The use of an injected neutral beam—either a dedicated diagnostic beam or the main heating beams—to localize and enhance plasma spectroscopic measurements can be exploited for a number of key physics issues in magnetic confinement fusion research, yielding detailed profile information on thermal and fast ion parameters, the radial electric field, plasma current density, and turbulent transport. The ability to make these measurements has played a significant role in much of our recent progress in the scientific understanding of fusion plasmas. The measurements can utilize emission from excited state transitions either from plasma ions or from the beam atoms themselves. The primary requirement is that the beam “probe” interacts with the plasma in a known fashion. Advantages of active spectroscopy include high spatial resolution due to the enhanced localization of the emission and the use of appropriate imaging optics, background rejection through the appropriate modulation and timing of the beam and emissio...

Beams, brightness, and background: Using active spectroscopy techniques for precision measurements in fusion plasma researcha)

Controlled thermonuclear fusion is the main goal of plasma physics. At the Swiss Plasma Center, the Tokamak \`a Configuration Variable (TCV) constitutes the main experiment on fusion research, where high temperature plasmas are confined by means of magnetic fields. The confinement of plasma energy and particles is limited by transport arising from the gradients between the hot-dense plasma core and the cold-rarefied plasma edge. Due to the tokamak topology, plasma can rotate in the toroidal and poloidal directions. Plasma rotation has a strong influence on confinement and stability, which makes its understanding a priority. There are many discrepancies between the theoretical rotation description and experiments, which stimulated research in the field. In this context this work provided experimental results of unprecedented accuracy, where plasma impurity parameters are measured with the charge exchange recombination spectroscopy (CXRS) diagnostic. CXRS exploits the CX signal induced by a diagnostic neutral beam injector (DNBI), permitting localised measurements of impurity rotation, density and temperature. During this work, the CXRS diagnostic was extended with the development of a new high resolution system, termed CXRS-EDGE, devoted to the study of edge profiles. The accuracy improvements with respect to the legacy systems were obtained through an high throughput lens spectrometer and numerical aperture matching optics, resulting in rotation uncertainties <1 km/s. The upgraded CXRS diagnostic was used in the study of the impact of the sawtooth (ST) magneto-hydrodynamic (MHD) instability on rotation in L-mode limited plasmas and on the changes in edge impurity parameters at the onset of the H-mode in diverted discharges. The evolution of impurity profiles during a canonical ST were studied with a 2 ms time resolution. Rotation profiles suddenly change at the ST crash, passing from peaking counter-current to hollow co-current in the core, showing evidence of a co-current torque at the crash of increasing magnitude with the ST period. This explains the Ip scaling observed in TCV L-mode plasmas. This effect must be included in realistic theoretical models and might be exploited in tailoring rotation using ST period and radius as actuators. The crash is also characterised by expulsion of impurity from the core, resulting in hollow post-crash profiles, that can be exploited in a reactor scenario for avoiding impurity accumulation and for ash removal. The first accurate measurement of the changes in impurity flow across the transition to ELM-free H-mode were performed in this work. A narrow and deep poloidal rotation well develops at the last closed flux surface (LCFS) at the transition, together with a density pedestal and an increase of the temperature. From the kinetic profiles the radial electric field was computed, revealing the development of the well characteristic of H-mode as observed in other devices. The evolution of its components suggests involvement of the poloidal rotation in the triggering of the transition. A reasonable agreement was found from the comparison of the impurity rotation with neoclassical predictions. The observations reported in this work constitute a strong constrain for theoretical models, demonstrate the strong influence that fast MHD events have on transport and set more stringent conditions for edge impurity behaviours.

Poloidal CX visible light plasma rotation diagnostics in TCV

Charge-transfer $n$ partial cross sections have been calculated for collisions of ${\mathrm{Be}}^{4+}$ with $\mathrm{H}(1s)$ by means of a versatile lattice method that is applicable in a wide energy range (between 1 and 500 keV/u). The cross sections, which include up to the high-lying $n=8$ level, are compared to existing semiclassical calculations in order to quantify the accuracy of the results. The reliability of the lattice method at high impact energies is confirmed by comparison with classical trajectory Monte Carlo calculations. It is found that the $n$ partial cross sections larger than ${10}^{\ensuremath{-}18} {\mathrm{cm}}^{2}$, calculated using the lattice method, agree with differences smaller than 15% with those from the method considered the most accurate at each energy. The calculation yields as well accurate total electron capture cross sections, which are studied in detail at $E=100$ keV/u to obtain a converged value.

