The influence of electric fields and neutral particles on the plasma sheath at ITER divertor conditions

The purpose of this thesis is to support the optimization of the ‘exhaust-pipe’, or so-called ‘divertor’, of the nuclear fusion experiment ITER, a large international fusion reactor now under construction in the south of France. We focus particularly on two ‘tools’ for optimization of the plasma conditions in the divertor: electric fields and neutral particles. We look at how these ‘tools’ affect the plasma conditions at divertor surfaces. These conditions determine the type and rates of plasma-surface interaction processes and ultimately the lifetime of these surface materials. A plasma boundary phenomenon that can be changed by the presence of electric fields is the so-called ‘Debye sheath’. This is a voltage drop in the transition between plasma and surface. Extremely localized, it will extend only a few micrometers from the ITER divertor plates into the plasma. However, its voltage is a crucial parameter for the interaction of plasma with these plates, since it determines the impinging ion energy. The change in sheath voltage may be particularly large in ITER where conditions are conducive to the development of large electric fields. This is partly due to the low electron temperature, such that electrical resistivity is high. It is also partly due to the high ion fluxes, which allow large currents to flow since electric currents through the divertor plates are limited by the ion flux. We will see that neutral particles will also influence the boundary conditions in the ITER divertor. Their influence is important, because in contrast to existing tokamak divertors, the ion flux will be so high that the plasma will not be transparent for neutral atoms. Atoms will exchange energy and momentum with plasma particles. Clearly, experiments are required to study the consequences of neutral particles and also electric fields on divertor boundary conditions in the plasma conditions foreseen for ITER. To perform these experiments systematically, we created the projected ITER divertor plasma conditions as closely as possible in a linear laboratory experiment, Pilot-PSI. Not only is this linear experiment unique in its production of the particle and heat fluxes expected in the ITER divertor, it is also able to produce parameters corresponding to the whole range of present day tokamak divertors. As a linear machine, it has large advantages over tokamak divertor experiments. The diagnostic accessibility is significantly improved and plasma parameters can be controlled much more directly. We began the project with the development of two non-intrusive diagnostic techniques for the study of electric fields and atomic neutral density in the Pilot-PSI beam. The first diagnostic developed uses optical emission spectroscopy to probe radial electric fields via the E X B ion rotation drift. Although these rotating ions do not emit radiation, we could estimate their drift velocity by observing radiation from excited neutral atoms. These atoms are coupled to both the hot, rotating ions as well as to cold non-rotating neutrals. A procedure was developed to obtain the ion rotation velocity as well as the ion temperature from measured spectra. Radial electric field strengths could then be deduced. We measured electric field strengths in Pilot-PSI of up to 16 kV/m. Secondly, we needed measurements of the atomic neutral density. Laser induced fluorescence (LIF) is generally well-suited for this purpose, since it probes ground state atomic densities directly and with high spatial resolution. However we found, at the high electron density conditions in Pilot-PSI, the fluorescence signal to be severely limited in strength and the background emission signal to be large. Sensitive LIF measurements were not possible. Absorption spectroscopy provided a good alternative. With this diagnostic we determined an upper limit on the atomic density in the centre of the beam, from which we could calculate the ionization degree (> 85% near the plasma source).We also found that as the electron density in Pilot-PSI was increased to ITER relevant values, there was a strong rise in the neutral atomic density in the beam and also in the proportion of molecules in the vessel that were strongly rovibrationally excited. Electric fields and ion temperatures could also be determined, and were in line with values from optical emission spectroscopy. Finally, we also obtained estimations for the dissociation degree in the vessel (~ 7%) and the proportion of rovibrationally excited molecules entering the plasma beam (~ 30%). The next step was to learn to understand and manipulate the radial electric fields in the beam of Pilot-PSI. We found that the radial electric field at the plasma source exit increased with nozzle diameter of the source and with magnetic field strength. The electric fields (and associated electric current) were found to penetrate into the beam outside of the plasma source with a characteristic length increasing with magnetization of the beam. We could then imitate the situation in a tokamak where electric fields in the plasma interface with electrically conducting divertor surfaces. We experimentally verified that electric current will flow through these conducting surfaces. Furthermore, we confirmed that the local sheath voltage can increase substantially from its typical value without biasing, 2.5kTe/e up to the total voltage difference applied. The sheath voltage increases at positions for which the current into the target is positive. Since the sheath voltage determines ion energies at the target, this may have negative consequences for the lifetime of divertor materials. Especially when there are heavy impurities present in the divertor, the threshold energy for physical sputtering may be surpassed. Experiments confirmed that sheath voltage increase at a floating target (for which the electrical potential is free to change) is avoided if an insulator inhibits surface current. We conclude that material damage reduction can be obtained by placing insulating inserts between electrically floating divertor plates. Finally, we addressed the issue of heating by neutral particles that are reflected from the divertor plates back into the plasma, carrying energy from the sheath. This heating effect will be important in ITER because of the strong ion-neutral coupling projected. We studied its effect in Pilot-PSI, where we amplified its impact by increasing the sheath voltage with target biasing. The result was an increase in the electron temperature measured close to the target. Also, the electron density was observed to decrease while ion flux to the target remained constant. Since electron and ion densities are equal in a quasineutral plasma, this implies rarefication caused by plasma acceleration. We attribute this acceleration to the Bohm criterion, which states that the plasma must accelerate to at least the sound velocity at the sheath edge. Since an increased temperature corresponds to an increase in the sound velocity, extra acceleration close to the target must result. These results are significant because they show that neutral atoms reflected from divertor plates in ITER will have a significant influence on the plasma boundary conditions. This will affect the rates of a range of processes at the plasma-wall interface. One important example is the redeposition rate of eroded divertor plate material. The observed effects are particularly striking when sheath voltages are enhanced either by electric fields in the plasma or by negative plate biasing, but will also play a role when divertor plates are floating or grounded. In conclusion, this thesis presents an experimental study of the influence of electric fields and neutral particles on the plasma conditions close to tokamak divertor plates. Since diagnostic access to tokamak divertors is limited and measurements of densities, temperatures, velocities and ion energies are minimal, good care should be taken in predicting values for these parameters. The predicted effects will be particularly strong at ITER divertor relevant conditions, where electric fields can be large and ion-neutral coupling strong

