Simulation of the motional stark effect on C-MOD using MSESIM and PERF

The Motional Stark Effect (MSE) is used to measure the internal topology of the magnetic field and radial electric field in a tokamak plasma. MSESIM, a MSE simulation code originally developed for MAST, was transformed to simulate the MSE effect on C-MOD. This code was extended to simulate the Paschen Back Effect and the non statistical population of the energy levels. MSESIM was benchmarked against PERF, a different MSE simulation code developed for JET and C-MOD, to gain more confidence in the results of both codes. MSESIM has been used to investigate the influence of the collection optics, beam divergence, collection volume, Paschen Back Effect, non statistical population of the energy levels and the narrow bandpass filter on the spectrum and polarisation angle of C-MOD. The MSESIM simulation results were compared with measurement to assess how well the code simulates reality

Validation of the OFIT technique for the detection of the plasma boundary at MAST : traineeship report

A plasma boundary reconstruction method based on optical images was introduced by Hommen et al.[1]. Consistency of the boundary with the boundary from magnetic measurements was demonstrated for a few cases, but systematic validation over a wide range of plasma parameters was lacking. Here we present a systematic comparison of the EFIT (magnetic) and OFIT (optical) boundaries for more than two hundred MAST (Mega Ampere Spherical Tokamak) discharges. For this comparison, the OFIT concept had to be implemented in the MAST data analysis chain and needed to be improved on a number of points. A dimensionless figure of merit has been introduced comparing the non-overlapping regions with the overlapping regions. With this criterion, overall discrepancies of about 6-8% were found between the optical and magnetic boundary. These dimensionless discrepancies translate to overall distances between the EFIT and OFIT boundaries of about 2-3 cm. Upper-limits on the error of OFIT, of 2:7 cm for the DND case and 4:1 cm for the SND case, were found. No (strong) correlation was found with plasma parameters such as normalized pressure ß or self induction li. We show that a significant part of the discrepancy is likely caused by faults in the EFIT reconstruction. This leads to the conclusion that the OFIT technique, if implemented and used correctly, is usable for reliable real-time detection of plasma position and shape, with an overall error less than 3-4 cm

The MAST motional Stark effect diagnostic

A motional Stark effect (MSE) diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of ~ 2.5 cm, and time resolution of ~ 1 ms at angular noise levels of ~ 0.5°. Conventional (albeit very narrow) interference filters isolate p or s polarized emission. Avalanche photodiode detectors with digital phase-sensitive detection measure the harmonics of a pair of photoelastic modulators operating at 20 and 23 kHz, and thus the polarization state. The p component is observed to be significantly stronger than s, in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on p. Trials with a wide filter that admits the entire Stark pattern (relying on the net polarization of the emission) have demonstrated performance almost as good as the conventional channels. MSE-constrained equilibrium reconstructions can readily be produced between pulses. © 2010 EURATO

Overview of Active Beam Spectroscopy developments for ITER

Active Beam Spectroscopy diagnostics on ITER measure spectra of light emitted due to plasma interaction with the beams of neutral hydrogen isotopes injected into the plasma. They fall in roughly 3 groups: -Charge Exchange Recombination Spectroscopy (CXRS), analyses line emission of plasma ions that receive electrons from the neutral beam atoms. The intensity, Doppler width and Doppler shift reveal information on the plasma ion density (typically an impurity), the ion temperature and the bulk ion velocity, respectively. -Motional Stark Effect (MSE), analyses Stark split line emission of the beam atoms itself due to collision with the background plasma. The Stark split is a result of the Lorentz electric field experienced by the beam atoms due to their motion across the magnetic field. Both the polarization of and the wavelength separation of the emission lines reveal information about the local magnetic field that provides powerful constraints to the plasma equilibrium reconstruction. -Beam Emission Spectroscopy fluctuations (BES), provides a fast (~ several 100 kHz bandwidth) measurement of the (filtered) intensity of the above mentioned MSE emission. The intensity fluctuations are proportional to electron density fluctuations due to turbulence and MHD modes. On ITER, active beam spectroscopy is faced with several challenges that are less present in current day devices: dominating background emission, partly due to reflection on the metallic walls and potentially polarized, low signal due to strong neutral beam attenuation, deposition of coatings on first mirrors, compliance with nuclear regulations and remote handling, limited access for in-vessel calibration et cetera. Presented in this contribution are an overview of the main challenges and the current status in research, development and design of the ITER active beam spectroscopy systems. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization

