摘要:

Single‐ and multiframe gated x‐ray images with time resolution as fast as 150 ps are described. These systems are based on the gating of microchannel plates in a stripline configuration. The gating voltage comes from the avalanche breakdown of a reverse biasedp–njunction producing high‐power voltage pulses as short as 70 ps. Results from single‐ and four‐frame x‐ray cameras used on Nova are described.

ISSN:0034-6748    

DOI:10.1063/1.1140115

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The Mirnov coil array, the horizontal soft x‐ray camera, and the ECE (electron cyclotron emission) grating polychromator are used to unfold the structure ofm≥2 coherent magnetic fluctuations (assumed to be tearing modes) observed in TFTR. The modes are found predominantly in the enhanced confinement, or supershot, regime, and when present seriously degrade confinement. The poloidal and toroidal mode numbers (mandn) are determined with the Mirnov coil array. The radial structure is found by calculating the theoretical radial eigenmode (based on the calculated current‐density profile) and scaling the resulting linear eigenmode with the measured fluctuation amplitude at the plasma edge. The presence of rotating magnetic islands is detected as fluctuations on both the chord‐averaged soft‐x‐ray emissivity measurements and the fast grating polychromator electron temperature measurements. The ECE and soft‐x‐ray systems are used to identify coherent modes as islands and confirm estimates of the islands’ location inferred from the Mirnov coil data and calculated current‐density profiles.

ISSN:0034-6748    

DOI:10.1063/1.1140116

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Experiments to obtain time‐resolved, soft‐x‐ray emission from laser‐driven plasmas are succinctly described. The spectra (0.19 keV ≤h&ngr; ≤ 1.3 keV) at various times have been deconvolved and energy integrated to obtain time‐resolved yields from Au disk targets. The temporal profiles of the total thermal x‐ray output power follow the overall laser temporal shape but do not show the high‐frequency fluctuation observed in the laser pulse. The behavior of the ratio of the instantaneous x‐ray yields over the laser absorption is studied. The studies were done at the LLNL Nova laser facility. Single pulses and pulses in ‘‘picket fence’’ configuration were used to heat the gold targets.

ISSN:0034-6748    

DOI:10.1063/1.1140117

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have used GaAs and GaAs:Cr photoconducting detectors (PCDs) to measure the absolute x‐ray output from 1 to 10 keV fromZ‐pinch plasma radiation sources. We calibrated the detectors over the spectral range of 900–3000 eV and found the spectral response to be constant to within the accuracy of the measurements. The flat spectral response, increased sensitivity (compared with bolometers), and good time response of the PCDs, coupled with their reliability and long lifetimes, have allowed us to acquire absolute x‐ray data of significantly higher quality than possible with resistive bolometers or x‐ray diodes (XRDs). An added attraction of PCDs is their smooth saturation at high incident x‐ray fluxes. Unlike other types of electrical x‐ray detectors, this feature allows a PCD to operate over a wide range of incident x‐ray powers (10–104W/cm2). We present results from gas puffZ‐pinch experiments on Proto‐II and Supermite comparing PCD, XRD, and resistive bolometer performance.

ISSN:0034-6748    

DOI:10.1063/1.1140118

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A toroidal soft x‐ray imaging system is being added to the currently installed poloidal soft x‐ray system on the DIII‐D tokamak. The poloidal array is used to determine the poloidal mode structure and location of internal helical MHD perturbations in the plasma. The new array will add toroidal mode identification capability. The four detector arrays are toroidally spaced in a manner that allows identification of toroidal mode numbers up to 24. Beryllium vacuum windows separate the detectors from the tokamak vacuum and also serve as low‐energy filters. The separate detector vacuum chambers can be filled with a gas that changes the low‐energy cutoff of the system. By proper selection of the gas and pressure the low‐energy cutoff can be chosen over the entire range of the detector sensitivity (500–1200 eV). This capability can be used to produce crude x‐ray spectra for the entire imaging system or for gain control.

