Poul Michelsen

Observations of Solitary Structures in a Magnetized, Plasma Loaded Waveguide

Two types of solitary structure were investigated experimentally and numerically in a magnetized, plasma-loaded waveguide. One was identified as an ordinary KdV soliton and its properties were investigated with particular attention to the damping by resonant particles. The other type of pulse was identified as a purely kinetic phenomenon being associated with a vortex in phase space.

Double layer dynamics in a collisionless magnetoplasma

Investigations of double layer dynamics are performed in a Q-machine plasma by applying a positive step potential to a cold end-plate collector. The double layer created at the grounded plasma source just after the pulse is applied propagates towards the collector with the plasma flow speed. Large oscillations occur in the plasma current which is related to a recurring formation and propagation of the double layer. The current is limited during the propagation by a growing negative potential dip formed on the low-potential tail. Similar phenomena appear on the low-potential tail of the stationary double layer formed by applying a potential difference between two plasma sources.

On velocity space interrogation regions of fast-ion collective Thomson scattering at ITER

The collective Thomson scattering (CTS) diagnostic proposed for ITER is designed to measure projected 1D fast-ion velocity distribution functions at several spatial locations simultaneously. The frequency shift of scattered radiation and the scattering geometry place fast ions that caused the collective scattering in well-defined regions in velocity space, here dubbed interrogation regions. Since the CTS instrument measures entire spectra of scattered radiation, many different interrogation regions are probed simultaneously. We here give analytic expressions for weight functions describing the interrogation regions, and we show typical interrogation regions of the proposed ITER CTS system. The backscattering system with receivers on the low-field side is sensitive to fast ions with pitch |p| = |v∥/v| 0.6–0.8. Additionally, we use weight functions to reconstruct 2D fast-ion distribution functions, given two projected 1D velocity distribution functions from simulated simultaneous measurements with the back- and forward scattering systems.

Comparison of collective Thomson scattering signals due to fast ions in ITER scenarios with fusion and auxiliary heating

Auxiliary heating such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) will accelerate ions in ITER up to energies in the MeV range, i.e. energies which are also typical for alpha particles. Fast ions of any of these populations will elevate the collective Thomson scattering (CTS) signal for the proposed CTS diagnostic in ITER. It is of interest to determine the contributions of these fast ion populations to the CTS signal for large Doppler shifts of the scattered radiation since conclusions can mostly be drawn for the dominant contributor. In this study, distribution functions of fast ions generated by NBI and ICRH are calculated for a steady-state ITER burning plasma equilibrium with the ASCOT and PION codes, respectively. The parameters for the auxiliary heating systems correspond to the design currently foreseen for ITER. The geometry of the CTS system for ITER is chosen such that near perpendicular and near parallel velocity components are resolved. In the investigated ICRH scenario, waves at 50 MHz resonate with tritium at the second harmonic off-axis on the low field side. Effects of a minority heating scheme with 3He are also considered. CTS scattering functions for fast deuterons, fast tritons, fast 3He and the fusion born alphas are presented, revealing that fusion alphas dominate the measurable signal by an order of magnitude or more in the Doppler shift frequency ranges typical for fast ions. Hence the observable CTS signal can mostly be attributed to the alpha population in these frequency ranges. The exceptions are limited regions in space with some non-negligible signal due to beam ions or fast 3He which give rise to about 30% and 10–20% of the CTS signal, respectively. In turn, the dominance of the alpha contribution implies that the effects of other fast ion contributions will be difficult to observe by CTS.

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? to the magnetic field we find a drop in the fast-ion distribution of 20?40%. For a resolved angle of 83? to the magnetic field the drop is no larger than 20%.

Unstable electrostatic ion cyclotron waves excited by an ion beam

Electrostatic ion cyclotron waves were observed in a quiescent cesium plasma into which a low‐energy beam of sodium ions was injected. The instability appeared when the beam velocity was above 12 times the ion thermal velocity. The waves propagated along the magnetic field with a velocity somewhat smaller than that of the beam. The dispersion relation was in good agreement with theory.

Dynamics of fast ions during sawtooth oscillations in the TEXTOR tokamak measured by collective Thomson scattering

Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering on TEXTOR are presented. Time-resolved measurements of localized 1D fast-ion distribution functions allow us to study fast-ion dynamics during several sawtooth cycles. Sawtooth oscillations interact strongly with the fast-ion population in a wide range of plasma parameters. Part of the ion phase space density oscillates out of phase with the sawtooth oscillation during hydrogen neutral beam injection (NBI). These oscillations most likely originate from fast hydrogen ions with energies close to the full injection energy. At lower energies passing fast ions in the plasma centre are strongly redistributed at the time of sawtooth collapse but no redistribution of trapped fast ions is observed. The redistribution of fast ions from deuterium NBI in the plasma centre is found to vary throughout velocity space. The reduction is most pronounced for passing ions. We find no evidence of inverted sawteeth outside the sawtooth inversion surface in the fast-ion distribution function.

The current-driven, ion-acoustic instability in a collisionless plasma

The current-driven, ion-acoustic instability was investigated by means of an experiment performed in a collisionless plasma produced in a single-ended Q-machine. Reflections at the ends of the plasma column gave rise to a standing wave. Parameters of the instability were investigated, and it was demonstrated that the fluctuations in the plasma column behave as a classical Van der Pol oscillator. Accurate measurements of the growth rate of the instability can be performed by making explicit use of the particular properties of such a system.

Tomography of fast-ion velocity-space distributions from synthetic CTS and FIDA measurements

We compute tomographies of 2D fast-ion velocity distribution functions from synthetic collective Thomson scattering (CTS) and fast-ion Dα (FIDA) 1D measurements using a new reconstruction prescription. Contradicting conventional wisdom we demonstrate that one single 1D CTS or FIDA view suffices to compute accurate tomographies of arbitrary 2D functions under idealized conditions. Under simulated experimental conditions, single-view tomographies do not resemble the original fast-ion velocity distribution functions but nevertheless show their coarsest features. For CTS or FIDA systems with many simultaneous views on the same measurement volume, the resemblance improves with the number of available views, even if the resolution in each view is varied inversely proportional to the number of views, so that the total number of measurements in all views is the same. With a realistic four-view system, tomographies of a beam ion velocity distribution function at ASDEX Upgrade reproduce the general shape of the function and the location of the maxima at full and half injection energy of the beam ions. By applying our method to real many-view CTS or FIDA measurements, one could determine tomographies of 2D fast-ion velocity distribution functions experimentally.

A method for measuring fast time evolutions of the plasma potential by means of a simple emissive probe

A method is presented for obtaining the temporal evolution of the plasma potential, which is assumed to be given by the floating potential of a simple emissive probe. The construction of the probe is also described. The method avoids the slow time response of the usual technique where the floating potential is measured across a high resistance. During each sweep of the probe voltage, the changing of the sign of the probe current, which is sampled at a specific time, gives rise to a negative pulse, driving the pen-lift of an X-Y recorder. Since the real floating potential is measured where the probe current is zero, the disturbance of the plasma is kept as low as possible.