Measurements Of Ion Temperature Profiles Research Articles

Charge exchange recombination spectroscopy on the alkali beam of Wendelstein 7-X.

Measurements of ion temperature profiles are required to assess the energy and particle transport processes in the Wendelstein 7-X stellarator. This device is equipped with a diagnostic alkali beam, which can be utilized to determine local impurity temperatures and densities by Charge Exchange Recombination Spectroscopy (CXRS). It could provide such profiles in the edge plasma, where other diagnostics are less efficient. With this contribution, first results of CXRS measurements on the sodium beam from the scientific operation phase OP2.1 are presented. The spectroscopic system was in commissioning phase lacking some of the final optical components. Thus, the aim of the diagnostics during this campaign was to explore the measurement capabilities. Based on the processed spectra, the prospects of C5+ and C6+ ion temperature and concentration measurements are discussed. The results indicate that with the final optical setup under installation, the diagnostics could provide ion temperature profiles in the edge with 3mm radial resolution and at least 1s temporal resolution.

Первые результаты исследований ионного теплопереноса на сферическом токамаке Глобус-М2

The paper presents the first ion temperature profile measurements results in the new spherical tokamak Globus-M2, which were obtained in discharges with neutral beam injection using the Charge eXchange Recombination Spectroscopy (CXRS) and the Neutral Particle Analyzer (NPA). Ion heat transport modeling using ASTRA code was carried out based on the obtained experimental ion temperature. The results of the modelling indicate the predominantly neoclassical ion heat transport.

Exploration of ion temperature profile measurements at JET using the upgraded neutron profile monitor

The neutron profile monitor (NPM), routinely used at the Joint European Torus for neutron emissivity profile measurements, consists of two fan-shaped arrays of collimators and each line of sight (LOS) is equipped with a NE213 liquid organic scintillator for simultaneous measurements of the 2.5 MeV and 14 MeV neutrons. A digital system developed in ENEA has replaced the analog acquisition electronics and now enables the NPM to perform spatially resolved neutron spectrometry by providing neutron pulse height spectra (PHS) for each LOS. However, the NPM was not originally designed as a spectrometer and, therefore, lacks several key features, such as detailed measurements of the detector response functions and the presence of detector stability monitors. We present a proof of principle of ion temperature profile measurements derived from the NPM PHS in high plasma current discharges using simulated detector response functions.

Ion Temperature Profile Measurements in ITER by Means of Neutron Spectroscopy

In the next generation of magnetic fusion experiments, such as the International Thermonuclear Experimental Reactor (ITER), information on ion temperature profiles will be needed for burn optimization and transport studies. The feasibility of obtaining these profiles for the core plasma (r < 0.75 of minor radius) directly from the width of measured 14-MeV neutron energy spectra is demonstrated for Maxwellian ion distributions. Neutron energy spectra are calculated using the Monte Carlo technique. Reaction kinematics and velocity distribution of the reacting ions are taken into account, which enables the resulting neutron flux and energy distribution entering a defined collimator to be calculated. Energy spectra of neutrons emitted along a line of sight (LOS) are obtained by adding the contributions from a large number of subvolumes. The associated correction factor (peak temperature over LOS measured temperature) depends on the ion temperature and on the shapes of the temperature and density profiles. The resulting accuracy in the evaluated ion temperature profiles is expected to be better than ± 10%. However, this can be improved to ±5% provided that the ion density profile shape is known. The relative accuracy is estimated to be better than ±5%. Features of several spectrometer candidates are briefly described in relation to ITER conditions and measurement requirements. A time-of-flight (TOF) neutron spectrometer is outlined. Experiments with a test device confirm the calculated energy resolution and separation of neutron from gamma events. The spectrometer is shown to be applicable to ITER under both ohmically heated and ignited conditions. A feedback system will be used to control the detector count rate at high neutron flux levels to accommodate the large dynamic neutron flux range from 5 × 106 to 5 × 1010 n/(cm2 · s). An array of five to nine TOF spectrometers provides ion temperature profiles that satisfy ITER measurement requirements, i.e., Ti ≥ 2.5 keV; 10% accuracy; and spatial and temporal resolutions of 30 cm and 100 ms, respectively.

