A. P. Zwicker

Layered synthetic microstructures for soft x‐ray spectroscopy of magnetically confined plasmas (invited)

With the recent advances in layered synthetic microstructure (LSM) technology, it is now possible to build a simple, high‐throughput, near‐normal incidence soft x‐ray spectrometer as a diagnostic for magnetically confined plasmas. Such spectrometers could be used for radiative power loss measurements from intrinsic impurities, an impurity concentration monitor, or ion temperature measurements from Doppler broadening of high charge‐state metallic impurities. LSMs have been developed as either flat or curved multilayer mirrors (MLMs) or as coatings for conventional gratings. Flat multilayer mirrors can have near‐normal incidence reflectivities greater than 50% throughout the entire soft x‐ray region with bandpasses that can be less than 4 A. Coated gratings are being developed that will combine the high soft x‐ray reflectivity of the LSM with the high resolution of the grating. Specific applications of LSMs as dispersive elements will be discussed. As an example, LSM‐based low‐resolution spectra of both a l...

Peak reflectivity measurements of W/C, Mo/Si, and Mo/B 4 C multilayer mirrors in the 8–190-Å range using both Kα line and synchrotron radiation

Peak reflectivity measurements of W/C, Mo/Si, and Mo/B(4)C multilayer mirrors have been performed using line and synchrotron radiation in the 8-190 A wavelength range. Short wavelength measurements using a line source were corrected for nonmonochromatic and divergent incident radiation. Reflectivities of Mo/Si mirrors, measured with synchrotron radiation, ranged from 25 to 44% but decreased significantly around the Si absorption edge. Mo/B(4)C multilayer mirrors were measured that had peak reflectivities from 10 to 25% between 90 and 200 A and bandpasses as small as 3 A.

High throughput multilayer mirror based soft x‐ray spectrometer for metallic impurity emission from tokamaks

A near‐normal incidence high throughput spectrometer with a flat multilayer mirror (MLM) as the dispersive element has been built and operated on the Texas Experimental Tokamak (TEXT). Using a Mo/B4C MLM that has a reflectivity of 20–30% and a bandpass of 4–9 A (FWHM), the instrument has a wavelength range of 90–180 A. The spectra obtained with this instrument have been compared to an artificial spectrum based upon a collisional‐radiative model. From this comparison it is possible to identify the dominant titanium emission lines responsible for the broadband shape of the low‐resolution experimental spectrum. This work is part of our continuing development of simple, near‐normal incidence high throughput spectrometers for tokamak plasma diagnostics.

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.

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...

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.

A soft x‐ray multilayer mirror scanning monochromator for magnetically confined fusion plasmas

A high throughput scanning soft x‐ray monochromator that uses a flat multilayer mirror in a near normal incidence configuration was built and tested in the Plasma Spectroscopy laboratory at Johns Hopkins University and operated on the DIII‐D tokamak. Depending upon the mirror used, the monochromator covers the range 10–170 A, with a resolution of 0.25 to 10.8 A. The instrument was photometrically calibrated between 10 and 45 A. Spectra from DIII‐D were obtained under a variety of discharge conditions over the entire range of the instrument. This paper describes the monochromator, the photometric calibration, and presents initial results.

Evaluation of a multilayer mirror spectrometer in the 230–304 Å wavelength range

The reflective properties of a flat Mo/Si multilayer mirror (MLM) optimized for the 230 A<λ<304 A range (2d=373.2 A, N=25, 25 A<Δλ<50 A) have been measured. A near‐normal incidence spectrometer using the MLM as the dispersive element, a beryllium foil as a light blocking filter, and a channel electron multiplier as the photodetector has been built. The interest in such a device is related to its possible application as a monitor of alpha particles by imaging the whole plasma using the Lyman series of He ii. In the laboratory experiment described here, the Lyman series of He ii, emitted from a Penning ionization discharge, were scanned with a θ‐2θ goniometer. In parallel, high‐resolution (Δλ≊1 A) spectra were also recorded using a 1 m grazing incidence (GI) spectrometer. Comparisons are made between the high‐resolution (GI) spectrum modeled to the bandpass of the MLM and the low‐resolution (MLM) spectrum in the range of interest.

Analysis of Impurity Content and Transport in Tokamak Plasmas Using Low-Resolution XUV Spectra

Two experiments were performed to determine whether impurity content and transport information could be extracted from low-resolution XUV spectra recorded from a simple spectroscopic diagnostic that utilized a flat multilayer mirror as the dispersive element. The first experiment, at the DIII-D tokamak, compared MLM spectra to higher-resolution spectra and found that the low-resolution MLM spectra were sufficient to distinguish changes in impurity emission patterns. The results demonstrated the feasibility of building simple MLM-based diagnostics for impurity monitors in the harsh environment of future tokamaks. The second experiment, at the Texas Experimental tokamak, compared MLM spectra to those produced by an impurity transport code coupled to a collisional-radiative model. The comparison showed that it is possible to distinguish changes in impurity transport from low-resolution MLM spectra.