From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT



Alcator C-Mod Weekly Highlights
June 7, 1999

Last week was a maintenance week at Alcator C-Mod. No plasma
operations were scheduled. This week will also be a maintenance week.

Physics and Analysis
---------------------

Impurity screening during recent divertor bypass experiments has been
analyzed. A number of views of the plasma were monitored during the bypass
opening in both the VUV (Ar-IV) and visible (AR-II) to see if the ionization
source due to the bypass opening could be localized poloidally. The evidence
shows that a) the ionization source change is primarily in the divertor
region; and b) the cross-correlation of different views (and different
spectrometers) indicates that the ionization change brought about by the
bypass opening occurs either above the x-point or at smaller major radius at
the inner divertor near the separatrix. This is interesting because one would
expect the increased source to be near the bypass opening - thus near the
outer midplane. Further experiments are needed to confirm this preliminary
conclusion.

Improved ion impurity spectra have been obtained from the Omegatron probe. A
combination of data acquisition techniques and signal processing now permit
unambiguous identification of resonances with intensities 0.1% of the
non-resonant current. Improvement in the signal to noise ratio has revealed
resonances with M/Z of 1, 5.5, 8, and 12, and has improved the resolution of
the resonances previously reported, with M/Z=2, 4, 6, 10, and 11. Candidate
ion species for the newly observed resonances include: doubly charged boron11
(M/Z=5.5) and oxygen16 (M/Z=8), and singly charged carbon12 (M/Z=12). Unlike
many other resonances, the resonance with M/Z=1 is non-degenerate and can
correspond only to singly charged hydrogen. The intensity of the M/Z=1
resonance is consistent with H/D ratios determined by spectroscopic
methods for the same shot, but further analysis of the Omegatron data is
required to give a quantitiative estimate of the hydrogen density.

High frequency magnetic fluctuations have been observed during ICRF heating in
the 1999 campaign at frequencies of 300 kHz and 800 kHz as well as at 440 kHz
and 940 kHz. The pairs of modes are observed when both D- and E-port antennas
are turned on and they are separated by exactly the 500 kHz difference
frequency of the two RF transmitters (80 MHz and 80.5 MHz). These high
frequency modes are correlated in time with up to 200 kW of reflected power
from the D-port antenna. The modes are observed only in EDA H-mode and not in
ELM-free H-mode or in L-mode (or in any Ohmic plasmas). They have nearly
constant frequency for up to a couple of hundred msec. When there are
particularly large sawteeth, the frequency is modulated by the sawteeth by
about 20 - 30 kHz. The frequency rises sharply just after a sawtooth collapse
and then slowly returns to the pre-collapse frequency during the ramp of the
next sawtooth. These modes are in many ways similar to the modes that were
observed in 1997 - 1998 during EDA H-mode at 600 kHz, except that these modes
have even larger amplitudes with ~Btheta/Btheta about 5 x 10^-7 and these
modes shut off abruptly as soon as the ICRF switches off rather than decaying
away after 10 - 15 msec for the previous years' modes. The different
frequencies also present another difference that makes it somewhat difficult
to associate these new modes with possible TAE modes. In order to have the
TAE frequency, omega_TAE = V_A/(2qR) = mode frequency, we would need a
resonant q value of about 1.75 in the case of the 300 kHz mode. This would be
well outside the sawtooth inversion radius and so at a sawtooth collapse, the
density would increase at that radius. Then, the frequency should decrease
right after the sawtooth collapse, if it were a TAE mode, but the frequency is
observed to increase sharply after the sawtooth collapse. So, it is still
unclear what type of modes these are and whether or not they are related to
the high frequency modes observed in 1997 - 1998. One possibility might be
that the ICRF transmitters are putting out some power at 80.8 MHz, which is
then beating with the power at 80.5 and 80.0 MHz to give the 300 kHz and 800
kHz beat waves. However, the fact that the frequency is modulated by the
sawteeth suggests that this is not the case and that the modes are driven by
the plasma. While RF coupling changes due to the sawteeth could easily change
the amplitude of an RF interaction, it is not easy to see how this could
change the frequency. It is proposed to put a spectrum analyzer on the
directional coupler that measures the D port reflected power to look at what
frequencies are generating the reflected power when these modes are observed.

ICRF Systems:
-------------

Transmitter #4's DC power crowbar system has now passed the wire test.
This completes the testing required to validate the refurbished protection
circuitry on this transmitter. Testing has begun on FMIT#3 crowbar.

The transmitter cooling system for all four transmitters has been improved.
The water system was flushed and cooling channels cleared. An additional
screen filter has been installed on the water inlet and a check valve removed.
Improvements planned for later in the summer include installation of an
additional filter screen and a better check valve.

Testing was begun on the RF control system that protects both the antenna and
transmitter. Some wiring errors in the primary control board have been corrected
in FMIT#4. This system must be operational before RF testing into the dummy
load can begin.

Visitors and Travel
-------------------

Ben Carreras is visiting us from ORNL. He is currently analysing Te
fluctuations measured by GPC2 (the PPPL polychromator) in recent ohmic
discharges, and will also look at signals from other diagnostics. His
visit will continue this week.

Several students from the University of Wurzburg are visiting the
PSFC for the summer as part of an exchage program. They started June 1. Two
students are working at C-Mod. They are Dominique Huebner, working under
Bruce Lipschultz's supervision on spectroscopic problems, and Stefan Krotz,
working for Steve Wukitch on ICRF problems.