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
May
10, 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:
---------------------
The
high resolution (0.7 mm at plasma edge) toroidal Z-Meter array, which
images
free-free bremsstrahlung emission from the plasma, has been used to look
at
EDA H-mode discharges. A clear pedestal in the emission forms, with tanh
fits
giving the pedestal foot location at or near to ( within about 2 mm) the
outboard
midplane location of the last closed flux surface derived from EFIT
reconstructions.
Typical emissivity full-widths range from 4 to 6 mm.
Analysis of
the rangefinder data indicates a linear trend in differential
motion of
the inner wall with halo current. We
have measured up to 47 um of
movement with a halo current of 170 kA. These results are being incorporated
into
the simulations needed to guide the design of the inner divertor upgrade.
ICRF
Systems:
-------------
As a result the tube failure in FMIT#2
and another suspected failure in FMIT#1
(tests are still ongoing), a
review was conducted to investigate whether the
tubes are adequately
protected and whether our operating practices were a
contributing factor
in the loss of these two tubes. This
review was
undertaken in collaboration with PPPL (Chris Brunkhorst, Joel
Hosea, Gerd
Schilling, and Randy Wilson).
As a result of this review, several
modifications and additions to
the protective circuitry are being undertaken,
including installation of
an additional "soft-start" resistor, a screen
crowbar circuit,
and a low-stored energy grid bias regulator circuit. In
addition, a
circuit was installed to disable the rf drive during a
crowbar. These
modifications are being made to all four transmitters. We plan
to have
FMIT#3 and #4, which are connected to the new J-port antenna, prepared
for
testing early this week with the aim of beginning operation into plasma
next
week. FMIT#1 has had an 8973 tube
installed (should provide ~ 1 MW) and
FMIT#2 has had a reconditioned 2274
tube installed (should provide ~2
MW). Both of these units will also
operate once all of the above modifications
have been made to them.
In
addition to the ICRF engineering issues, Joel Hosea and Randy Wilson
visited
to discuss the ICRF priorities for the remainder of the current
campaign.
Diagnostic
Neutral Beam
------------------------
During the past week it
was necessary to divert technical and engineering
resources from the DNB
to the ICRF transmitter modifications. This resulted
in a schedule slip of
nearly one week for the completion of the DNB. We
still anticipate that
the DNB will be ready for testing at the commencement
of the scheduled
outage this summer and that the installation onto the
tokamak and final
comissioning of the beam will be completed in time for
the beginning of
the FY 2000 operating campaign.
Travel and Vistors
-------------------
Marco
Brambilla has been visiting the PSFC and working with the Alcator C-Mod
group
since April 19, 1999. His visit will
continue until May 14, 1999. He
has been working mostly with Paul Bonoli
on the implementation of a new
version of the toroidal ICRF code TORIC on
our local workstation cluster. The
improved TORIC module now computes the
electron damping of ion Bernstein waves
using a new formulation which
relies far less on a numerical broadening (ad
hoc) of the ion-ion hybird
layer as did the previous version. The code has
also been coupled to an
ICRF current drive module that was developed at
MIT. The new code is being
tested and compared with the old code as as well
as with experimental data
from mode conversion electron heating in C-Mod.
Some effort has also been
devoted to the implementation of an IDL
post-processor for TORIC that can
be used both at Garching and MIT. This will
facilitate the transfer of
future versions of the ICRF code between MIT and
the US under the ASDEX
Upgrade - US collaboration agreement.
Perry Phillips (U. Texas) will
be at M.I.T. for the rest of this month to work
on the high resolution ECE
system.