Modeling of a diagnostic neutral beam for ITER

A neutral hydrogen beam has been modeled to serve as an energetic particle source for the measurement of ion temperature profiles via charge-exchange recombination spectroscopy. Calculation of the neutral beam attenuation uses the multistep ionization cross sections of Boley, Janev, and Post. Whereas beam penetration toward the center of the discharge increases with increasing beam energy, the relevant charge-exchange cross section for the familiar C 5+ n=8–7 transition decreases above 50 keV, leading to an optimum beam energy that yields the maximum charge exchange brightness for each set of plasma conditions. This energy is in the range of 100–150 keV. As a consequence of the high plasma density, temperature, and machine size, the detected signal is dominated by visible bremsstrahlung integrated along the detector sightline, placing a premium on beam intensity, beam current density, and beam chopping with synchronous detection.

Confinement properties of jet plasmas with different temperature and density profiles

The confinement properties of plasmas with substantially different temperature and density profiles have been analysed. The effect of fast particles and energy pedestals on the overall confinement of plasma energy in limiter (L-mode) and X-point (L- and H-modes) discharges heated by NBI or ICRH or both are determined. The importance of the bootstrap current when such energy pedestals are formed is noted. Using sets of consistent experimental data, including ion temperature profile measurements, the local transport properties are compared in the L- and H-phases of a single null X-point medium density NBI heated discharge, the 'enhanced' confinement phase of a limiter high density pellet-fuelled and ICRF heated discharge, the hot-ion phase of a double null X-point low density NBI heated discharge and the hot-ion and H-phases of a double null X-point low density high temperature NBI heated discharge.

Transport and stability studies on TFTR

During the 1987 run, TFTR reached record values of QDD, neutron source strength, and Ti(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3*105 Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalant QDT of 0.25 to close to break-even conditions in the coming run.

Ion temperature profile measurements using the far line wings of Hα

We present ion temperature profiles for the outer half of the TEXT plasma using the Doppler broadened far line wings of the Hα line profile. The technique depends on the assumption that the hydrogen neutrals have equilibrated with the ions through charge-exchange collisions. Data reduction depends upon going far enough out on the line wing that the remaining intensity in the profile originates in the hottest portion of the line of sight, thus removing the need for an inversion procedure. A scanning Fabry–Perot interferometer is used to measure the line profiles because of its superior instrumental profile. This technique and its results will be compared with the charge-exchange technique and impurity Doppler broadening results. The application to ion temperature measurements using a diagnostic neutral beam will also be discussed.

High time resolution ion temperature profile measurements on PBX

Ion temperature profiles with a time resolution of 2–5 ms have been measured on the Princeton Beta Experiment tokamak (PBX) by charge-exchange recombination spectroscopy (CXRS) and a neutral particle charge-exchange analyzer (NPA). The sightlines of both diagnostics crossed the trajectory of a near-perpendicular heating beam, which enhanced the local neutral density (∝ signal strength) and provided spatial resolution. The time resolution of these two independent techniques is sufficient to see sawtooth oscillations and other magneto- hydrodynamic activity. Effects of these phenomena on the toroidal rotation velocity profile, vφ(r), are clearly observed by CXRS. For example, a sharp drop in the central vφ occurs at the sawtooth crash, followed by a linear rise during the quiescent phase. The NPA results are compared with those from CXRS.

Experimentally inferred ion thermal diffusivity profiles in the Doublet III tokamak: Comparison with neoclassical theory

Measurements of ion temperature profiles, obtained on a single-shot basis, have been coupled with transport analysis to obtain spatial profiles of the ion thermal diffusivity χi for a series of 18 neutral-beam-heated discharges in the Doublet III tokamak. The expanded boundary divertor discharges examined in this study had the following range of parameters: Bt = 2.53 T, Ip = 0.35–0.89 MA, vertical elongation = 1.4–1.9, n̄e = (3.7–7.9) × 1013 cm−3, PT = 3.5–6.6 MW, and νi* = 0.03–0.22 , for r/a = 0.2–0.7. The experimentally inferred values of χi have been compared with the values of the ion thermal diffusivity predicted by neoclassical theory. On the basis of a detailed error analysis, the results for all discharges examined are the same: χi is approximately equal to near the plasma centre, but for r/a between about 0.2 and 0.8, χi is both larger than and almost certainly has a spatial variation different from that of . The differences between χi and are larger than can be explained by errors in the experimental measurements and indicate that there are processes operative for the loss of ion heat which are not included in the usual transport models for ion confinement.