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
April 10, 2000

Plasma operations continued on Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 39 plasma shots were produced with a
reliability of about 80%. The bus work had been re-connected in the normal
field direction (toroidal field and plasma current both negative). The primary
goals were to re-establish operation with normal (negative) field; continue
machine conditioning with tokamak plasmas; and proceed with checkout and
commissioning of diagnostics.

Plasma operation was readily obtained, but conditioning is still
incomplete. High disruptivity was experienced except at low density
(nebar<1.e20), and numerous molybdenum injections were observed on the
bolometer and spectroscopic diagnostics. The observed behavior is not atypical
of previous campaigns at this stage of the startup process.

This week will be a maintenance week at Alcator C-Mod. No plasma runs are
scheduled; electron cyclotron discharge cleaning (ECDC) will be carried out
consistent with other activities. The principal focus will be on readying the
ICRF and DNB systems for operation into plasma.


Physics
---------

A second array looking at neutral deuterium Lyman alpha radiation is now
operational. This array uses a photodiode array and a very narrow filter at
1215 Angstroms. The radiation gives the local ionization rate and neutral
density with information of the electron density and temperature. Initial
results indicate that the Lyman alpha radiation is approximately 10 times
larger near the inner wall compared to the radiation near the outer midplane.
Detailed analysis of the measured profiles is under way.

The new limiter particle-flux probes are now operating. Particle flux
densities to the sides of the A/B and G/H limiters are recorded at 8 different
locations (4 on each limiter) by Langmuir probes operated in an
ion-saturation mode (Isat). The probe geometry is designed so that each
probe element integrates the cross-field profile of Isat from the leading
edge of the limiter far into the limiter shadow.

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

Efforts continued in bringing up the ICRF transmitters for the resumption of
operation into plasma. Transmitter tuning for maximum output power had been
hampered by residual telemetry waveform and calibration issues, but these are
being resolved. Lack of transmitter access resulting from C-Mod plasma
operation has added delays, but this will be alleviated by the scheduled
maintenance week of 4/10. Chris Brunkhorst from the PPPL rf engineering group
will come for the week once more to help.

DNB System:
-----------

Preparation for operating the DNB into C-Mod continued. A problem
with the arc crowbar was bypassed, allowing consistent formation of the
source plasma. An interaction between the C-Mod magnet supplies and the
DNB HV supplies was corrected, allowing a beam to be fired into the dump
plate during a C-Mod shot. The DNB software was further checked during
C-Mod operation. The RGA was made operational in preparation for opening
the DNB gate valve.

Diagnostic activities this week included installing the full set of BES fibers
in the MSE/BES image dissector, installing the MSE polarizer, checking the PEM
control units, and calibrating the filter temperature controls. The
BES system is now ready to make measurements of the radial profile of the
absolute level and fluctuations of the beam emission. Howard Yuh,
an MIT graduate student, is continuing to write software to do calibrations
and set the timing for automated and interactive data taking. The filter
control calibrations were both tests of Howard's programs and system
calibrations aimed at anticipated 5.3 T plasma and 45 keV hydrogen DNB
operations.

Lower Hybrid Fabrication
-------------------------

Work on the lower hybrid test stand continued. Mechanical assembly is now 98%
complete, and wiring 90% complete. Power was applied to the focus magnet, and
testing of the fast-off fault circuits was begun.

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

Randy Wilson (PPPL) came to MIT for the week to help with transmitter tuning
and debugging. Joel Hosea(PPPL) came 4/6-7 to contribute to the ICRF
startup and for discussions with Ron Parker on the LHCD project.
Norton Bretz(PPPL) was at MIT 4/3-5 for the MSE work.

Ron Bravenec (UTx) visited C-Mod to work on the BES system. Perry Phillips
also visited C-Mod and worked on hardware for the Texas ECE system. Dave
Winslow is presently at C-Mod working on hardware for the Texas turbulence
probe.

Dr. Nobuyuki Asakura, from the JAERI JT-60U experiement, visited the PSFC
for the period March 28 through April 8. He spent his time primarily on
issues to do with transport in the SOL. More specifically the question of
how wall recycling on different machines scales was addressed. This
involved comparing data on midplane pressure and D_alpha, as well as plasma
profiles in the SOL for different experiments. We compared not only C-Mod
and JT-60U, but also data found in the literature for ASDEX Upgrade and
JET. For the most part midplane pressures, which represent the ion flux to
the walls (ion flux to wall = neutral flux back towards the plasma) scale
with the operating density of the experiment. Some differences among
experiments became apparent. All of the data will be analyzed further.

Montgomery Grimes (MIT) traveled to CPI (the manufacturer of the LH high-power
klystrons) to discuss the klystron operation in this application.

Miklos Porkolab, Ian Hutchinson, and Earl Marmar attended the OFES Field Work
Proposal Meeting in Germantown, Md., on April 5,6 .

Bob Childs attended the Executive Council Meeting 85, chaired the Vacuum
Science Division meeting of IUVSTA and attended the Program Committee meeting
for the International Vacuum Congress 15 to be held in San Fransisco October
2001. These meetings where held in Namur, Belgium from Friday March 31, to
April 2, 2000




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