Neutral beam driven hydrogen spectroscopy in fusion plasmas

ITER, the next generation fusion reactor, is currently being constructed with the challenge of demonstrating the technical feasibility of a nuclear fusion driven power plant. In ITER, the generated fusion power will exceed the external heating by a factor 10, generating a large population of fusion-born helium in the plasma that needs to be measured and eventually controlled to prevent fuel dilution. The only method capable of measuring the local density of fully ionized helium in a fusion plasma is charge exchange spectroscopy (CXS), which relies on populating excited levels of plasma ions by charge transfer collisions with hydrogen atoms, injected by a powerful neutral hydrogen beam. The work described in this thesis is focussed on 2 open questions: 'Can the emission by the neutral beam itself be reliably combined with CXS to obtain absolute helium concentrations?' and 'Can CXS be used to measure density profiles of non-thermal ions?'. The quantitative interpretation of beam emission (BES), i.e. relating the observed excited population to the ground state density, has been the subject of investigation by several groups over the last 20 years, leading to different results. In this thesis, collisional-radiative models from different modellers have been compared and inconsistencies, both in the models and in the cross sections that were used, have been solved. The resulting emission rates have been used to compare expected beam densities with measured densities using Balmer-alpha and Balmer-beta beam emission spectra from the JET tokamak. Excellent agreement between the Balmer-alpha and -beta spectra, and reasonable agreement with the expected ground state density was found. The Stark line intensities within the beam emission multiplet have been compared with a newly developed sublevel resolved collisional-radiative model, showing good agreement, and providing an explanation to earlier failed experiments of using the line ratios to obtain information about the magnetic pitch angle. As a side result, beam emission spectroscopy has been used to correct and validate the determination of the fractional energy components in the neutral beams. A setup combining CXS and BES, similar to the one on ITER, has been used on the TEXTOR tokamak to validate the helium concentration measurements during strong helium gas puffs against the increase in electron density. The TEXTOR helium concentration data as well as the beam densities obtained on JET give confidence that the proposed CXS analysis scheme on ITER combining beam emission and charge exchange data will not induce systematic uncertainties larger than 20-30%. Purely statistical errors from fitting the helium and MSE spectrum on ITER are obtained from modelling the spectra and are expected to be lower than 10 over the entire radial range. Measuring fast ions in tokamak plasmas with CXS combines challenges on both the hardware and analysis side to measure and interprete a faint, broad and anisotropic spectrum, polluted by parastic emission and distorted by cross section effects. A high-resolution, high-throughput spectrometer has been commissioned on TEXTOR and the capability to detect the slowing down spectrum of beam injected ions has been demonstrated. The measured spectral shape agrees well with a slowing down model for the beam ions, but strong passive charge exchange emission was detected. Faster detectors and beam modulation is proposed for the TEXTOR diagnostic. Preliminary fast ion density profiles are obtained by sustracting the first frame after the NB switch off, showing a flattening of the core fast ion profile. Fast beam ion spectra have also been obtained on JET, but the signal to noise ratio is rather small due to the limited optical throughput of the diagnostic. Fast helium CX spectra on ITER are modelled and the feasibility to extract the fast ion density profiles is discussed as a function of the optical throughput that could be achieved on the diagnostic. Prospects are limited. Only with coarse wavelength binning and very low contamination with impurity lines could a reasonable signal to noise ratio be obtained

Feasibility of charge exchange spectroscopy fast helium measurements on ITER

\u3cp\u3eThe feasibility to measure fast alpha particles using Active Charge Exchange Recombination Spectroscopy (CXRS) on ITER is investigated. Through modelling of the charge exchange spectral line for fast ions together with the expected background emission, the signal-to-noise ratio has been calculated as a function of the diagnostic design parameters. Combining the CXRS data from both the heating and the diagnostic neutral beams on ITER, information on the fast ion energy spectrum up to 1 MeV can be obtained for the parameters of the ITER core CXRS diagnostic design, provided that the signal is binned in 100 keV bins and a time resolution of Isec is used.\u3c/p\u3

Analysis of ELM stability with extended MHD models in JET, JT-60U and future JT-60SA tokamak plasmas

\u3cp\u3eThe stability with respect to a peelingballooning mode (PBM) was investigated numerically with extended MHD simulation codes in JET, JT-60U and future JT-60SA plasmas. The MINERVA-DI code was used to analyze the linear stability, including the effects of rotation and ion diamagnetic drift (w∗i), in JET-ILW and JT-60SA plasmas, and the JOREK code was used to simulate nonlinear dynamics with rotation, viscosity and resistivity in JT-60U plasmas. It was validated quantitatively that the ELM trigger condition in JET-ILW plasmas can be reasonably explained by taking into account both the rotation and w∗i effects in the numerical analysis. When deuterium poloidal rotation is evaluated based on neoclassical theory, an increase in the effective charge of plasma destabilizes the PBM because of an acceleration of rotation and a decrease in w∗i. The difference in the amount of ELM energy loss in JT-60U plasmas rotating in opposite directions was reproduced qualitatively with JOREK. By comparing the ELM affected areas with linear eigenfunctions, it was confirmed that the difference in the linear stability property, due not to the rotation direction but to the plasma density profile, is thought to be responsible for changing the ELM energy loss just after the ELM crash. A predictive study to determine the pedestal profiles in JT-60SA was performed by updating the EPED1 model to include the rotation and w∗i effects in the PBM stability analysis. It was shown that the plasma rotation predicted with the neoclassical toroidal viscosity degrades the pedestal performance by about 10% by destabilizing the PBM, but the pressure pedestal height will be high enough to achieve the target parameters required for the ITER-like shape inductive scenario in JT-60SA.\u3c/p\u3