Collisionality and safety factor scalings of H-mode energy transport in the MAST spherical tokamak

A factor of 4 dimensionless collisionality scan of H-mode plasmas in MAST shows that the thermal energy confinement time scales as [formula]. Local heat transport is dominated by electrons and is consistent with the global scaling. The neutron rate is in good agreement with the ¿* dependence of tE,th. The gyrokinetic code GYRO indicates that micro-tearing turbulence might explain such a trend. A factor of 1.4 dimensionless safety factor scan shows that the energy confinement time scales as [formula] . These two scalings are consistent with the dependence of energy confinement time on plasma current and magnetic field. Weaker qeng and stronger ¿* dependences compared with the IPB98y2 scaling could be favourable for an ST-CTF device, in that it would allow operation at lower plasma current

OVERVIEW OF CORE DIAGNOSTICS FOR TEXTOR

The diagnostic system of TEXTOR comprises about 50 individual diagnostic devices. Since the start of the Trilateral Euregio Cluster collaboration, part of the emphasis in the experimental program has shifted toward the study of physics processes in the plasma core. To aid these studies several new and advanced core diagnostics have been implemented, whereas a number of other core diagnostics have been upgraded to higher resolution, more channels, and better accuracy. In this paper a brief overview is given of the present set of plasma core diagnostics at TEXTOR.X1114sciescopu

Overview of the JET preparation for deuterium-tritium operation with the ITER like-wall

\u3cp\u3eFor the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D-T mixtures since 1997 and the first ever D-T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D-T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D-T preparation. This intense preparation includes the review of the physics basis for the D-T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D-T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the three-ions scheme), new diagnostics (neutron camera and spectrometer, active Alfven eigenmode antennas, neutral gauges, radiation hard imaging systems...) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D-T campaign provides an incomparable source of information and a basis for the future D-T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas.\u3c/p\u3

14 MeV calibration of JET neutron detectors-phase 1:calibration and characterization of the neutron source

\u3cp\u3eIn view of the planned DT operations at JET, a calibration of the JET neutron monitors at 14 MeV neutron energy is needed using a 14 MeV neutron generator deployed inside the vacuum vessel by the JET remote handling system. The target accuracy of this calibration is 10% as also required by ITER, where a precise neutron yield measurement is important, e.g. for tritium accountancy. To achieve this accuracy, the 14 MeV neutron generator selected as the calibration source has been fully characterised and calibrated prior to the in-vessel calibration of the JET monitors. This paper describes the measurements performed using different types of neutron detectors, spectrometers, calibrated long counters and activation foils which allowed us to obtain the neutron emission rate and the anisotropy of the neutron generator, i.e.The neutron flux and energy spectrum dependence on emission angle, and to derive the absolute emission rate in 4π sr. The use of high resolution diamond spectrometers made it possible to resolve the complex features of the neutron energy spectra resulting from the mixed D/T beam ions reacting with the D/T nuclei present in the neutron generator target. As the neutron generator is not a stable neutron source, several monitoring detectors were attached to it by means of an ad hoc mechanical structure to continuously monitor the neutron emission rate during the in-vessel calibration. These monitoring detectors, two diamond diodes and activation foils, have been calibrated in terms of neutrons/counts within ± 5% total uncertainty. A neutron source routine has been developed, able to produce the neutron spectra resulting from all possible reactions occurring with the D/T ions in the beam impinging on the Ti D/T target. The neutron energy spectra calculated by combining the source routine with a MCNP model of the neutron generator have been validated by the measurements. These numerical tools will be key in analysing the results from the in-vessel calibration and to derive the response of the JET neutron detectors to DT plasma neutrons starting from the response to the generator neutrons, and taking into account all the calibration circumstances.\u3c/p\u3

Efficient generation of energetic ions in multi-ion plasmas by radio-frequency heating

We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed `three-ion' scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of 3He ions to high energies in dedicated hydrogen-deuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast 3He ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3He-rich solar flares

Toroidal plasma rotation induced by the Dynamic Ergodic Divertor in the TEXTOR tokamak

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer