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


Alcator C-Mod Weekly Highlights
June 14, 1999

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

The tokamak is presently in a maintanance/standby mode, with LN2 cooling and
vessel heaters turned off. All components are now at or close to room
temperature. We will remain in this state until the ICRF system is ready to
resume operation. A backfill in D2 and/or H2 for Thomson calibration will be
scheduled during this interval. ECDC and a fresh boronization will also be
scheduled prior to resumption of tokamak operation.

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

The behavior of RF antenna phase balance faults (an imbalance in the current
phase in the top and bottom parts of the antenna straps) vs. plasma shape has
been investigated for the last seven RF run days. There seems to be a trend
of an increasing fault rate for larger up- down asymmetry, defined as the
difference in distance from the top of the RF limiter to the plasma and
the bottom to the plasma. Questions still remain regarding the effect of
overall distance between plasma and limiter (the right gap). Further
investigation, including looking at older data, is continuing to see what
impact this may have on plasma shape and position consistent with reliable RF
operation.

The Rydberg series up to n=14 of helium-like chlorine, argon and sulphur have
been observed in Alcator C-Mod plasmas. High n satellites to these lines of
the form 1s$^2$2s -- 1s2snp and 1s$^2$2p -- 1s2pnp with 3 $\le$ n $\le$ 12
have also been seen for chlorine and argon. Accurate wavelengths of these
satellites have been obtained and comparison has been made with code
calculations. Line intensities have also been compared with collisional
radiative modeling that includes the contribution from dielectronic
recombination and inner shell excitation rates to each line's emission.

Analysis of the high resolution visible continuum array data has been extended
to generate profiles of the quantity n_e*sqrt(Z_eff), by taking the square root
of the emissivity profiles of the free-free bremsstrahlung, after taking out
the weak electron temperature dependence of this emission. This signal will now
be routinely available after each discharge. The time history of Z_eff in the
core of the plasma, based on this signal in combination with the cenral density
measured with the core Thomson scattering system, is also being calculated and
written to the tree. Analysis of a shot which exhibited an ohmic H-mode phase,
followed by an enhanced neutron rate L-mode phase, clearly shows the peaking of
the density profile which results from the prompt loss of electron density in
the outer half of the discharge following the back-transition. This, in turn,
appears to lead to the spontaneous formation of a core transport barrier. The
effects of subsequent sawteeth, which lead to a return to the normal L-mode
profiles, are also clearly observed.

ICRF Systems
------------

Final testing (wire test) of the protection circuits on the DC power systems
of transmitters #3 and # 4 was successfully completed. Initial RF tests into
dummy load were performed on both of these units. FMIT#4 produced
1.4 MW into dummy load for 0.5 sec; from FMIT#3, 0.4 MW was injected
into the dummy load. The maximum power output was limited by the reliability
of the RF control system, which is now being improved, along with other
diagnostic instrumentation.

Preliminary testing of the screen crowbars and grid bias regulators for FMIT#1
and #2 was completed. Installation of these devices is necessary before
beginning testing of FMIT#1's and #2's crowbar circuits.

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

Chris Brunkhorst (PPPL RF engineer) was onsite at C-Mod all last week, helping
with the ICRF work.

Gabriele Manduchi from the RFX experiment in Padua visited Tom Fredian and Josh
Stillerman to discuss future directions of the MDSplus data handling system. Dr.
Manduchi, a member of the original MDSplus development team, has been developing
java based tools to extend the graphical interfaces of MDSplus to other
non-X-window/Motif platforms.

Dr. Rejean Boivin visited Chalmers University of Technology, Goteborg, where
he presented a talk on "Neutral Particle Physics in Alcator C-Mod Plasmas". He
also spent time at W7-AS where he worked with Dr. Louis Giannone on AXUV diodes
for bolometry.

Ron Bravenec, U.Tx., has been at C-Mod since Thursday calibrating the BES
filter/spectrometer for D-alpha profile measurements. This was done by
shining a hydrogen lamp into one end of a test fiber and progressively moving
the other end channel-to-channel. For each channel the bandpass filter was
tuned to maximize the signal. The result: a relative calibration of the
detector electronics, bandpass-filter attenuations, neutral-density filter
attenuations, etc. Since the system has already been absolutely calibrated at
660 nm, and we know the variation of bandpass-filter attenuation with
wavelength, the system will be absolutely calibrated at D-alpha as well. Ron
will leave this Wednesday, and plans to take data remotely during the first
few days following resumption of plasma operations . The D-alpha profiles
will be compared with a model which uses density and temperature profiles from
the TRANSP database. He will return afterwards, install new neutral-density
filters into the spectometer, and move all nine fibers to look at the edge
plasma. We will then take D-alpha fluctuation data, hoping to detect poloidal
propagation of the fluctuations (among other things) which would give us a
measure of the radial electric field.

Martin Greenwald travelled to Brookhaven National Lab to attend a meeting of
the ESnet Steering Committee. Topics discussed included network requirements
for the DOE-NGI which looks likely to be funded for FY99-00, and DOE-SSI which
faces a more uncertain future. A good deal of time was devoted to computer
security issues, which have been raised to very high visibility in recent
months. Bob Fink briefed the group on implementation plans for IPV6 - a new
version of the internet protocol which has the alleviation of the address
space crunch as its principal aim. Also discussed were upgrades to
connections to Internet2 and international interconnection points, which
provide connectivity to universities and international sites respectively.