Interaction of supra-thermal ions with turbulence in a magnetized toroidal plasma

This thesis addresses the interaction of a supra-thermal ion beam with turbulence in the simple magnetized toroidal plasma of TORPEX. The first part of the thesis deals with the ohmic assisted discharges on TORPEX. The aim of these discharges is the investigation of the open to closed magnetic field line transition. The relevant magnetic diagnostics were developed. Ohmic assisted discharges with a maximum plasma current up to 1kA are routinely obtained. The equilibrium conditions on the vacuum magnetic field configuration were investigated. In the second part of the thesis, the design of the fast ion source and detector are discussed. The accelerating electric field needed for the fast ion source was optimized. The fast ion source was constructed and commissioned. To detect the fast ions a specially designed gridded energy analyzer was used. The electron energy distribution function was obtained to demonstrate the efficiency of the detector. The experiments with the fast ion beam were conducted in different plasma regions of TORPEX. In the third part of the thesis numerical simulations are used to interpret the measured fast ion beam behavior. It is shown that a simple single particle equation of motion explains the beam behavior in the experiments in the absence of plasma. To explain the fast ion beam experiments with the plasma a turbulent electric field must be used. The model that takes into account this turbulent electrical field qualitatively explains the shape of the fast ion current density profile in the different plasma regions of TORPEX. The vertically elongated fast ion current density profiles are explained by a spread in the fast ion velocity distribution. The theoretically predicted radial fast ion beam spreading due to the turbulent electric field was observed in the experiment

Sources and detectors of fast ions for basic devices

The physics of supra thermal test ions in turbulent plasmas can be conveniently studied in basic plasma physics devices, which allow high-resolution measurements of plasma and fast ion parameters and wave fields throughout the whole plasma cross-section. We describe recent advances in the development of an experimental setup consisting of a non-perturbative Li 6+ miniaturized ion source and a detector for the investigation of the interaction between supra thermal ions and drift/interchange–driven turbulence on the TORPEX device. We present first measurements of the spatial and energy space distribution of the fast ion beam in different plasma scenarios, in which the plasma turbulence is fully characterized. We also discuss the possibility of using the fast ion source in basic plasma devices for fusion. This work is partly funded by the Fonds National Suisse de la Recherche Scientifique

Helicon wave-generated plasmas for negative ion beams for fusion

In the next generation of fusion reactors, such as DEMO, neutral beam injectors (NBIs) of high energy (0.8-1 MeV) deuterium atom swith high wall-plug efficiency (>50%) will be required to reach burning plasma conditions and to provide a significant amount of current drive.The present NBI system for DEMO assumes that 50 MW is delivered to the plasma by 3 NBIs. In the Siphore NBI concept, negative deuterium ions are extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center, a novel radio frequency helicon plasma source, based on a resonant network antenna source delivering up to 10 kW at 13.56 MHz, has been developed and is presently under study on the Resonant Antenna Ion Device (RAID). RAID is a linear device (1.9 m total length, 0.4 m diameter) and is equipped with an extensive set of diagnostics for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources areintroduced. Wepresent absolute spectroscopy, Langmuir probe, and interferometry measurements on helicon plasmas. We characterize the performanceof the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power. Furthermore, first results with the helicon birdcage antenna installed on the Cybele negative ion source at CEA-IRFM arepresented, as a first step towards the validation of the Siphoreconcept

A Rubidium Vapor Source for a Plasma Source for AWAKE

We present the scheme for a rubidium vapor source that is used as a plasma source in the AWAKE plasma wakefield acceleration experiment. The plasma wakefield acceleration process requires a number of stringent parameters for the plasma: electron density adjustable in the (1-10)$\times$10$^{14}$cm$^{-3}$ range, 0.25% relative density uniformity, sharp ($<$10cm) density ramps at each end, density gradient adjustable from -3 to +10% over 10m, and %-level density step near the beginning the plasma column. We show with analytical and direct Simulation Monte Carlo results that the rubidium density in the proposed source should meet these requirements. Laser ionization then transfers the above neutral vapor parameters to the plasma

Beam-Plasma Interaction Simulations for the AWAKE Experiment at CERN

The AWAKE experiment at CERN will be the first proof-of-principle demonstration of the proton-driven plasma wakefield acceleration using the 400 GeV proton beam extracted from the SPS accelerator. The plasma wakefield will be driven by a sequence of sub-millimeter long micro-bunches produced as a result of the self-modulation instability (SMI) of the 12 cm long SPS proton bunch in the 10 m long rubidium plasma with a density corresponding to the plasma wavelength of around 1 mm. A 16 MeV electron beam will be injected into the developing SMI and used to probe the plasma wakefields. The proton beam self-modulation in a wide range of plasma densities and gradients have been studied in detail via numerical simulations. A new configuration of the AWAKE experiment with a small plasma density step is proposed

