M. Finkenthal

Near normal incidence spectroscopy of a Penning ionization discharge in the 110-180 A range with flat multilayer mirrors.

Al III to Al v spectra emitted from a Penning ionization discharge have been recorded in the 110-180 i range using two flat multilayer mirrors (Mo/Si and Mo/B(4)C) as dispersive elements in a near normal incidence configuration.

Measured conversion efficiencies of P45, paraterphenyl, tetraphenyl butadiene, and sodium salicylate phosphors in the soft-x-ray wavelength range

The measured conversion efficiencies at 9.89, 23.6, 44.7, and 160 Å of P45 phosphor screens, as well as those of paraterphenyl, tetraphenyl butadiene, and sodium salicylate at 9.89, 44.7, and 67.6 Å, are presented. The conversion efficiency is defined as the ratio of photoelectrons ejected from the photocathode of a visible detector, which are excited by the scintillated photons that are emitted from the phosphor in a solid angle of 2π, to the number of soft-x-ray photons incident on the phosphor. The effect of the phosphors thickness on the conversion efficiency was studied. The P45 phosphor converts the soft-x-ray photon (10-200 Å) into an order of magnitude more visible photons than the low-Z phosphors. The P45 phosphor screen used in conjunction with a photomultiplier tube offers a soft-x-ray photodetector with a conversion efficiency that ranges from 0.5 at 160 Å to 12 at 9.89 Å and a high electronic gain.

MLM-based monochromator for molybdenum impurity monitoring on the Alcator C-MOD tokamak in the 30-130 Å range

Molybdenum, from the vessel wall armor tiles and divertor plates, is expected to be the dominant high‐Z impurity in the Alcator C‐MOD tokamak plasma. To monitor the molybdenum emission in the extreme ultraviolet (XUV), a monochromator utilizing multilayer mirrors (MLMs) as dispersive elements will be installed on the C‐MOD tokamak with a 10° above the mid‐plane view across the plasma’s center. Much of the strong Mo emission under the C‐MOD experimental conditions will be emitted between 30–50 A and 65–90 A by M‐shell ions (n=3–3 and n=3–4 transitions), at 116 A from MoXXXI and at 127 A from MoXXXII. The monochromator will have three MLMs and either three‐channel electron multipliers or three XUV silicon photodiodes mounted on a single θ‐2θ goniometer. Each MLM and detector unit will simultaneously monitor a separate molybdenum emission range with a 1–10‐ms temporal resolution. The resolution of the MLMs range from 1 to 5 A depending on the wavelength, and each MLM will be optimized for one of the above sp...

Energy resolved fast two-dimensional x-ray imaging for MFE plasmas (invited)

A new approach to time resolved two-dimensional x-ray imaging is presented. It is based on a micropattern gas detector with gas electron multiplier as amplifying stage and pixel readout. It allows x-ray photon counting at high detection efficiency, high signal to noise ratio, extremely high time resolution (framing rate up to 100 kHz), and with an innovative capability: the energy resolution. The detector has a limited number of pixels (up to tens of thousands), but each pixel behaves like a low-resolution spectrometer, allowing quasimonochromatic images or spectral scans. This new approach is proposed as an alternative to the traditional static, spectral integrated, noiser, high definition (millions of pixels) x-ray imaging as performed, for example, with charge coupled device based devices. All these innovative features have been tested on the National Spherical Torus Experiment (Princeton, USA) but the authors are firmly convinced that it will be applied to many other fields of research.

Development of phosphor scintillator-based detectors for soft x-ray and vacuum ultraviolet spectroscopy of magnetically confined fusion plasmas

Specialized soft x-ray and vacuum ultraviolet (VUV) diagnostics used to monitor impurity emissions from fusion plasmas are often placed in a very challenging experimental environment. Detectors in these diagnostics must be simple; mechanically robust; immune to electromagnetic interference, energetic particles, and magnetic fields up to several tesla; ultra-high-vacuum compatible; and able to withstand bakeout temperatures up to 300 °C. The design and the photometric calibration of a detector consisting of a P45 phosphor (Y2O2S:Tb), two incoherent fiber-optic bundles coupled with a vacuum feedthrough fiber-optic faceplate, and a photomultiplier tube (PMT) are reported. We have successfully operated the detectors of this type in novel soft x-ray and VUV diagnostics on several fusion plasma facilities. Measurements of the visible photon throughput of the silica/silica incoherent fiber-optic bundle, and the light loss associated with the coupling of the two fibers with the faceplate are presented. In additio...

Photometric calibration of soft x-ray and p -terphenyl coated visible photodiodes in the 180--1500 eV range for fusion plasma spectroscopy

The efficiencies of x‐ray ultraviolet silicon, and p‐terphenyl coated visible photodiodes have been measured in the 180–1500 eV range using a K radiation‐Manson source. It is found that the quantum efficiency (electrons/photon) of the silicon diode varies between 25 and 400 in the above‐mentioned range; the p‐terphenyl coated diode is by two orders of magnitude less performing at the high‐energy end of the range considered, but approaches the efficiency of the silicon diode at 100 A. Such diodes with built‐in amplifiers, coated with scintillator and thin layers of metal films, can be efficiently used in spectroscopic diagnostics of magnetically confined plasmas.

Spectroscopic diagnostics on low aspect-ratio tokamaks

Abstract Low aspect-ratio tokamaks (LARTs) have intrinsic features which require specific diagnostics for particle transport and confinement studies. Such spectroscopic diagnostics are developed in collaboration between the CDX-U group at Princeton Plasma Physics Laboratory and the Plasma Spectroscopy group at Johns Hopkins University. This paper presents results obtained from the low aspect-ratio CDX-U tokamak, as well as future diagnostic developments for CDX-U and other LARTs. These results are based on spectroscopy in the VUV (500 A - 2000 A) using a multiple grating normal incidence spectrometer equipped with an optical multichannel analyzer, and in the XUV range on emission recorded using multilayer mirror devices.

Photopumping of a soft x-ray laser by narrow quasicontinuous band radiation

We propose a laser scheme in which the upper levels of a lasing iron are pumped by narrow, quasi-continuous band radiation. In particular, we consider pumping of neon-like chlorine ions by N band radiation of Pb from 41 angstroms to 44 angstroms. We calculate small signal gain coefficients for the n equals 4 - n equals 3 transitions in neon-like chlorine. Both steady-state and time-dependent calculations have shown that the largest gain is found for the 2s22p-11/24f5/2(J equals 2) - 2s22p-11/23d3/2(J equals 1) transition at a wavelength of 264 angstroms, with a value of 5 cm-1. Achieving this value for the gain would require a pump radiation that has a brightness of 0.01 photons/mode, corresponding to an intensity of 1.6 X 1012 Wcm-2.

Plasma imaging in the XUV wavelength range (175 Å) using curved layered-synthetic-microstructure coated surfaces

A recently constructed calibration facility utilizing a Manson soft x-ray line source in the wavelength range of 8 - 114 angstroms and a Penning ionization discharge (PID) in the 100 - 350 angstroms range, has been used to map the reflectivity across a curved layered synthetic microstructure (LSM) coated surface. This calibrated mirror was also used to image the Al III emission ((lambda) equals 170 - 175 angstroms) from the PID.