Determining the fuel ion ratio for D(T)and T(D) plasmas at JET using neutron time-of-flight spectrometry

The fusion fuel ion ratio, nΤ/nD, is an important plasma parameter that needs to be tuned to maximize the power of a tokamak type fusion reactor. It is recognized as a parameter required for optimizing several ITER operating scenarios, and will likely be continuously monitored in future high-performance fusion devices such as DEMO. Tritium was recently introduced in the Joint European Torus (JET) plasma for the first time since the 1997 DTE1 and 2003 TTE campaigns, enabling the possibility to investigate fuel ion ratios. We present a method for measuring nΤ/nD using neutron time-of-flight (TOF) spectrometry. By fitting the measured neutron spectral features, the relative reaction rate intensities between different ion species can be inferred, from which the fuel ion ratio can be extracted for a corresponding modeled reactivity. Unlike previous measurements of nT/nD using neutron spectrometry, we utilize the neutron energy continuum produced in the three-body TT reaction to determine the fuel ion ratio for plasmas with large concentrations of tritium. Furthermore, the use of neutron TOF spectrometry has never previously been demonstrated for evaluating nT/nD. The method is applied to TOF spectra acquired with TOFOR (JET name KM11) and shown to be consistent with the optical JET diagnostic KT5P which uses optical spectroscopy of a modified Penning gauge plasma to measure tritium and deuterium concentrations in the divertor exhaust gas

RF Sheath-Enhanced Plasma Surface Interaction Studies using Beryllium Optical Emission Spectroscopy in JET ITER-Like Wall

A dedicated study on JET-ILW, deploying two types of ICRH antennas and spectroscopic observation spots at two outboard, beryllium limiters, has provided insight on long-range (up to 6m) RFenhanced plasma-surface interactions (RF-PSI) due to near-antenna electric fields. To aid in the interpretation of optical emission measurements of these effects, the antenna near-fields are computed using the TOPICA code, specifically run for the ITER-like antenna (ILA); similar modelling already existed for the standard JET antennas (A2). In the experiment, both antennas were operated in current drive mode, as RF-PSI tends to be higher in this phasing and at similar power (∼0.5 MW). When sweeping the edge magnetic field pitch angle, peaked RF-PSI effects, in the form of 2-4 fold increase in the local Be source,are consistently measured with the observation spots magnetically connect to regions of TOPICAL-calculated high near-fields, particularly at the near-antenna limiters. It is also found that similar RF-PSI effects are produced by the two types of antenna on similarly distant limiters. Although this mapping of calculated near-fields to enhanced RF-PSI gives only qualitative interpretion of the data, the present dataset is expected to provide a sound experimental basis for emerging RF sheath simulation model validation

Fast-ion redistribution due to sawtooth crash in the TEXTOR tokamak measured by collective Thomson scattering

Here we present collective Thomson scattering measurements of 1D fast-ion velocity distribution functions in neutral beam heated TEXTOR plasmas with sawtooth oscillations. Up to 50% of the fast ions in the centre are redistributed as a consequence of a sawtooth crash. We resolve various directions to the magnetic field. The fast-ion distribution is found to be anisotropic as expected. For a resolved angle of 39 degrees to the magnetic field we find a drop in the fast-ion distribution of 20-40%. For a resolved angle of 83 degrees to the magnetic field the drop is no larger than 20%

Active beam spectroscopy for ITER

Since the first feasibility studies of active beam spectroscopy on ITER in 1995 the proposed diagnostic has developed into a well advanced and mature system. Substantial progress has been achieved on the physics side including comprehensive performance studies based on an advanced predictive code, which simulates active and passive features of the expected spectral ranges. The simulation has enabled detailed specifications for an optimized instrumentation and has helped to specify suitable diagnostic neutral beam parameters.Four ITER partners share presently the task of developing a suite of ITER active beam diagnostics. which make use of the two 0.5 MeV/amu 18 MW heating neutral beams and a dedicated 0.1 MeV/amu, 3.6 MW diagnostic neutral beam. The IN ITER team is responsible for the DNB development and also for beam physics related aspects of the diagnostic. The RF will be responsible for edge CXRS system covering the outer region of the plasma (1 > r/a > 0.4) using an equatorial observation port, and the EU will develop the core CXRS system for the very core (0 < r/a < 0.7) using a top observation port. Thus optimum radial resolution is ensured for each system with better than a/30 resolution. Finally, the US will develop a dedicated MSE system making use of the HNBs and two equatorial ports. With appropriate modification, these systems could also potentially provide information on alpha particle slowing-down features..On the engineering side, comprehensive preparations were made involving the development of an observation periscope, a neutron labyrinth optical system and design studies for remote maintenance including the exchange of the first mirror assembly, a critical issue for the operation of the CXRS diagnostic in the harsh ITER environment.Additionally, an essential change of the orientation of the DNB injection angle and specification of suitable blanket aperture has been made to avoid trapped particle damage to the first wall. (C) 2010 Elsevier B.V. All rights reserved

Development of novel fuel ion ratio diagnostic techniques

To overcome the challenge of measuring the fuel ion ratio in the core (