Rotation of a strongly magnetized hydrogen plasma column determined from an asymmetric Balmer-beta spectral line with two radiating distributions

A potential buildup in front of a magnetized cascaded arc hydrogen plasma source is explored via .vector.E * .vector.B rotation and plate potential measurements. Plasma rotation approaches thermal speeds with max. velocities of 10 km/s. The diagnostic for plasma rotation is optical emission spectroscopy on the Balmer-beta line. Asym. spectra are obsd. A detailed consideration is given on the interpretation of such spectra with a two distribution model. This consideration includes radial dependence of emission detd. by Abel inversion of the lateral intensity profile. Spectrum anal. is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting. [on SciFinder (R)

Chemical erosion of different carbon composites under ITER-relevant plasma conditions

\u3cp\u3eWe have studied the chemical erosion of different carbon composites in Pilot-PSI at ITER-relevant hydrogen plasma fluxes (∼10\u3csup\u3e24\u3c/sup\u3e m \u3csup\u3e-2\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e) and low electron temperatures (T\u3csub\u3ee\u3c/sub\u3e∼1 eV). Optical emission spectroscopy on the CH A-X band was used to characterize the chemical sputtering. Fine grain graphite (R 6650, SGL Carbon Group), ITER-reference carbon fiber composite material (SNECMA NB31 and NB41; Dunlop 3D), nano- and micro-crystalline diamond coatings on molybdenum and SiC (Silit® SKD Reaction-Bonded, Saint-Gobain Ceramics) were compared. The chemical sputtering was similar for the different composites under comparable plasma conditions, except for SiC, which produced a ten times lower rate. The CH emission was constant at electron temperatures T\u3csub\u3ee\u3c/sub\u3e>1 eV and ion fluxes ranging between 10\u3csup\u3e23\u3c/sup\u3e and 10\u3csup\u3e24\u3c/sup\u3e m\u3csup\u3e- 2\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e, but decreased at lower temperatures. This decrease is possibly due to changes in the excitation of CH and not due to a change in the chemical erosion rate.\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

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

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