D. Wagner

Comparison of fast ion collective Thomson scattering measurements at ASDEX Upgrade with numerical simulations

Collective Thomson scattering (CTS) experiments were carried out at ASDEX Upgrade to measure the one-dimensional velocity distribution functions of fast ion populations. These measurements are compared with simulations using the codes TRANSP/NUBEAM and ASCOT for two different neutral beam injection (NBI) configurations: two NBI sources and only one NBI source. The measured CTS spectra as well as the inferred one-dimensional fast ion velocity distribution functions are clearly asymmetric as a consequence of the anisotropy of the beam ion populations and the selected geometry of the experiment. As expected, the one-beam configuration can clearly be distinguished from the two-beam configuration. The fast ion population is smaller and the asymmetry is less pronounced for the one-beam configuration. Salient features of the numerical simulation results agree with the CTS measurements while quantitative discrepancies in absolute values and gradients are found.

A high-efficiency quasi-optical mode converter for a 140-GHz 1-MW CW gyrotron

A highly efficient quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK). The converter consists of a dimpled-wall waveguide launcher, one quasi-elliptical mirror and two toroidal mirrors. The coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher; the radiated fields from the cut of the launcher have been calculated by the scalar diffraction integral. Simulation results show that the advanced dimpled-wall launcher generates a well-focused Gaussian radiation pattern with low diffraction losses. In this case, toroidal mirrors are sufficient to obtain a desired output beam pattern. An efficiency of more than 98% has been achieved to convert the rotating TE/sub 28,8/ cavity mode at 140 GHz into a fundamental Gaussian beam. Experimental measurements show close agreement with theoretical predictions.

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.

Possibilities for multifrequency operation of a gyrotron at FZK

We investigate the possibility of multifrequency operation of a 140-GHz gyrotron, which is designed to operate in the TE/sub 22,8/ mode at 140 GHz and the TE/sub 19,6/ mode at 111 GHz or the TE/sub 17,6/ mode at 105 GHz, for which existing equipment can be used. The present calculations compute beam properties for a given set of coil currents, accelerating voltage and current, and then use these beam properties to compute the output power and efficiency. These calculations are performed separately for each mode.

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.

Magnetic island localization for NTM control by ECE viewed along the same optical path of the ECCD beam

Abstract Neoclassical tearing modes (NTMs) deteriorate high-pressure tokamak plasma confinement and can be suppressed by electron cyclotron current drive (ECCD). In order to obtain efficient suppression, the ECCD power needs to be deposited at the center of an NTM magnetic island. To enhance efficiency, this power also needs to be synchronized in phase with the rotation of the island. The problem is that of real-time detection and precise localization of the island(s) in order to provide the feedback signal required to control the ECCD power deposition area with an accuracy of 1 to 2 cm. Existing schemes based on mode location, equilibrium reconstruction, and plasma profile measurements are limited in positional and temporal accuracy and moreover will become very complex when applied to ITER. To overcome these limitations, it is proposed to provide the feedback signal from electron cyclotron emission (ECE) measurements taken along the identical line of sight as traced by the incident ECCD millimeter-wave beam but in reverse direction. Experiments on TEXTOR have demonstrated a proof of principle. These measurements motivate the further development and the implementation of such an ECCD-aligned ECE system for NTM control in larger fusion machines. Possible implementation of such a system on ASDEX-Upgrade, based on waveguides equipped with a fast directional switch, is presented in this paper. Possible further development for ITER is also discussed.

First Operation of a Step-Frequency Tunable 1-MW Gyrotron With a Diamond Brewster Angle Output Window

Experimental results using a step-frequency tunable D-band gyrotron are reported. The short pulse (~3 ms) gyrotron is equipped with an elliptically brazed chemical vapor deposition (CVD) diamond Brewster angle output window. It is designed for the operation in the frequency range from 111.6 up to 165.7 GHz. Operating parameters for ten different frequencies corresponding to an equal number of different cavity operating modes has been measured. A minimum output power of 830 kW and a peak output power of 1.3 MW have been realized. For all frequencies, the parameters of the RF beam generated by the internal quasioptical converter, such as fundamental Gaussian contents and beam waist, are sufficiently good to allow an efficient coupling of the RF power out of the window. This is the first time a diamond Brewster angle window has been used in a high power gyrotron (~1 MW). Such a system offers the path to a simple and compact window solution for high power broadband applications using gyrotrons.

Selective excitation of high-order modes in circular waveguides

The excitation of very high-order modes in circular waveguides has been performed in a cavity with a connected up-taper with a geometry similar to those used in gyrotrons. A Gaussian beam was coupled to the cavity which was made translucent by an array of holes. With the help of a special optics, the amplitude as well as the phase distribution of the beam was matched to the mode to be excited in the resonant cavity. By simple rotation of one mirror to adjust the phase distribution together with the change of frequency to match the resonance condition, a large number of modes could be produced with one experimental set-up. Field measurements in the output waveguide show a high mode purity of the radiation and confirm the calculations. The method can be used for cold tests of electrodynamic systems operating with these modes, e.g. quasi-optical converters for gyrotrons.

Plans for a new ECRH system at ASDEX Upgrade

Abstract A new ECRH system is being constructed for ASDEX Upgrade with a total power of 4 MW, generated by four gyrotrons, and a pulse duration of 10 s. Particular features are the use of gyrotrons which can work at various frequencies in the range 104–140 GHz and correspondingly broad band transmission components. The transmission will be at normal air pressure, and at the torus we will have a tunable double disk vacuum window. A further aim is the installation of fast moveable mirrors for a feedback controlled localized power deposition.

First results and analysis of collective Thomson scattering (CTS) fast ion distribution measurements on ASDEX Upgrade

Experimental knowledge of the fast ion physics in magnetically confined plasmas is essential. The collective Thomson scattering (CTS) diagnostic is capable of measuring localized 1D ion velocity distributions and anisotropies dependent on the angle to the magnetic field. The CTS installed at ASDEX-Upgrade (AUG) uses mm-waves generated by the 1 MW dual frequency gyrotron. The successful commissioning the CTS at AUG enabled first scattering experiments and the consequent milestone of first fast ion distribution measurements on AUG presented in this paper. The first fast ion distribution results have already uncovered some physics of confined fast ions at the plasma centre with off-axis neutral beam heating. However, CTS experiments on AUG H-mode plasmas have also uncovered some unexpected signals not related to scattering that required additional analysis and treatment of the data. These secondary emission signals are generated from the plasma-gyrotron interaction therefore contain additional physics. Despite their existence that complicate the fast ion analysis, they do not prevent the diagnostics capability to infer the fast ion distribution function on AUG.