DESIGN OF A 32 CHANNEL HETERODYNE ELECTRON CYCLOTRON EMISSION SYSTEM FOR INSTALLATION ON ALCATOR C-MOD

Introduction

Toroidal plasma confinement devices have high electron heat conductivity above predictions of classical and neoclassical theories. It is believed that fluctuation-induced transport can explain the discrepancies between theory and measurements. To calculate the electrostatic fluctuation-induced particle and energy transport fluxes,

,
local fluctuation measurements of the electron density, , electrostatic potential, , and electron temperature, , are necessary. Recently, techniques to measure have been developed on both tokamaks and stellerators by correlating Electron Cyclotron Emission (ECE) from a localized sample volume. Extending this technique to reactor-relevant discharge parameters is will help identify any new fluctuation features and verify previous observations on other machines.
This describes the present designs for an ECE heterodyne system to be installed on Alcator C-Mod. A brief description of the tokamak and diagnostic requirements is followed by a depiction of the proposed ECE diagnostic hardware.

Experiment Description

Alcator C-mod is a compact, high-field tokamak ( a = 0.225 m, R= 0.67 m, < 1.8 ) capable of generating discharges with central magnetic field strengths, 2.5 T < B0 < 8.0 T, peak densities, ne0 < 9 x 1020 m-3, and total plasma currents, Ip < 1.4 MA. A representation shown below shows the high triangularity and elongation of the single null diverted discharge. Main auxiliary heating is 4 MW of ion cyclotron resonance frequency (ICRF) heating at 80 MHz.


Crossectional view of Alcator C-Mod with ECE optics and central ray projection.

Crossectional view of Alcator C-Mod with ECE optics.

Typical plasma operating parameters are Bo = 5.3 T, Ip = 0.8 MA, and line-averaged density, neo = 2.5 x 1020 m-3. Using these parameters, second harmonic X-mode propagation for the ECE system was the optimal choice to avoid the refractive effects of the density cutoff. This magnetic field configuration has a magnetic field range from 8.0 T on the inboard plasma midplane to 4.0 T on the outboard plasma midplane which corresponds to a second harmonic frequency bandwidth of 220 GHz to 450 GHz. Frequencies above 300 GHz require expensive sub millimeter wave equipment and precise alignment, so the bandwidth of 220 GHz to 306 GHz was chosen to view from the outboard edge density pedastal region to a position on the high field side of the plasma core.

Diagnostic Hardware

The design of the Alacator ECE system consists of the collection antenna assembly, broadband radiometer stages, and modular intermediate frequency (IF) sections connected to a CAMAC data acquisition system. The following paragraphs detail the antenna subsystem.


Close-up of ECE Antenna system.

Antenna Subsystem

The antenna system is pictured above. This system includes an elliptical cylindrical collection mirror, a hyperbolic paraboloidal collimating mirror, two plane mirrors, and an overmoded waveguide.
The collection antenna is a section of an elliptical cylinder with one focus in the plasma and one coincident with the hyperbolic paraboloid collimation mirror. This allows the focus position (the position of the minimum plasma beam waist) to be independent of frequency. The size of the mirror is approximately 20 cm in elliptical arc by 5 cm of cylindrical width. This large arc length allows for narrow Gaussian beam waists. Positioning of the elliptical mirrors at the back end of the port extension allows for an enhanced depth fo focus as well as protection from coating, sputtering, and disruptions. The cylindrical portion of the mirror allows for several collimating mirrors to view similar poloidal locations with a slight toroidal change in position.
There are two different elliptical collection mirrors each located on either side of the port extension midplane. One is set to focus on the edge density pedestal region (R= 0.85 m) and the other has a focus at R= 0.75 m. These correspond to 230 GHz and 260 GHz respectively for the edge and gradient region of a B0 = 5.3 T discharge. Calculated 90% power spot sizes over the radiometer bandwidths are presented in figure 3 for a B0 = 5.3 T discharge. Note that refractive effects from density are not included. The emission width or radial extent of the plasma spot sizes has been estimated to be 0.5 cm.


Calculated 90% power spot sizes for edge and interior viewing antenna systems.

The hyperbolic paraboloid or `saddle' mirror has a coincident poloidal focus with the elliptical collection mirror. The parabolic part of the mirror collimates or extends the focal length of the collected radiation from the poloidal extent of the sample volume. The hyperbolic part of the mirror localizes the toroidal spot size. The saddle mirror is small (2.4 cm x 3.2 cm x 3.1 cm) and several can be positioned to view the same elliptical collection mirror. The overmoded waveguide has the advantage of low attenuation of the ECE radiation while traveling to the radiometers. Present designs of the waveguide are a 1 cm ID circular cylindrical copper tubing with a helical corrugation pattern machined into the inner surface. Polarization shifts due the windings will be small and will be further minimized by reversing the helical pitch angle in each half of the waveguide. The Teflon vacuum break for the system will be placed within the waveguide.

Radiometer (RF) and Intermediate Frequency (IF) Subsystems

The RF and IF sections of the heterodyne ECE system are to be provided by the Fusion Research Center from the University of Texas at Austin as part of an inter-university collaboration. Initially, the radiometer system will consist of two separate heterodyne radiometers: one gradient region radiometer with bandwidth of 234 - 270 GHz, and a core radiometer with bandwidth of 270 - 306 GHz. Each output an IF signal of 2 - 34 GHz. A modular IF section is planned with the 18 - 34 GHz bandwidth downshifted to 2 - 18 GHz to allow the use of identical bandwidth multiplexer sections. The multiplexer modules will separate the signals into eight equal frequency bands covering the bandwidth. The separated signals are amplified and filtered according to whether the signal is used for correlation radiometry or temperature profile measurement. Correlation radiometry signals are divided, narrow bandpass filtered with disjoint frequency bands. The filtered signals enter a modular detector/amplifier section before it enters a CAMAC data acquisition system.

Status and Future Plans

The heterodyne ECE diagnostic is to be built and installed for the winter 1997 Alcator C-Mod run period. The diagnostic will provide 32 channel electron temperature profiles on the low field side, in addition to being able to look for microturbulent electron temperature fluctuations using the principle of correlation radiometry. The detailed mounting specifications are being finalized. Fabrication of the mirror system and spot size testing will be performed over the summer at Auburn University. Alcator C-Mod venting in September will allow installation of antenna assembly into the vacuum vessel. The first temperature fluctuation measurements will be made during the Alcator C-Mod winter run time.