M. Hirata

New findings of X-ray energy responses of silicon surface barrier detectors and their generalized theoretical extension to X-ray responses of position sensitive detectors

Abstract X-ray energy responses of silicon surface barrier (SSB) semiconductor detectors are found to be explained neither by the commonly believed conventional model using the depletion layer thickness of an SSB detector nor by a recently proposed model using its wafer thickness as the X-ray sensitive layer. Our new theory using both the depletion layer sensitivity and the X-ray response due to a three-dimensional charge diffusion effect in the field-free substrate region of a SSB detector can well fit the response data. This theory is newly extended to analyse each channel response of a multichannel semiconductor X-ray detector fabricated on one silicon wafer; these detectors are widely utilized as position sensitive X-ray detectors for nuclear fusion oriented plasma research as well as for high energy elementary particle studies. A numerical simulation of multichannel detector outputs using our three dimensional diffusion theory is carried out; generalised theoretical formulae are also presented.

A new end‐loss‐ion energy analyzer with obliquely placed multigrids for open‐ended plasma diagnostics

A new type of end‐loss‐ion energy analyzer (ELA; a multigridded Faraday cup) is applied to measure plasma ion temperatures Tip and plasma potentials ΦP in the GAMMA 10 tandem‐mirror device. As compared with conventional‐type ELA, this new ELA has obliquely placed ion‐ and secondary‐electron‐repeller grids and a collector plate with respect to external tandem‐mirror magnetic fields. One of the most useful capabilities of this new ELA for precise ion measurements is to greatly reduce the current of high‐energy electrons onto the collector plate. Also, this compact‐sized ELA is scannable in the plasma to obtain radial profiles of Tip and ΦP without providing any perturbations of the tandem‐mirror magnetic fields because it neither adds its own magnetic field nor needs to shield against the ambient magnetic field. Ion trajectories in the new ELA are numerically calculated so as to make a further understanding of its physics principles and to optimize its design. The energy‐calibration experiments for the new ...

Observation of AIC-mode in ICRF-heated plasmas in the central cell of GAMMA 10 tandem mirror

Alfven ion-cyclotron modes are generated in an ICRF heated plasma in the central cell of the GAMMA 10 tandem mirror. Identification of the mode, wave characteristics and its effects for the plasma parameters are described. General results of the GAMMA 10 experiments are also reviewed including the experimental scaling of the potential confinement.

DEVELOPMENT AND CHARACTERIZATION OF SILICON SEMICONDUCTOR X-RAY DETECTORS FOR PLASMA DIAGNOSTICS

The x‐ray energy responses of silicon semiconductor detectors, including surface‐barrier and photodiode‐type detectors, have been investigated, using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory (KEK) in order to study the fundamental physics mechanism of the output signals. These studies are essential to obtain the principles of the future designs of plasma x‐ray detectors, as well as of their plasma data analyses. The characterization experiments and their physics interpretations were made using (i) a group of fully depleted detectors and (ii) another group of partially depleted detectors characterized by obviously different depletion‐layer thicknesses and minority‐carrier diffusion lengths, but with the same wafer thickness. Both results are well interpreted by our newly proposed formula for a semiconductor x‐ray‐detector response. The importance of these investigations for the plasma x‐ray diagnostics is highlighted by significant errors for the estimation of plasma...

Plasma diagnostics using vacuum ultraviolet radiation and soft X-rays in the GAMMA 10 tandem mirror device

Abstract Plasma diagnostics using vacuum ultra violet (VUV) and X-ray radiation in the world largest tandemmirror plasma-confinement-device GAMMA 10 are reported. In particular, the detection characteristics of microchannel plates (MCP) with 50 channels (anodes) are investigated using synchrotron radiation ranging from VUV to an X-ray regime (12 eV to 82.5 keV) for the purpose of detector fundamental researches as well as their development studies. These investigations also provide applications of MCP as imaging detectors including applications for spatially and temporally resolved plasma VUV and soft X-ray diagnostics. It is found that the detection efficiency of MCP remains within one order of magnitude through dozens eV to several tens keV along with the physical mechanism interpretations of the MCP energy response, and the response uniformity of each channel for an imaging use is experimentally investigated. Using these detailed and precise MCP calibration data, we apply the MCP not only for a plasma-radiation imaging detector so as to reconstruct spatial profiles of emissivity but for a plasma diagnostic tool to measure plasma-electron velocity distribution functions.

A novel analysis method for plasma light and soft X-ray tomography data including the three-dimensional diffusion in a semiconductor detector array

Abstract The three dimensional diffusion effect in a semiconductor photon detector is directly demonstrated using a spatially distributed charge profile produced by a monochromatic energy beam in a multichannel semiconductor detector. When we substitute the value of the diffusion length obtained by the data on the charge profile and the depletion layer thickness from the data on its capacitance vs applied voltages into our theoretical formula, then the photon energy response data are well fitted by the theory. The effect of this diffusion on the plasma VUV and X-ray data is also demonstrated for obtaining actual plasma data.

Experimental verification of the three‐dimensional diffusion effect of x‐ray‐produced charges in semiconductor x‐ray detectors on the quantum‐efficiency enhancement

One of the characteristic physical principles of our recently proposed theory on the x‐ray‐energy response of a semiconductor x‐ray detector is the effect of the three‐dimensional diffusion of x‐ray‐produced charges in a semiconductor field‐free substrate region. It is found and reported that the quantum efficiency of a semiconductor x‐ray detector is enhanced when this phenomenon is taken into account, and the data on the x‐ray‐energy response of the semiconductor detector using monochromatized synchrotron radiation at the Photon Factory are actually well fitted by the theory. In addition, we measure the diffusion length L of the detector using a precisely collimated x‐ray narrow beam and a multichannel detector array setting on a μm‐order position‐controllable stage. The verification of the consistency between this observed value of L and the predicted value of L from our theory on the quantum efficiency, has been achieved.

Photoelectron spectroscopy for plasma x‐ray measurements

A new x‐ray spectrometer using the principle of a photoelectron spectroscopy method is developed for the purpose of the observations of plasma‐electron temperatures in the range of a hundred of eV. X‐ray photoelectron spectroscopy is a widely utilized method for surface‐physics analyses; here, we employ a parallel‐plate energy analyzer. This new type spectrometer is calibrated using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory (KEK). Theoretical calculations for photoelectron spectra from plasmas with various electron temperatures are carried out using the calibration data. This spectrometer is set up at the central cell of the GAMMA 10 tandem mirror for the measurements of potential‐confined electron temperatures. Experimentally observed spectra are compared with the calculated spectra as well as the data from the other x‐ray detection method.