A study on heat flux predictions for re-entry flight analysis

The analysis of the risk from re-entry objects has become an important topic. Concerning the uncertainties and different flow regimes, the object-oriented tools have been developed and used for various re-entry mission scenarios because of the simplified computation process and low calculation burden, which allow a probabilistic analysis. In this study, the heat flux correlations that are used in the object-oriented tools are investigated and compared to better understand the heat flux predictions with respect to re-entry survivability. Often these tools calculate the continuum stagnation-point heat flux using the correlation formulae of Lees and Fay-Riddell, which usually assume local thermo-chemical equilibrium with a fully-catalytic wall condition in the shock layer. Based on this observation, heat flux measurements were conducted in a hypersonic wind tunnel, and validated with the theory of Fay-Riddell considering the equivalent velocity gradient. For the comparison, in total, 11 different heat flux correlations were examined. It is shown that, based on the formula of Fay-Riddell, the differences in the final integrated heat flux were less than 16% which led to a large discrepancy in the survivability estimations for the cases of small spheres made of aluminum. This shows the importance of considering the heat flux correlation as well as the velocity gradient effect for such re-entry analysis

Ion heating and flows in a high power helicon source

We report experimental measurements of ion temperatures and flows in a high power, linear, magnetized, helicon plasma device, the Resonant Antenna Ion Device (RAID). Parallel and perpendicular ion temperatures on the order of 0.6 eV are observed for an rf power of 4 kW, suggesting that higher power helicon sources should attain ion temperatures in excess of 1 eV. The unique RAID antenna design produces broad, uniform plasma density and perpendicular ion temperature radial profiles. Measurements of the azimuthal flow indicate rigid body rotation of the plasma column of a few kHz. When configured with an expanding magnetic field, modest parallel ion flows are observed in the expansion region. The ion flows and temperatures are derived from laser induced fluorescence measurements of the Doppler resolved velocity distribution functions of argon ions. Published by AIP Publishing

Helicon wave-generated plasmas for negative ion beams for fusion

In the next generation of fusion reactors, such as DEMO, neutral beam injectors (NBIs) of high energy (0.8-1 MeV) deuterium atoms with high wall-plug efficiency (>50%) will be required to reach burning plasma conditions and to provide a significant amount of current drive. The present NBI system for DEMO assumes that 50 MW is delivered to the plasma by 3 NBIs. In the Siphore NBI concept, negative deuterium ions are extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center, a novel radio frequency helicon plasma source, based on a resonant network antenna source delivering up to 10 kW at 13.56 MHz, has been developed and is presently under study on the Resonant Antenna Ion Device (RAID). RAID is a linear device (1.9 m total length, 0.4 m diameter) and is equipped with an extensive set of diagnostics for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources are introduced. We present absolute spectroscopy, Langmuir probe, and interferometry measurements on helicon plasmas. We characterize the performance of the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power. Furthermore, first results with the helicon birdcage antenna installed on the Cybele negative ion source at CEA-IRFM are presented, as a first step towards the validation of the Siphore concept

First B-dot measurements in the RAID device, an alternative negative ion source for DEMO neutral beams

International audienceAs a new concept of Neutral Beam Injectors (NBI) for DEMO-like reactors, SIPHORE (IRFM, CEA, in France) expects to extract negative deuterium ions and photo-neutralize the accelerated D-. The Swiss Plasma Center (SPC) of EPFL is involved in this project by developing an innovative helicon source, which could provide the adequate D 2 negative ion blade-shaped plasma, in terms of density and homogeneity along the axial direction. In the Resonant Antenna Ion Device (RAID), the test bed, a helicon wave is sustained by a resonant antenna plasma source at 13.56 MHz (input power ≤ 10 kW), connected to a cylindrical vacuum chamber (1.5 m long, 0.4 m diameter) and is surrounded by 6 Helmholtz coils, providing a DC magnetic field up to 800 G on axis. To characterize the helicon wave propagation, RAID has been recently equipped with a three-axis magnetic probe (B-dot). The paper describes the RAID experiment and its helicon source, including a 3D characterization of density and temperature, together with the B-dot design and calibration. It presents measurements of helicon wave propagation; in typical H 2 plasmas (0.3 Pa), preliminary results show a helicon wave right-handed polarized with a wavelength of approximately 240 mm