/pdf/application-of-a-grid-numerical-method-to-calculate-state-2vln06vul1.pdf

Application of a grid numerical method to calculate state-selective cross sections for electron capture in Be 4 + +H (1 s ) collisions

A feasibility study for the use of core charge exchange recombination spectroscopy on ITER has shown that accurate measurements on the helium ash require a spectrometer with a high etendue of 1mm 2sr to comply with the measurement requirements [S. Tugarinov, Rev. Sci. Instrum. 74, 2075 (2003)]10.1063/1.1537443. To this purpose such an instrument has been developed consisting of three separate wavelength channels (to measure simultaneously He/Be, C/Ne, and H/D/T together with the Doppler shifted direct emission of the diagnostic neutral beam, the beam emission (BES) signal), combining high dispersion (0.02 nm/pixel), sufficient resolution (0.2 nm), high efficiency (55%), and extended wavelength range (14 nm) at high etendue. The combined measurement of the BES along the same sightline within a third wavelength range provides the possibility for in situ calibration of the charge eXchange recombination spectroscopy signals. In addition, the option is included to use the same instrument for measurements of the fast fluctuations of the beam emission intensity up to 2 MHz, with the aim to study MHD activity. © 2012 American Institute of Physics.

/pdf/a-high-etendue-spectrometer-suitable-for-core-charge-24s6iomy7z.pdf

A high etendue spectrometer suitable for core charge eXchange recombination spectroscopy on ITER.

The CXRS (Charge-eXchange Recombination Spectroscopy) diagnostic for the core plasma of ITER will be designed to provide observation of the dedicated diagnostic beam (DNB) over a wide radial range, roughly from a normalised radius r/a = 07 to close to the plasma axis The collected light will be transported through the Upper Port Plug #3 (UPP3) to a bundle of fibres and ultimately to a set of remote spectrometers The design is particularly challenging in view of the ITER environment of particle, heat and neutron fluxes, temperature cycles, electromagnetic loads, vibrations, expected material degradation and fatigue, constraints against tritium penetration, integration in the plug and limited opportunities for maintenance Moreover, a high performance (etendue × transmission, dynamic range) is expected for the port plug system since the beam attenuation is large and the background light omnipresent, especially in terms of bremsstrahlung, line radiation and reflections The present contribution will give an overview of the current status and activities which deal with the core CXRS system, summarising the investigations which have taken place before entering the actual development and design phase

/pdf/status-of-the-r-d-activities-to-the-design-of-an-iter-core-48zinzhvbx.pdf

Status of the R&D activities to the design of an ITER core CXRS diagnostic system

Overview on R&D and design activities for the ITER core charge exchange spectroscopy diagnostic system

Charge eXchange Recombination Spectroscopy (CXRS) will play a crucial role in the diagnosing of burning plasmas: items like helium ash, transport barriers, impurity content or fuel ratio can all be assessed with CXRS. In fact this is the only direct method to obtain information about the light impurity ions, such as temperature, concentrations and rotation, in the core of the plasma. Two of such systems are foreseen at ITER, one in the upper port plug and one on the equatorial level, both viewing the 100 keV diagnostic H-beam. Challenges for this diagnostic development project are: constructing a mirror close to the plasma able to keep its reflectivity under the extreme bombardment by neutrons, erosion by fast neutrals and deposition of sputtered material, design of an optical labyrinth with well enough neutron shielding, developing spectrometers with a throughput about a factor 100 larger than on present day devices, with a similar or better dispersion, calibration and alignment techniques and last but not least, finding ways to obtain the physics relevant information from the measured complicated spectra in real time. With all these issues in mind, an ITER-CXRS pilot experiment has been developed and became operational on the TEXTOR tokamak, to address some of the above aspects. The results of this show the feasibility and the sensitivity of the ITER system: the simulation code for ITER signal levels could be verified, the sensitivity to the current profile has been tested, the beam emission has been compared to beam attenuation calculations and used to obtain absolute impurity concentrations and a high throughput spectrometer became operational.

The CXRS diagnostic for ITER and the CXRS- Pilot Experiment on TEXTOR

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Overview on R&D and design activities for the ITER core charge exchange spectroscopy diagnostic system

The CXRS diagnostic for ITER and the CXRS- Pilot Experiment on TEXTOR