T. Cho

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.

Generalization and consolidation of scaling laws of potential formation and associated effects in the GAMMA 10 tandem mirror

Generalized scaling laws for the formation of plasma confining potentials and the associated effectiveness of the potentials produced are systematically investigated to find the physics essentials common to the representative tandem mirror operational modes of GAMMA?10, and to explore novel extended operational modes from the scaling bases constructed. (a)?The potential formation scalings are generalized using a novel finding of wider validity of Cohens strong ECH theory covering the representative modes. (b)?The potentials produced, in turn, provide a favourable novel scaling of the increase in the central cell electron temperatures Te with increasing thermal barrier potentials b, limited by the available ECH power. The scaling of Te with b is well interpreted in terms of the generalized Pastukhov theory of plasma potential confinement. A detailed comparison of the results from several related modified theories is also made. (c)?Consolidation of the two major scalings of (a) and (b) in a tandem mirror is carried out by the use of an electron energy balance equation for the first time. In addition, (d)?an empirical scaling of c with ECH power in the plug region and the central cell densities are studied to discover whether there is the possibility of extending these theoretically well interpreted scaling data to parameters in the future scalable regime. There is also a discussion about numerical scalings in the three dimensional parameter spaces.

Investigation of x-ray-energy responses of semiconductor detectors under deuterium–tritium fusion-produced neutron irradiation

For the purpose of investigating fusion-produced neutron effects on semiconductor x-ray detectors, detection characteristics of x-ray tomography detectors used before and after deuterium–tritium (DT) and/or DD fusion-plasma experiments in the Joint European Torus (JET) tokamak are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Degradations in the responses after neutron exposure into the detectors are found to have functional dependence on x-ray energy. Changes in the depletion thicknesses of the detectors are investigated by means of impedance analyses. The Fusion Neutronics Source (FNS) facility of the Japan Atomic Energy Research Institute is also employed for well calibrated DT fusion-produced neutron irradiation onto these semiconductor x-ray detectors. Recovery of the response degradation is found due to a method for supplying the operational bias to the degraded detector. Our theory is applied to interpret these detector characteristics under the irr...

Simultaneous observations of temporally and spatially resolved two-dimensional profiles of ion-confining potentials and ion fluxes using novel ion-energy spectrometer arrays

Newly designed spectrometer arrays for ion-energy-spectrum observations are proposed and constructed in both end regions of the GAMMAu200210 tandem mirror so as to obtain two dimensionally resolved radial profiles of end-loss-ion fluxes IELA, ion temperatures, and ion-confining potentials φc by the use of a single plasma discharge alone. Each spectrometer unit in the array has a specific structure with obliquely placed multiple grids with respect to the direction of the ambient plasma-confining magnetic fields. This structure is proposed to obtain precise ion-energy spectra without the disturbance of simultaneously incident energetic electrons into the array, since widely distributed “multigridded Faraday cup” signals have significant electron disturbances on ion spectra even if over a few tens of kV are applied to the electron-repeller grid. In tandem-mirror experiments, plasma-confining potentials produced by electron cyclotron heatings play one of the most critical roles in the improvement of simple-mirror...

Tomographic reconstruction of plasma electron temperature profiles using semiconductor detector arrays in the elliptic transition region and the circular central cell of the GAMMA 10 tandem mirror

Abstract Elliptically shaped transition-region plasmas located between circularly shaped central cell and fish-tail shaped anchor-cell plasmas have been observed using X-ray tomography techniques for the first time in the world largest tandem mirror device, GAMMA 10. These three regions are connected through the lines of magnetic forces. Electrons are, therefore, anticipated to move easily through these regions along the magnetic field lines when neither appreciable electron losses into the transverse direction across the magnetic field lines nor the formation of electron transport barrier potentials in the parallel direction are expected. From this viewpoint, comparisons of electron temperature ( T e ) profiles in these regions, thus, give information on one of the most critical issues in tandem mirror plasma confinement. The first data on good agreement in T e profiles of the central cell and the transition region have supported this basic and essential concept of tandem mirror confinement. Development of position sensitive semiconductor-detector arrays and an upgraded X-ray tomography algorithm for analysing elliptically shaped transition-region plasmas are also reported.

A proposal for both plasma ion- and electron-temperature diagnostics under simultaneous incidence of particles and x-rays into a semiconductor on the basis of a proposed model for a semiconductor detector response

A method is proposed for obtaining radial profiles of plasma temperatures of both plasma ion (Ti) and electron (Te) simultaneously by the use of a semiconductor detector array. The method is based on our developed particle-response model for a semiconductor detector; in particular, the response theory is constructed for giving the applicability in particle energies ranging down to a kiloelectronvolt. Calculated results from our model are in fairly good agreement with experimental data on the detector response of incident particle beams with energies in the range 100 eV to a few kiloelectronvolts. On the basis of the verification of the proposed model, an idea of the use of a developed semiconductor detector array covered with `reliably unbreakable ultrathin SiO2 `dead-layer filters having various nanometre-order thicknesses is applied for simultaneous Ti and Te analyses by using charge-exchange neutral particles and x-rays from plasmas. Radial profiles of Ti and Te are obtained in a single plasma discharge alone, and the data reliability is independently cross-checked by a radial scan of a conventional charge-exchange neutral-particle analyser system as well as a 50-channel microchannel plate x-ray diagnostics system in the GAMMA 10 tandem mirror.

