S. P. Regan

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.

Laboratory test of a LSM‐based narrow bandpass and high throughput camera for Tokamak plasma imaging between 100 and 200 Å

A narrow‐bandpass (∼7 A) and high‐throughput (f/5.5) extreme ultraviolet (XUV) camera using layered synthetic microstructure (LSM) coated optics has been built and tested at the Johns Hopkins University laboratory in order to image tokamak plasma emission between 100 and 200 A. The LSM camera was tested with a Penning ionization discharge emission source. The test measures the spectral bandpass, the spatial resolution, and the relative photosensitivities. The XUV camera will be used to measure the O vi 150 A line brightness on the Phaedrus‐T tokamak with a tangential view to investigate the impurity transport.

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 polychromator monitoring soft‐x‐ray spectral line emissions of C, O, and Fe used in transport studies of the Phaedrus‐T tokamak plasma during ohmic, rf‐heated, and H‐mode discharges

The MLM (multilayer mirror) polychromator has two modes of operation. In the first mode, it simultaneously monitors the Ly α and β emissions of H‐like O at 19.0 and 16.0 A, respectively, the Ly α emission of C at 33.7 A, and the singlet and triplet transitions of He‐like C at 40.5 A. In the second mode, it simultaneously monitors the Ly α and β emissions of H‐like C at 33.7 and 28.5 A, respectively, as well as the 15.6 and 93.9 A emissions of F‐like Fe. The MLM polychromator has a wavelength resolution that varies from 0.3 A at 16.0 A to 2 A at 40.5 A and 7 A at 93.9 A. It was mounted on the midplane of the Phaedrus‐T tokamak with its line of sight along the major radius. On a shot by shot basis, the MLM polychromator, with a temporal resolution of 1 ms and a spatial resolution of 1 cm, scanned the plasma in the z direction ±15 cm from the toroidal magnetic axis. Vertical asymmetries in the soft x‐ray spectral line brightness profiles were measured and found to depend on the direction of BT. The brightnes...

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.

Measurements of molybdenum radiation in the Alcator C-Mod tokamak using a multilayer mirror soft x-ray polychromator

A photometrically calibrated polychromator utilizing layered synthetic microstructure coated flats (also known as multilayer mirrors, MLMs) as dispersive elements is operating on the Alcator C‐Mod tokamak to measure the molybdenum emissions in the XUV. Molybdenum, the first wall material in C‐Mod, is the dominant high Z impurity in the plasma. Three spectral regions are measured by three separate MLM‐detector channels. The characteristic charge states in the region between 30–40 A are Mo xv to Mo xx, between 65–90 A are Mo xxiv to Mo xxvi, and between 110–130 A are Mo xxxi and Mo xxxii. The instrument’s spectral resolution varies from 0.4 A at λ=30 A to 7 A at λ=130 A. The temporal resolution is typically 1.0 ms, but sampling rates of less than 1 ms are possible. The instrument was photometrically calibrated at The Johns Hopkins University using a Manson soft x‐ray light source. Power loss estimates from Mo xxiv to Mo xxvi, Mo xxxi, and Mo xxxii have been obtained during ohmic and ICRF plasmas using the m...

Line emission tomography for CDX-U using filtered diodes

Electron density and temperature in the CDX-U low aspect ratio tokamak are too low to allow observation of fast magnetohydrodynamic activity using soft x-ray continuum emission. However, spectroscopic measurements show that extreme ultraviolet (XUV) line emission of intrinsic impurities is bright enough to observe such activity. In addition, a fast monitoring system for local temperature changes in the plasma core is required for planned auxiliary heating experiments. We present a spectrally resolved tomographic system for fast imaging of O VI 2s–np, 2p–nd (n⩾3), C V 1s2–1s2p and C VI 1s–2p XUV transitions. Using this emission, we can study both core and edge MHD activity, while the C VI to C V intensity ratio can indicate local changes in electron temperature. To achieve maximal throughput together with the needed spectral resolution, we use arrays of surface barrier diodes filtered with bandpass elemental filters. Using M edge filters (Zr, Pd, and Ag), we achieve both good discrimination between the abo...

Multilayer mirror based line emission tomography for spherical Tokamaks

Due to their highly shaped plasma and possible poloidal asymmetry in impurity concentration, spherical Tokamaks will require tomographic reconstruction of local emissivities to assess impurity content and transport. To collect in an effective manner the data required for such reconstruction, we develop arrays of high throughput “mini-monochromators” using extreme ultraviolet multilayer mirrors as dispersive elements and filtered surface barrier diodes as detectors. We discuss monochromator optimization and show that by working at near normal incidence throughput and spectral resolution are simultaneously maximized. A system proposed for tomographic reconstruction of C V and C VI resonance emission at 33.7 and 40.5 A respectively, achieves 0.9 A spectral resolution, 2 cm spatial resolution, and 0.2 ms temporal resolution, together with good sensitivity and background rejection. Preliminary results obtained from CDX-U low aspect ratio tokamak are also presented.