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 5, 2000

Operations
----------

Plasma Operations continued on Alcator C-Mod last week. Three run days were
scheduled and completed, following a scheduled boronization. A total of 50
plasma shots were obtained, with a startup reliability of 75%.

A fresh boronization was carried out over Tuesday night, using 144 psi of
diborane for an average layer thickness of 2106 Angstroms. Boronization was
followed by over 6 hours of ECDC in D2.

This weeks runs were devoted to ICRF Systems (evaluation of J-port
antenna performance), ICRF-induced rotation experiments, H-mode pedestal
scaling, and fluctuations and transport in the scrape-off layer (SOL).

Plasma operations are scheduled to continue this week.


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

Thursday's run was divided between two MiniProposals, both of which had been
unable to complete their proposed experiments earlier in the campaign. The
resumption of MP#269 on ICRF rotation again was unsuccessful in obtaining good
results with the ICRF resonance moved far to the low-field side of the axis. A
combination of plasma startup difficulties and ICRF coupling using the D-port
antenna frustrated these experiments, although one additional LFS data point
was obtained at 5.7T, which corresponds to a resonance at R=R0 + 5.6cm. For
this case, the observed rotation remained in the co-current direction. The
completion of this MiniProposal will again be rescheduled.

The second half of Thursday's run continued the study of scaling of pedestal
parameters. A dataset at Ip=1.0MA was obtained, with varying RF power
steps. These data, together with the lower current datasets obtained last
week, are being analyzed.

Friday's run was the second scheduled run day devoted to MP#267, "Dependence
of SOL Fluctuations and Particle Transport on Collisionality". The purpose of
these experiments is to explore the dependence of SOL fluctuation
characteristics and attendant cross-field particle transport on the
collisionality of the SOL. Fluctuation and plasma profiles were measured using
the A-port and F-port scanning probes operated in a fluctuation mode (The A-port
probe was used in both a spatially scanning and a fixed position mode);
divertor probes operated in a fluctuation mode; fast visible diodes viewing
helium puffed from the A/B limiter capillary; the tangential Ly-alpha diode
array;
limiter particle flux probes; and edge Thomson scattering. The range of
parameter space to be investigated included 0.53<Ip<1.06MA,
0.5<NL04<1.3e20/m2, and 4<Bt<6T. We were able to cover all the Ip, Bt, and
NL04 parameter space with at least one acceptable discharge for each
permutation. It is immediately apparent from a quick inspection of the data
that the fluctuation character generally changes with increasing plasma
density, all else being the same. Determination of any trends with
collisionality (i.e., electron-ion mean free path normalized to the parallel
magnetic connection length), awaits detailed analysis.


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

Following boronization, the ICRF antennas were reconditioned and high target
density tests for J-port were performed. The D and E-port antennas were
operated at 3.1 MW (combined) into EDA H-mode plasmas. We are, however, still
experiencing reliability problems due to arcing. Rotation experiments this
past week were hampered by D-port arcing at 6 T. D-port ran without problems
at 5.4 T on following discharges and ran at 6 T by itself on Friday. This may
suggest a subtle conditioning problem following boronization or an unexpected
interaction with E-port.

The high target density discharge experiment for J-port was to investigate
whether the antenna could run at high target densities. In the previous
campaign, J-port would arc repeatedly for targets above a line density of
1.2x10^20 m^-2, about 20% higher than the fiducial density. Although the
experiment is incomplete, operation into target densities of 1.3x10^20 m^-2
has been achieved. In addition, the antenna straps nearest H-port behaved
more erratically (poorer reflected to forward power ratio) than the pair near
K-port. Further experiments should investigate this asymmetry.

DNB Systems:
------------

Progress continued on the suppressor and gradient grid voltage scans on
Thursday and Friday. Wednesday was lost to a PSFC chilled water problem
which shut down and warmed up the DNB cryo pumps.


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

Following the PSI conference, May 22-26, Bruce Lipschultz also attended a
tw-day workshop at Garching on Plasma Edge Issues for Next Step
Devices. There, he presented two talks on the details of tokamak operation
with a Mo first wall and on wall recycling. The latter is very controversial,
but it was apparent that all experiments are observing some of the
characteristics of this - ion fluxes to the wall that are comparable or
greater to the ion flux to the divertor.

On May 24, Amanda Hubbard presented the PPPL colloquium on "H-mode pedestal
Physics on C-Mod".

Alan Glasser (LANL) spent last week at the PSFC working with Steve Wolfe
on the adaptation of his Ideal MHD stability code DCON to MDSplus. The goal of
this work is to produce an MDSplus-aware version of DCON which will facilitate
the use of this code at C-Mod, DIII-D, NSTX, and other sites which employ the
MDSplus data system. The specification of the MDSplus tree for DCON was
refined, and a working copy of the Model tree generated. Nodes (with
descriptive commentary) for all of the user-relevant DCON inputs and outputs
are now represented in the tree. In addition to working on the MDSplus
adaptation, Dr. Glasser also identified and began implementing a fix for a bug
in the code which was causing invalid results for a particular class of C-Mod
equilibria. During his visit, Dr. Glasser also presented a PSFC seminar on the
DCON code, and held discussions with a number of physicists from the PSFC
Theory Group.

Norton Bretz was at MIT for three days this week and continued work on
readying the MSE diagnostic which will operate in conjunction with the DNB.
Data acquisition and I/O software now work, but issues with the reference
sync signal from the machine, an extension of the C-Mod DIAG link to include
MSE still need to be resolved. Problems with the AC power provided for the
experiment have now been resolved.

Stewart Zweben (PPPL) came to C-Mod this week to plan the installation of the
new 2-D edge turbulence imaging diagnostic. This diagnostic has been designed
to view along a magnetic field line to image the radial vs. poloidal structure
of the fluctuations in visible light emission in the edge and scrape-off
layer. Such fluctuations have been monitored routinely during this run with
the LANL fast camera, using a wide-angle view of the plasma. Strong
filamentation of the visible emission is seen at a 10 microsecond exposure
time all around the machine, and especially at high density. Many of these
images show a surprising spatially-periodic pattern of filamentation, but
with a period which varies from frame-to-frame.

Dr. Yantai Shu visited Tom Fredian, Stuart Sherman and Josh Stillerman to
discuss MDSplus. Dr. Shu is a professor of Computer Science at Tianjin
University in the Peoples Republic of China and has been contracted to
investigate the use of MDSplus on a new tokamak (HT7U) to be built at the
Institute of Plasma Physics in Hefei, China.