Observations of differences in the spatial-profile symmetry-of ion-confining potentials in the cases with and without the central-cell density increase during an ion-plugging period

Temporal evolution of radial profiles of end-loss-ion currents, ion temperatures, and ion-confining potentials φc is investigated in the GAMMA 10 tandem mirror by the use of our newly designed electrostatic ion-energy-spectrometer arrays. In tandem-mirror experiments, plasma-confining potentials produced by electron-cyclotron heatings (ECH’s) play one of the most critical roles in the improvement of simple-mirror plasma confinement. For the observations of these confining potentials, the development of ion-diagnostics equipment providing clear energy spectra without disturbances from ECH produced energetic electrons is of essential importance, since conventional multigrided Faraday cup signals are seriously influenced by such incident electrons. Each spectrometer unit in our developed array has a specific structure with obliquely placed multiple grids with respect to the direction of the ambient lines of magnetic force for plasma confinement. The relation between the spatial profiles of end-loss-ion fluxe...

Effects of nuclear fusion produced neutrons on silicon semiconductor plasma X-ray detectors

Abstract The effects of nuclear fusion produced neutrons on the X-ray energy responses of semiconductor detectors are characterized. The degradation of the response of position-sensitive X-ray tomography detectors in the Joint European Torus (JET) tokamak is found after neutron exposure produced by deuterium–deuterium and deuterium–tritium plasma fusion experiments. For the purpose of further detailed characterization of the neutron degradation effects, an azimuthally varying-field (AVF) cyclotron accelerator is employed using well-calibrated neutron fluence. These neutron effects on the detector responses are characterized using synchrotron radiation from a 2.5xa0GeV positron storage ring at the Photon Factory (KEK). The effects of neutrons on X-ray sensitive semiconductor depletion thicknesses are also investigated using an impedance analyser. Novel findings of (i) the dependence of the response degradation on X-ray energies as well as (ii) the recovery of the degraded detector response due to the detector bias application are found and interpreted by systematically combined methods using a synchrotron facility, a cyclotron facility, a fusion plasma device, material analysis methods along with our recent theory on semiconductor X-ray responses.

A novel method for simultaneous observations of plasma ion and electron temperatures using a semiconductor-detector array

A new method for a simultaneous observation of both plasma ion and electron temperatures is proposed using one semiconductor-detector array alone. This method will provide a new application of semiconductor-detector arrays for monitoring the key parameter set of nuclear-fusion triple product (i.e., ion temperatures, densities, and confinement time) as well as for clarifying physics mechanisms of energy transport between plasma ions and electrons under various plasma confining conditions. This method is developed on the basis of an alternative “positive” use of a semiconductor “dead layer”; that is, an SiO2 layer is employed as a reliable ultra-thin energy analysis filter for low-energy charge-exchanged neutral particles from plasmas ranging in ion temperatures from 0.1 to several tens of kilo-electron-volts. Using recent fabrication techniques for the thin and uniform SiO2 layers of the order of tens to hundreds of angstrom, our computer simulation and its experimental verification show the availability of such semiconductors for distinguishing neutral particles (for ion temperatures) from X-rays (for electron temperatures). These are simultaneously emitted from the plasmas into semiconductor detectors; however, we employ their quite different penetration lengths and the resultant different deposition depths and profiles in semiconductor materials. As a result, their output signals are distinguishable for these two different and fundamental species of plasmas.

A novel electrostatic ion-energy spectrometer by the use of a proposed “self-collection” method for secondary-electron emission from a metal collector

For the purpose of end-loss-ion energy analyses in open-field plasmas, a newly developed electrostatic ion-energy spectrometer is proposed on the basis of a “self-collection” principle for secondary-electron emission from a metal collector. The ion-energy spectrometer is designed with multiple grids for analyzing incident ion energies, and a set of parallelly placed metal plates with respect to lines of ambient magnetic forces in an open-ended device. One of the most important characteristic properties of this spectrometer is the use of our proposed principle of a “self-collection” mechanism due to E×B drifts for secondary electrons emitted from the grounded metal-plate collector by the use of no further additional magnetic systems except the ambient open-ended fields B. The proof-of-principle and characterization experiments are carried out by the use of a test-ion-beam line along with an additional use of a Helmholtz coil system for the formation of open magnetic fields similar to those in the GAMMA 10 end region. The applications of the developed ion-energy spectrometer for end-loss-ion diagnostics in the GAMMA 10 plasma experiments are demonstrated under the conditions with simultaneous incidence of energetic electrons produced by electron-cyclotron heatings for end-loss-plugging potential formation, since these electrons have contributed to disturb these ion signals from conventional end-loss-ion detectors.For the purpose of end-loss-ion energy analyses in open-field plasmas, a newly developed electrostatic ion-energy spectrometer is proposed on the basis of a “self-collection” principle for secondary-electron emission from a metal collector. The ion-energy spectrometer is designed with multiple grids for analyzing incident ion energies, and a set of parallelly placed metal plates with respect to lines of ambient magnetic forces in an open-ended device. One of the most important characteristic properties of this spectrometer is the use of our proposed principle of a “self-collection” mechanism due to E×B drifts for secondary electrons emitted from the grounded metal-plate collector by the use of no further additional magnetic systems except the ambient open-ended fields B. The proof-of-principle and characterization experiments are carried out by the use of a test-ion-beam line along with an additional use of a Helmholtz coil system for the formation of open magnetic fields similar to those in the GAMMA 10 ...