FRCECE Diagnostic, A unique high frequency ECE
diagnostic for high magnetic field tokamaks (comparable to
ITER fields)
The FRCECE system is unique due to its high frequency
(234-306 GHz), large number of channels (32), high spatial
resolution (~0.6 cm x 2 cm) and wide video bandwidth (1
MHz), which facilitates measurements of complete
temperature profiles (full coverage of the low field side)
with a toroidal field of 5.4 T and of fluctuation
amplitudes. The diagnostic has been installed on a radial
port of the Alcator C-Mod tokamak. It consists of a set of
in-vessel collection optics that focus the ECE emission
onto a set of overmoded waveguides that then carry the
emission to the radio frequency receivers.
Two heterodyne receivers are used to cover the 80 GHz
bandwidth. Each detects one of the bands (234-270 GHz and
270-306 GHz) which are then down-converted to 4-40 GHz
bands, using the local oscillator frequencies of 115 GHz
and 133 GHz (a frequency doubled 66.5 GHz LO), which then
feed two second harmonic mixers. The mixer blocks have a
net RF to IF gain of 10 dB, and employ an anti-parallel
diode pair configuration to enhance the second harmonic
mixing efficiency and eliminate the fundamental mixing
response. The mixers are fed by an integrated diagonal feed
horn structure coupled to a fundamental WR-4 waveguide
machined into the mixer block. The IF signal from these
mixers are split into 32 channels, band-pass filtered and
detected using square-law detectors. The output of the
detector is amplified by a video amplifier with a 1 MHz
response and digitized. The wideband video amplifiers were
developed at the University of Texas to reduce the cost of
the IF section.
The ECE radiometer that UT-FRC provides for Alcator C-Mod
yields 32 channels of high temporal and spatial resolution
data on each shot. At 5.4 T, the most common field for
C-Mod experiments, the channels cover the plasma from the
magnetic axis to the lowfield- side separatrix. At lower
fields, the measurement range shifts away from the magnetic
axis but still provides measurements through the pedestal.
At higher fields, the pedestal data is lost. An extension
of the frequency range either to higher or lower frequency
would benefit C-Mod experiments. We plan to increase the
range of the measurements by extending the low end of the
frequency range of the diagnostic.