ISSN:0034-6748    

DOI:10.1063/1.1140119

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Because soft‐x‐ray pulse‐height‐analysis spectra contain chordal information, the electron temperature and the radiation intensity (enhancement factor) measurements do not represent the local values. The correction factors for the electron temperature and the enhancement factor as a function of the temperature and density profile parameters and the energy are obtained. The spectrum distortion due to pulse pileup effects is also evaluated. A set of curves is given from which the distortion of the spectrum can be obtained if the electron temperature, the Be filter thickness, and the electronic parameters of the acquisition system are known.

ISSN:0034-6748    

DOI:10.1063/1.1140120

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A Bragg x‐ray spectrograph was developed for pulsed diagnostics. It covers the wavelength region 2–100 A˚ and accommodates two convex crystals and a time‐resolving x‐ray intensifier. The x‐ray detector is a gated intensified camera incorporating a custom‐designed microchannel plate (MCP) built into a microwave circuit for fast pulsing of four or seven separate images. Various photocathodes (Au, Ni, CsI, and MgF2) have been deposited directly onto the microchannel plate to form the strip lines. The MCP is mounted above a high‐quality phosphor screen coated on top of a fiber‐optic faceplate. Time resolution is obtained by sequentially gating the independent strip electrodes on the MCP for subnanosecond electrical gating. A one‐dimensional time‐resolved spectral image can be obtained.

ISSN:0034-6748    

DOI:10.1063/1.1140068

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Temporal and spatial profiles of the x‐ray spectra in the range 3–400 keV have been measured for divertor plasmas on the JFT‐2M tokamak with a recently developed pulse‐height analyzer system. This detection system incorporates an array of six vertically placed sets of a high‐purity germanium spectrometer. Each x‐ray energy spectrum can be measured simultaneously with a sampling interval of 1–255 ms. Dynamic change of the electron temperature profile, such as time behavior with H‐mode transition, can be measured with this system. It was observed that the electron temperature profile became broad just after the H‐mode transition during neutral‐beam heated discharge. In the lower‐hybrid current drive experiments, the x‐ray emission profile depended strongly on the density. The x‐ray intensity from the peripheral region became strong with increase in electron density.

ISSN:0034-6748    

DOI:10.1063/1.1140069

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Two 32‐channel side‐viewing soft x‐ray diode arrays will be installed on PBX‐M which will provide profile information in the horizontal and vertical directions with a time resolution up to 1 &mgr;s. The information from these arrays can be reconstructed to provide a high time‐resolution poloidal emission profile using a maximum‐entropy‐based technique which incorporates the reconstructed profile from a tangentially viewing soft x‐ray pinhole camera. The pinhole camera provides lower time‐resolution (about 5 ms) shape information which has been reconstructed with the assumption of toroidal symmetry. The pinhole camera information supplements the information from the diode arrays, allowing a reconstruction which can resolve MHD fluctuations of the equilibrium profile. The pinhole camera information need only be a better approximation to the real emission profile than the assumption of a flat profile since the algorithm uses it as a first‐order solution which is perturbed by the information from the diode arrays. The algorithm can also explicitly include the bean‐shaped PBX‐M vacuum vessel and the vignetting of some detectors by the pusher coil.

ISSN:0034-6748    

DOI:10.1063/1.1140070

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

X‐ray spectroscopy is an essential tool in high‐temperature plasma research. We describe a time‐resolved x‐ray spectrometer suitable for measuring spectra in harsh environments common to many very high‐energy density laboratory plasma sources. The spectrometer consisted of a doubly curved Si(111) crystal diffraction element, a WL‐1201 (ZnO:Ga) phosphor, a coherent fiber‐optic array, and two visible streak cameras. The spectrometer design described here has a minimum time resolution of 1.3 ns with 2.8‐eV spectral resolution over a 200‐eV‐wide bandpass in the 6–7‐keV region of the spectrum. Complete system spectral throughput calibrations were done at the Cornell High Energy Synchrotron (CHESS). Details of the design and calibration results are presented.

ISSN:0034-6748    

DOI:10.1063/1.1140071

出版商:AIP    

年代:1988

数据来源: AIP