R. Barnsley

Localized bulk electron heating with ICRF mode conversion in the JET tokamak

Ion cyclotron resonance frequencies (ICRF) mode conversion has been developed for localized on-axis and off-axis bulk electron heating on the JET tokamak. The fast magnetosonic waves launched from the low-field side ICRF antennas are mode-converted to short-wavelength waves on the high-field side of the 3He ion cyclotron resonance layer in D and 4He plasmas and subsequently damped on the bulk electrons. The resulting electron power deposition, measured using ICRF power modulation, is narrow with a typical full-width at half-maximum of ?30?cm (i.e. about 30% of the minor radius) and the total deposited power to electrons comprises at least up to 80% of the applied ICRF power. The ICRF mode conversion power deposition has been kept constant using 3He bleed throughout the ICRF phase with a typical duration of 4?6?s, i.e. 15?40 energy confinement times. Using waves propagating in the counter-current direction minimizes competing ion damping in the presence of co-injected deuterium beam ions.

Versatile high resolution crystal spectrometer with x-ray charge coupled device detector

A family of Johann configuration curved crystal spectrometers has been designed to share the basic engineering features of compactness, modularity, facility of alignment and focus, and incorporation of solid-state charge coupled device detector arrays. These detectors have intrinsically low noise, useful energy resolution, two-dimensional position sensitivity, and readout modes that are programmable. The spectrometers, although relatively compact, with a Rowland circle diameter in the range 0.5–2 m, can still have sufficient resolving power, dispersion, and throughput to be invaluable in high resolution studies of atomic and plasma sources. This article discusses the basic design features and performance of these doubly dispersive spectrometers and illustrates their versatility by applications to studies of a wide range of laboratory x-ray sources such as line emission from highly ionized atoms in the extended plasmas of Tokamaks and nearly point plasmas produced by focused laser irradiation of solids and...

Bragg rotor spectrometer for tokamak diagnostics

A high‐throughput broadband (1–24‐A) x‐ray spectrometer has been demonstrated on the divertor injection tokamak experiment (DITE) tokamak. A hexagonal rotor supporting six diffractors may be driven in several modes, ranging from a full spectral survey at ∼10 Hz to a stationary, monochromator mode. Wavelength resolution, 500≲λ/Δλ≲1000, is governed by gridded or slotted collimators. A multiwire gas proportional counter provides a measure of energy discrimination, which together with the large instrument aperture, gives sufficient sensitivity and signal/noise ratio to allow measurement of the continuum radiation from the tokamak. The instrument has a self‐contained vacuum system which allows full spatial scans of the DITE plasma. Data acquisition and drive mechanisms for the rotor and filter selection, are controlled remotely from a computer. Results are presented of fast spectral surveys and time evolution of impurity emission during impurity injection.

Ionization balance in EBIT and tokamak plasmas

The equilibrium state in tokamak core plasmas has been studied using the relative intensities of resonance x-ray lines, for example Lyα (H-like), “w” (He-like), and “q” (Li-like) from test ions such as Ar+15, Ar+16, and Ar+17. A full spatial analysis involves comparison of the line intensities with ion diffusion calculations, including relevant atomic rates. A zero-dimensional model using a global ion loss rate approximation has also been demonstrated by comparison with the data collected from a Johann configuration spectrometer with a charged coupled device (CCD) detector. Since the lines are nearly monoenergetic, their intensities are independent of the instrument sensitivity and are directly proportional to the ion abundances. This method has recently been applied to Ar in the Oxford electron beam ion trap (EBIT) with a beam energy in the range 3–10 keV. Taking into account the cross sections for monoenergetic electron collisions and polarization effects, model calculations agree with the observed line...

Bragg spectroscopy of impurities during the JET preliminary tritium experiment

Two Bragg spectrometers shared a shielded beamline, and monitored all significant plasma impurities throughout the Joint European Torus (JET) preliminary tritium experiment. The JET Active Phase Double Crystal Monochromator and a Bragg rotor spectrometer together monitored K‐shell lines of Be, C, O, and Cl, and L‐shell lines from metals such as Ni and Zn, in the wavelength range 0.2–11 nm. Background radiation was recorded during and after discharges, to assess the signal‐to‐noise ratio, and to monitor any activation or tritium ingress. Bragg spectroscopy was shown to be suitable for all impurities in magnetically confined D–T plasmas, using a thin foil to isolate tritium, and a low f‐number shielded beamline. Future improvements to the signal‐to‐noise ratio are discussed in the context of a next‐step machine such as ITER.

