F. Meo

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.

Status of the new multi-frequency ECRH system for ASDEX Upgrade

Summary form only given. The first two-frequency GYCOM gyrotron Odissey-1 has been installed and put into operation in the new multi-frequency ECRH system at the ASDEX Upgrade tokamak experiment. It works at 105 GHz and 140GHz with output power 610kW and 820kW respectively at a pulse length of 10s. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140GHz. Such gyrotrons will require broadband vacuum windows. Construction and cold tests of a first broadband double-disc toms window are completed. The transmission to the tonis is in normal air, through corrugated aluminum waveguides with I.D.=87mm over a total length of about 70m. Calorimetric measurements gave a total transmission loss of only 12% at 105GHz and 10% at 140GHz. The variable frequency will significantly extend the operating range of the ECRH system, e.g. allow for central heating at different magnetic fields. Other experimental features, like the suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. The stabilization of NTMs requires a very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this reason fast movable launchers have been installed.

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%.

Design of the collective Thomson scattering diagnostic for International Thermonuclear Experimental Reactor at the 60 GHz frequency range

The physics feasibility study [H. Bindslev et al., ITER Report Contract No. EFDA 01.654, 2003, www.risoe.dk/euratom/CTS/ITER] concludes that the frequency option below the electron cyclotron resonance was the only system capable of meeting the International Thermonuclear Experimental Reactor (ITER) measurement requirements for the fusion alphas, with present or near term technology. This article presents the design of the collective Thomson scattering diagnostic for ITER at the 60 GHz range. The system is capable of measuring the fast ion distribution parallel and perpendicular to the magnetic field at different radial locations simultaneously. The design is robust technologically with no moveable components near the plasma. The article includes the upgrade requirements to provide temporally and spatially resolved measurements of the fuel ion ratio.

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.

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.

Design and performance of the collective Thomson scattering receiver at ASDEX Upgrade.

Here we present the design of the fast-ion collective Thomson scattering receiver for millimeter wave radiation installed at ASDEX Upgrade, a tokamak for fusion plasma experiments. The receiver can detect spectral power densities of a few eV against the electron cyclotron emission background on the order of 100 eV under presence of gyrotron stray radiation that is several orders of magnitude stronger than the signal to be detected. The receiver down converts the frequencies of scattered radiation (100-110 GHz) to intermediate frequencies (IF) (4.5-14.5 GHz) by heterodyning. The IF signal is divided into 50 IF channels tightly spaced in frequency space. The channels are terminated by square-law detector diodes that convert the signal power into DC voltages. We present measurements of the transmission characteristics and performance of the main receiver components operating at mm-wave frequencies (notch, bandpass, and lowpass filters, a voltage-controlled variable attenuator, and an isolator), the down-converter unit, and the IF components (amplifiers, bandpass filters, and detector diodes). Furthermore, we determine the performance of the receiver as a unit through spectral response measurements and find reasonable agreement with the expectation based on the individual component measurements.

Collective Thomson scattering of a high power electron cyclotron resonance heating beam in LHD (invited)

Collective Thomson scattering (CTS) system has been constructed at LHD making use of the high power electron cyclotron resonance heating (ECRH) system in Large Helical Device (LHD). The necessary features for CTS, high power probing beams and receiving beams, both with well defined Gaussian profile and with the fine controllability, are endowed in the ECRH system. The 32 channel radiometer with sharp notch filter at the front end is attached to the ECRH system transmission line as a CTS receiver. The validation of the CTS signal is performed by scanning the scattering volume. A new method to separate the CTS signal from background electron cyclotron emission is developed and applied to derive the bulk and high energy ion components for several combinations of neutral beam heated plasmas.

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

The collective Thomson scattering (CTS) diagnostic installed on ASDEX Upgrade uses millimeter waves generated by the newly installed 1 MW dual frequency gyrotron as probing radiation at 105 GHz. It measures backscattered radiation with a heterodyne receiver having 50 channels (between 100 and 110 GHz) to resolve the one-dimensional velocity distribution of the confined fast ions. The steerable antennas will allow different scattering geometries to fully explore the anisotropic fast ion distributions at different spatial locations. This paper covers the capabilities and operational limits of the diagnostic. It then describes the commissioning activities carried out to date. These activities include gyrotron studies, transmission line alignment, and beam pattern measurements in the vacuum vessel. Overlap experiments in near perpendicular and near parallel have confirmed the successful alignment of the system. First results in near perpendicular of scattered spectra in a neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) plasma (minority hydrogen) on ASDEX Upgrade have shown evidence of ICRH heating phase of hydrogen.