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


Alcator C-Mod Weekly Highlights
February 07, 2000


We continue to approach closeup of the C-Mod vacuum vessel as
installations and alignments are completed. Work on the RF, DNB,
and power systems also made good progress.


Engineering:

DNB power systems tests in the cell area were started on Thursday, 2/3/00.
The magnet supply and suppressor switch were tested. On Friday, 2/4/00 the
HV supplies were tested into the spark gap and the suppressor supply and
switch were tested unloaded. The HV supplies (lightly loaded) were run
successfully at 25kV with the spark gap set to 30kV. At 30kV, arcdown
occurred and the controls successfully rolled back the tube and did four or
five retries. The spark gap was reset to about 40kV and the supplies ran
successfully at 38kV. At 40kV, the arcdown caused the HV supply to crowbar
and some of the control system timers to latch up. More work is needed at
these levels to determine the cause of this problem and the solution.
These tests complete the third major DNB milestone approximately one
week early.

We began calibrations on some of the Hi-yard signals last week. These
signals are used to calculate the complex power load supplied by the
Hi-yard to the alternator, RF system, and the EF2 power supplies.

Invessel work last week included a calibration of the reflectometer
with a metal mirror scannable from about 60 to 83 cm in major radius. Some
problems with the 88 GHz channel were discovered and will require work on
the receiver. We also continued work on the installation of new
retro-reflectors for the rangefinder diagnostic. Some inner wall tiles
were found to be vignetting the view and had to be modified.
Norton Bretz of PPPL, Howard Yuh of MIT, and Matt Sampsell of
UTexas installed the external optics and fiber holder of the MSE/BES optical
system last week. In the process of alignment, it was discovered that two
of the mirrors and one lens of the in-vessel optics had come loose and were
chipped during the last campaign. The mirrors were replaced in improved
holders, and the entire optical train was aligned. The optical fibers were
backlit and digital photos were taken of the images on a screen along the
DNB axis. These will be analyzed to determine the view chords and sample
volumes. Later this week, Norton Bretz will return to install a structure to
stabilize the loose lens. (This will not require in-vessel access.)
Installation of the scannable mirror for the VUV spectrometer has been
completed. Over the weekend we began cleaning up invessel and removing tools
and other equipment no longer required. Beginning this week, all invessel
tools and equipment will be strictly monitored as we prepare for closeup
and pumpdown.

Physics:

Helium-three gas was puffed from the wall into ohmic L-mode discharges
discharges and the charged states were measured with the omegatron ion
mass spectrometer. Analysis of the data shows that the concentrations of
singly- and doubly-ionized helium at the edge are approximately equal.
The electron temperature and density at the omegatron are too low to
account for ionization of helium in the local flux tube, therefore the
helium is ionized in a hotter region of the edge plasma and is transported
to the omegatron. The measurements by the omegatron of the charge states
gives information about the transport in the edge plasma. If ion transport
is slow, then ion dwell time in the hot plasma is sufficiently long to
be ionized to the second charge state, and the omegatron should see
mostly doubly ionized helium. Conversely, if ion transport is rapid,
ions do not have sufficient time to be ionized again and the omegatron
should see mostly singly-ionized helium. A simple one-dimensional radial
transport model reproduces the observed values of charge state flux and density
only if rapid outward transport is included, increasing with distance from the
separatrix, with diffusion coefficients of order 2 m$^2$/s and outward
convection velocity of order 100 m/s. These conclusions are similar to
those of Umansky, but are based on completely independent measurements.

During some of Type III edge localized mode (ELM) bursts, significant ECE
signal drops (10 ~ 50 %) have been observed on grating polychromator (GPC).
Starting from the fact that the refraction effects are getting serious as the
refraction index (N) goes to 0 (cutoff condition), a plausible density profile
has been assumed with poloidal variations. Since the well-known conventional
ECH ray tracing code "TORAY" cannot accommodate such poloidal variations, a new
ray tracing code has been developed. Assuming a typical plasma without poloidal
consideration, the new code has been successfully benchmarked with "TORAY".
Using the new ray tracing code, the ray trajectories of two GPC channels were
found for a ELMy plasma with poloidal variations and compared with experiments.
As a result, 5.5 and 35 % reduced signals were expected respectively, while 25
and 42 % were observed in experiments. Consequently, we can infer the ELM
dimensions from the input parameters (width = 2cm, peaked ELM density=3.6 x
10^20 m^-3). Such result is expected to be compared with a theoretical
prediction (Wilson, H. et al, Phys. Plas., Vol.6 p1925, 1999). Nevertheless,
there are so many hidden subtleties which may be an issue in the end. For
example, such theoretical parameter is derived on the basis of limiter
geometry, though separatrix geometry is typical in experiments.
After the talk, there was an interesting point that the ELMs should be
described with the density loss, rather than density gain. This can solve
a rather conflicting mystery why the fundamental O-mode radiometer had signal
"spikes", while 2nd harmonic X-mode GPC had signal "drops". The detailed
follow-up investigation will be reported soon.


Travel and Visits:

Norton Bretz was at C-Mod 2/2-6/00 working on the MSE optical
problems. Raffi Nazikian was present 2/2-3/00 for ongoing
discussions with Yijun Lin and Jim Irby on the reflectometer upgrade
progress. Gerd Schilling is back at C-Mod as well, finishing
preparations for the PAC meeting 2/7-8/00.

Miklos Porkolab attended the FESAC MEETING in Gaithersburg on Feb. 2,3
and made a brief presentation during the "public comments" period
about the PSFC's efforts to establish stronger connections with
Astrophysics at MIT.

Tom Fredian and Josh Stillerman attended an MDSplus Developers Meeting at
General Atomics last week. The status of the port to unix and windows,
enhancement ideas and up coming installations were discussed. Participants at
the meeting included GA, UCSD, PPPL, LLNL, MIT, Lausanne and Padova.

Benjamin Carreras of ORNL has been here since 2/3. He has been collaborating
with us on the analysis of H-mode pedestal evolution, as well as analysis of
edge fluctuations. He gave a talk to the C-Mod group on 2/4 about "L to H
transition and edge pedestal formation".

Martin Greenwald attended a meeting of the ESnet Steering Committee.
Plans for the latest network deployment (ESnet3) as well as issues
involving network research and distributed computing were discussed.