J. S. Machuzak

Gyrotron collective Thomson scattering diagnostic for confined alpha particles in TFTR

A 200‐kW, ∼60‐GHz gyrotron will be used on TFTR during D–T operation for collective Thomson scattering diagnostics of confined alpha‐particle velocity distribution and density. Scattering angles of up to 120° are under consideration using x‐mode propagation to increase the density cutoff limit. Useful signal‐to‐noise ratios will be possible in plasmas where ne≳5×1013 cm−3, nα≳2×1011 cm−3, and the background emission is ≲30 eV equivalent blackbody level.

137‐GHz gyrotron diagnostic for instability studies in Tara

A narrow linewidth ( 97% TE11 mode output) have been built for collective Thomson scattering diagnostics. The main goal will be to study instability driven ion density fluctuations in the Tara plug such as the drift cyclotron loss cone (DCLC), the axial loss cone (ALC), harmonics of the DCLC and ALC, and the ion two‐stream instability. The heterodyne receiver and signal optics have been installed on Tara. Background electron cyclotron emission (ECE) at 139±1.5 GHz after electron cyclotron resonance heating (ECRH) in the Tara plug corresponded to equivalent blackbody temperatures of 453 and 70 eV for extraordinary and ordinary emission, respectively. The well‐collimated receiver field of view completely through the Tara plug has allowed for excellent polarization discrimination of the ECE. The high‐power capability of this gyrotron will allow weak fluctuation levels (n/n<10−6) to be detected above thi...

Implementation of collective Thomson scattering on the TEXTOR tokamak for energetic ion measurements

Knowledge of the energy spectrum of fast ions in magnetically confined fusion plasmas is important and fundamental for achieving fusion energy production. Collective Thomson scattering (CTS) of high-power electromagnetic radiation is one of the most promising diagnostic methods for measuring the energy spectrum of confined fast ions. A CTS diagnostic has been implemented on the TEXTOR tokamak for the study of fast ions. It uses a 400 kW, 110 GHz gyrotron source as the probe and a high-resolution heterodyne radiometer as the receiver. Details of the system are given in this article. First results from the diagnostic show that scattered radiation has been detected.

Fast ion collective Thomson scattering, JET results and TEXTOR plans

Diagnosis of MeV range fast ions by collective Thomson scattering (CTS) of millimetre waves was successfully demonstrated at JET. Building on the experiences from JET, a new millimetre wave based CTS fast ion diagnostic is being built for the TEXTOR tokamak. With this we intend to address both generic issues of energetic ion dynamics and specific issues for ion cyclotron resonance heating.

First results of collective scattering on JET (invited)

The fast ion and α-particle diagnostic at JET is based on collective Thomson scattering of high power millimeter-wave radiation. The principal aim of the diagnostic will be the measurement of the spatially resolved velocity distribution of fast α particles when tritium is introduced in JET plasmas, although several other applications are foreseen. The diagnostic uses a 140 GHz, 500 kW, gyrotron as the source of probing radiation and a heterodyne detection system. The diagnostic came into operation during the last JET operational campaign. First measurements were made of the thermal and mildly suprathermal (induced by ion cyclotron resonance heating) ion feature. The results confirmed expectations, indicating that the diagnostic should give the predicted performance for observation of α-particle populations in the DT phase. The signal-to-noise ratio is limited by the noise on the background radiation (electron cyclotron emission), which has a minimum around 140 GHz only when JET is operated at B∼3.4 T. To ...

Results from the low-power 60 GHz gyrotron collective Thomson scattering diagnostic on TFTR

A low-power 60 GHz gyrotron collective Thomson scattering diagnostic has been operating on TFTR to test the feasibility of detecting alpha particles when scattering perpendicular to the magnetic field. An enhanced scattered signal is predicted to result from the interaction of the energetic ions with plasma resonances in the lower hybrid frequency range. Millimeter-wave power levels at the plasma were approximately 200 W for typical pulse lengths of 50 ms. Deuterium and possible fusion product ion cyclotron frequencies and their harmonics were observed during neutral beam heating. These spectra are similar to ion cyclotron emission spectra which are detected with radio-frequency probes on TFTR at the plasma edge. Also, ion cyclotron resonance heating fundamental and harmonic fluctuations were observed. However, a signal has not been definitively detected in the lower hybrid frequency range which correlates to alpha particles. Broadband noise was observed during neutral beam heating which is greater than t...

TFTR 60 GHz alpha particle collective Thomson scattering diagnostic

A 60 GHz gyrotron collective Thomson Scattering alpha particle diagnostic has been implemented for the D-T period on TFM. Gyrotron power of 0.1-1 kW in pulses of up to 1 second can be launched in X-mode. Efficient corrugated waveguides are used with antennaes and vacuum windows of the TFTR Microwave Scattering system. A multichannel synchronous detector receiver system and spectrum analyzer acquire the scattered signals. A 200 Megasample/sec digitizer is used to resolve fine structure in the frequency spectrum. By scattering nearly perpendicular to the magnetic field, this experiment will take advantage of an enhancement of the scattered signal which results from the interaction of the alpha particles with plasma resonances in the lower hybrid frequency range. Significant enhancements are expected, which will make these measurements possible with gyrotron power less than 1 kW, while maintaining an acceptable signal to noise ratio. We hope to extract alpha particle density and velocity distribution functions from the data. The D and T fuel densities and temperatures may also be obtainable by measurement of the respective ion cyclotron harmonic frequencies.

Design of a third‐harmonic electron cyclotron emission diagnostic for ballooning mode fluctuations in PBX‐M

A third‐harmonic electron cyclotron emission diagnostic using ultrawide bandwidth (≊40 GHz) heterodyne receivers centered on 120 GHz with 14 channels per radial view is described for localized, long wavelength (5≲λ≲50 cm), fast time response (≊1 μs) fluctuation studies in the PBX‐M tokamak. The optically gray emission signal will have a γne/ne+(3/β)Te/Te dependence on temperature and density fluctuations where γ≤1 and 1≤β≤3 depending on local optical depth. Electron temperature fluctuation sensitivity is estimated to be 0.2%≤Te/Te ≤2.9% depending on local optical depth and fluctuation frequency in the 0.1–1 MHz range. Spatial resolution of approximately 3 cm radially and 5 cm vertically are estimated for 2 keV plasmas with low suprathermal electron emission.

Fast ECE correlation radiometry for fluctuation measurements in JET and PBX-M

We discuss the fluctuation characteristics expected from MHD modes including ballooning modes and show how they can be studied using measurements of ECE. A novel technique, ECE correlation radiometry, which enables the location and spatial structure of long wavelength (L≳0.1×a) MHD modes to be determined, is described. Measurements on JET with a 44‐channel ECE heterodyne radiometer are presented and show the existence of high‐frequency (high n number) MHD modes under high poloidal beta, pellet‐enhanced performance (PEP) plasma conditions. Similar measurements are planned for PBX‐M and the measurement system is described.

Gyrotron collective Thomson scattering from plasma fluctuations in a Tara axicell

Collective Thomson scattering in the Tara Tandem Mirror axicell at MIT was accomplished with a 137-GHz, approx.0.4-kW, 75-ms pulsed gyrotron. Ion cyclotron waves, ion Bernstein wave harmonics, and other plasma fluctuations possibly due to microinstabilities and magnetohydrodynamic (MHD) activity have been observed during ion cyclotron resonance frequency (ICRF) heating. The observation of ion Bernstein waves may be due to an enhanced ion thermal fluctuation spectrum in an ICRF heated plasma.