E. Takahashi

X-ray detection characteristics of gold photocathodes and microchannel plates using synchrotron radiation (10 eV–82.5 keV)

Abstract X-ray detection characteristics of gold photocathodes, and microchannel plates (MCP) have been investigated using synchrotron radiation in the energy range from 12 eV to 82.5 keV. The quantum efficiency of gold is compared with the published data which were reported at some discrete energies, and we add several new data points in the 12–35.9 eV. The detection response of MCP has been represented for photons from the VUV regime of 12 eV to the hard X-ray region of 82.5 keV along with its incident angle dependence. This MCP response has demonstrated the following remarkable sensitivity in this wide energy range: The energy response of MCP remains within about one order of magnitude through this broad range. This comes from the contribution of the combination of various ingredients of MCP; namely, C, O, Si, Ba and Pb. For the energy response, these ingredients, in turn, make some jumps and humps at their edge energies. Another useful feature of the MCP response we obtained is a rapid decrease in the MCP response for the energies less than about ten eV; this is desirable for avoiding the effect of visible light on the X-ray data. Also, the MCP response is compared with the detection efficiency of a silicon surface barrier detector for the application to plasma X-ray diagnostics.

Temporal behaviour of the potential confined electrons in the central cell and in the plug region during a period with thermal barriers

An increase with time of the central-cell electron temperature during a period with thermal barrier potentials has been observed. The increase is explained by an improvement of the electron energy confinement due to the presence of thermal barriers. Different evolutions of the electron energy in the plug region have been observed; these are closely related with the variation in time of the confining potential of the plug electrons. The observed behaviour of the potential confined electrons provides the first data set for the evolution of the effect of thermal isolation caused by the formation of a thermal barrier in the kilovolt range.

X‐ray detection characteristics of microchannel plates using synchrotron radiation in the energy range from 0.06 to 0.6 keV

The characteristics of microchannel plates (MCPs) for x‐ray detection have been investigated with continuous x‐ray energy variation using synchrotron radiation in the XUV and soft x‐ray region (from 60 to 600 eV). The current response data have shown x‐ray absorption fine structure (EXAFS or XANES) near the oxygen K absorption edge as well as a jump structure near the silicon L edge; these structures are explained by the surface composition of channel walls (SiO2). The data show that the surface layer of a microchannel is essential for x‐ray detection by MCPs in this energy range. The precise current response curve has been completed for the wide x‐ray energy range from 0.06 to 82 keV combined with the previously reported data [Kondoh et al., Rev. Sci. Instrum. 59, 252 (1988); Cho et al., ibid. 59, 2453 (1988); Yamaguchi et al., ibid. 60, 368, 2307 (1989)].

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 ...

A theory on the x‐ray sensitivity of a silicon surface‐barrier detector including a thermal charge‐diffusion effect

An analytical method based on a new theoretical model for the x‐ray energy responses of silicon surface‐barrier (SSB) detectors has been proposed. This method may address a recent confusing issue in the x‐ray detection characteristics of SSB semiconductor detectors; that is, the x‐ray responses of SSB detectors as well as p‐i‐n diodes used in underbiased operations were recently found to be contrary to the commonly held belief that the x‐ray sensitivity of an SSB detector is determined by the thickness of the depletion layer. The model presented includes a signal contribution from thermally diffusing charge that is created in the field‐free substrate region within a diffusion length from the depletion layer along with a signal contribution from charge created in the depletion layer. This model predicts a large signal contribution from the charge‐diffusion effect on the SSB responses to high‐energy x rays. Formulas and calculated results supporting SSB calibration data have been represented. These analytic...

X-ray detection efficiency of a silicon surface barrier detector using synchrotron radiation in the 0.06–0.9 keV energy region

Abstract Using synchrotron radiation from 0.056 to 0.900 keV, the detection efficiency of a silicon surface barrier (SSB) detector. η SSB , has been investigated. The experimental data are compared with the theoretically calculated results using the detector parameters of the depletion layer, the dead layer and the electrode. As a whole, the data points agree well with the theoretical results except for an oscillatory structure for the 0.07–0.17 keV region. In this energy region, sharp edge structures in η SSB are observed near the Al L II and L III absorption edges as well as the Al L I edge. In particular, this structure in η SSB near the Al L I edge is accompanied by an oscillatory behaviour like an X-ray absorption near-edge structure (XANES) on the high energy side of the edge.

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...

X-ray detector characterization and applications for plasma research

Abstract Using synchrotron radiation, the detection characteristics of microchannel plates, silicon surface barrier detectors and gold photocathodes are investigated; in particular, the detailed structures of the detection efficiency including an extended X-ray absorption fine structure (EXAFS), an X-ray absorption near-edge structure (XANES) and a chemical shift is reported along the with discussion on the physical mechanisms. Also, the applications of these detectors to X-ray tomography diagnostics for the observation of the cross-sections of fusion-oriented tandem-mirror plasmas are described. These data give information on the spatial distribution of the hot electrons which play an important role in the formation of thermal-barrier potentials φb. Also, the tomographic reconstructed X-ray data in the central cell show the effect of φb formation on the improvement in the central-cell electron confinement for the first time.

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.

Reply to ‘‘Comment on ‘A theory on the x‐ray sensitivity of a silicon surface‐barrier detector including a thermal charge‐diffusion effect’ ’’ [J. Appl. Phys. 72, 3363 (1992)]

The effect of thermal‐diffusion charge on the x‐ray energy response of silicon surface‐barrier (SSB) detectors have generally been ignored; consequently, the SSB response has been believed to be analyzed using the thickness of the depletion layer alone. Our new theory on the SSB x‐ray response [J. Appl. Phys. 72, 3363 (1992)] was prepared for addressing recent confusion on plasma x‐ray analyses using SSB detectors [Rev. Sci. Instrum. 59, 1380 (1988); 61, 693 (1990); 63, 4850 (1992)]. This approach was made under the assumption of a dominant contribution of the diffusion‐charge signal in the vicinity of the x‐ray incident location because of the strong reduction of the x‐ray produced charge within the thermal‐diffusion length. In this report, the comparison between this approximation (having an approximated solution) and the exact numerical calculation (using an integral form) is carried out. Necessity and importance of such three‐dimensional treatments for the data analyses as well as the design of multic...