Quantitative spectroscopy of x-ray lines and continua in Tokamaks

Crystal and synthetic multilayer diffractors, deployed either as flat Bragg reflectors, or curved, as in the Johann configuration, are used to study the spectrum of COMPASS-D and other tokamaks in the wavelength region of 1–100 A. In this article, we concentrate on the measurement of absolute photon fluxes and the derivation of volume emissivities of the lines and continua in the x-ray region. The sensitivities of these instruments to absolute photon flux have been constructed ab initio from the individual component efficiencies, including published values of the diffractor reflectivities, which have been checked or supplemented by measurements using a double-axis goniometer or from line branching ratios. For those tokamak plasmas, where the elemental abundances and effective ion charge are documented, the x-ray continuum intensity itself has been used as a calibration source to derive absolute instrument sensitivity, in reasonable agreement with the ab initio method. In the COMPASS-D Tokamak, changes in ...

Two‐axis goniometer for reflectivity measurements of x‐ray diffractors used in fusion researcha)

Quantitative measurements of the line and continua emissivities and the analyses of spectral line profiles are essential steps in the interpretation of the x‐ray emission from high‐temperature fusion plasmas. One method of placing the emissivities on an absolute basis is to use an absolutely calibrated spectrometer to record the data. The overall sensitivity of the spectrometer can be constructed in terms of the efficiencies of its separate components, the most intractable being Rc, the reflection integral of the diffractor. To this end, a new, compact, two‐axis diffractometer, incorporating modern robotic technology, such as direct‐drive servomotors with closed‐loop operation from built‐in arcsec optical encoders, has been constructed. Improved features of this double‐axis goniometer include the use of fixed line‐of‐sight x‐ray sources with the capability of operation in the (1,−1) parallel, nondispersive mode or the antiparallel, (1,+1), dispersive mode. The diffractometer is now being used to calibrate...

Pilot experiments for the International Thermonuclear Experimental Reactor active beam spectroscopy diagnostic

Supporting pilot experiments and activities which are currently considered or already performed for the development of the International Thermonuclear Experiment Reactor active beam spectroscopy diagnostic are addressed in this article. Four key issues are presented including optimization of spectral instrumentation, feasibility of a motional Stark effect (MSE) evaluation based on line ratios, “first-mirror” test-bed experiments at the tokamak TEXTOR, and finally the role of integrated data analysis for the conceptual layout of the change exchange recombination spectroscopy and MSE diagnostic.

The impurity control limiter experiment

The paper presents results from the impurity control limiter (ICL) experiment, which was installed in the DITE tokamak. The ICL was designed so that the majority of ions leaving the plasma strike the undersides of its graphite tiles, which are angled towards the wall. These experiments confirm that the impurities sputtered from the ICL are ionized predominantly in the scrape-off layer and that there is significant deposition of neutral carbon on the walls. Both the spatial distribution of ionization and the toroidal distribution of carbon on the walls are in good agreement with code predictions. In helium discharges, the insertion of the ICL resulted in a 50% reduction of the total radiation and a 37–66% reduction in the impurity contribution to Zeff compared with the values obtained with rail limiters of conventional geometry. In deuterium discharges, the reduction in the total radiation was only 20%; this is attributed to the greater importance of the wall sources of carbon and oxygen in deuterium plasma relative to those in helium.

Diagnostics for ITER

After an introduction into the specific challenges in the field of diagnostics for ITER (specifically high level of nuclear radiation, long pulses, high fluxes of particles to plasma facing components, need for reliability and robustness), an overview will be given of the spectroscopic diagnostics foreseen for ITER. The paper will describe both active neutral‐beam based diagnostics as well as passive spectroscopic diagnostics operating in the visible, ultra‐violet and x‐ray spectral regions.