA 10 tandem mirror (p-type silicon). Neutron effects on the x-ray-energy responses of these detectors are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Changes in the material properties of the detectors have been investigated using an impedance analyzer to estimate neutron effects on x-ray-sensitive depletion thicknesses. A cyclotron accelerator is employed for well-calibrated neutron irradiation onto these plasma x-ray detectors; a fluence of 2–5×1013 neutrons/cm2 is utilized for simulating the effects of fusion-produced neutrons in JET. Modifications of the x-ray responses after neutron exposure due to fusion plasma shots in JET as well as cyclotron-produced neutron irradiations are found to have ...

A diagnostic method for both plasma ion and electron temperatures under simultaneous incidence of charge-exchange particles and x rays into a semiconductor detector array

An idea for using semiconductor detectors to simultaneously observe both plasma ion Ti and electron Te temperatures is proposed. The idea is also experimentally verified in tandem-mirror plasma shots. This method is developed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange neutral particles from plasmas on the thickness of a thin (on the order of nm thick) SiO2 layer is employed for analyzing Ti in the range from hundreds to thousands of eV. Even under the conditions of simultaneous incidence of such particles and x rays into semiconductor detectors, the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish particles (for Ti) from x rays (for Te). In this letter, proof-of-principle plasma experiments for the proposed idea are carried out to verify the availability of this concept of distinguishing and identifying each value of Ti ...

Newly developed matrix-type semiconductor detector for temporally and spatially resolved x-ray analyses ranging down to a few tens eV using a single plasma shot

For the purpose of the measurements of temporally and spatially resolved electron temperatures (Te) during a single plasma shot alone, we propose and fabricate a new matrix-type semiconductor x-ray detector. This detector is fabricated using the precise formation of thin dead layers (SiO2) with six different thicknesses (from 10 to 5000 A) aligned in line on its surface compactly. Each “row” has seven channels for the measurements of plasma x-ray radial profiles so as to make x-ray tomographic reconstructions; namely, the compact-sized matrix detector having six rows and seven columns with a 5×5 mm2 active area for each matrix unit. These various SiO2 layers are proposed to be utilized as ultrathin “x-ray absorption filters” with different thicknesses, which are never obtained as “self-supporting material absorbers” because of their ultrathin properties. This novel idea enables us to analyze x-ray tomography data including in the Te region down to a few tens eV. The simultaneous comparison of each tomogra...

Recent results of divertor simulation experiments using D-Module in the GAMMA 10/PDX tandem mirror

Abstract This paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.76 × 1023 particles/·m2 at the end-mirror exit. Massive gas injection (H2 and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Hα emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is also found that Xe gas is much more effective in achieving detached plasma than Ar gas. Numerical simulation studies also have been performed toward the understanding of the cooling mechanism of divertor plasma. The above results will contribute to establishment of detached plasma control and clarification of the radiation cooling mechanism toward the development of future divertor systems.

Improved GAMMA 10 tandem mirror confinement in high density plasma

GAMMA 10 experiments have advanced in high density experiments after the last IAEA fusion energy conference in 2000 where we reported the production of the high density plasma through use of ion cyclotron range of frequency heating at a high harmonic frequency and neutral beam injection in the anchor cells. However, the diamagnetic signal of the plasma decreased when electron cyclotron resonance heating was applied for the potential formation. Recently a high density plasma has been obtained without degradation of the diamagnetic signal and with much improved reproducibility than before. The high density plasma was attained through adjustment of the spacing of the conducting plates installed in the anchor transition regions. The potential confinement of the plasma has been extensively studied. Dependences of the ion confinement time, ion-energy confinement time and plasma confining potential on plasma density were obtained for the first time in the high density region up to a density of 4?1018?m?3.

Simultaneous measurements of temporally and spatially resolved ion and electron temperatures using a semiconductor detector array in a single plasma discharge

A method is proposed for obtaining radial profiles of both plasma ion (Ti) and electron temperatures (Te) simultaneously using a semiconductor detector. This method for semiconductor Ti diagnostics is proposed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange (cx) neutral particles from plasmas on the thickness on the order of a nanometer-thick SiO2 layer is used for analyzing Ti in the range from hundreds to thousands of electron volts. Even under the circumstances of simultaneous incidence of such particles and x rays along the same lines of sight of a semiconductor detector array, it is found that the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish cx neutral particles for Ti diagnostics from x rays for Te diagnostics. Experimental verification of this concept of the simultaneous Ti and Te diagnostics is carried out in the ...

Development of gyrotrons for fusion with power exceeding 1 MW over a wide frequency range

Megawatt-class gyrotrons covering a wide frequency range (14 GHz?300 GHz) are in increasing demand for nuclear fusion. Recent electron cyclotron heating and electron cyclotron current drive experiments highlight a requirement of megawatt-scale gyrotrons at a relatively lower frequency (14?35 GHz) range of some plasma devices, like GAMMA 10/PDX of the University of Tsukuba, QUEST of Kyushu University, NSTX-U of Princeton Plasma Physics Laboratory, and Heliotron J of Kyoto University. Collaborative studies for designing a new 28 GHz/35 GHz dual-frequency gyrotron and a 14 GHz gyrotron have commenced. Operation above 1 MW of 28 GHz/35 GHz dual oscillation was demonstrated experimentally. Further in the design of dual-frequency gyrotron, operations with 2 MW 3 s and 0.4 MW CW (continuous wave) at 28 GHz, and power exceeding 1 MW for 3 s at 34.8 GHz have been shown to be feasible. The 14 GHz gyrotron is expected to operate above 1 MW. We are also developing higher frequency gyrotrons (77?300 GHz). The joint program of National Institute for Fusion Science and the University of Tsukuba developed two new 154 GHz gyrotrons for the large helical device after the demonstration of three 77 GHz gyrotrons. The 154 GHz gyrotrons achieved a maximum output power of 1.25 MW and quasi-CW operation of 0.35 MW for 30?min.