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


Alcator C-Mod Weekly Highlights
March 6, 2000


Major activities on Alcator C-Mod over the last week included completion
of the vacuum bake, continued work on the DNB, ICRF systems, and
diagnostics, preparations for resumption of ECDC, and construction of
the lower hybrid klystron test stand.

Engineering:

The vessel vacuum bake was ended on Sunday, 3/5, after nine days at
temperatures above 100 C. It is hoped that this long bake period will
reduce not only the general impurity levels in the machine during plasma
operation startup, but also reduce the hydrogen levels coming from
residual water vapor. High hydrogen levels adversely affect the ICRF
minority heating efficiency. The machine basepressure is currently
1.0e-8 Torr at a wall temperature of 35C.

Progress on the DNB last week included work on the PLC program that will
interface the main gatevalve to the C-Mod Torvac system. Control of the
probe plate used to diagnose the beam profile and energy deposition will also
be included in this program. Line filters have been added to some of the
control circuitry to better protect them during arc downs and noise
generated by other C-Mod systems. We expect to operate the beam this week
and verify its timer programming relative to the C-Mod shot cycles.

Now that we have approval for the lower hybrid current drive experiments,
manpower is being assigned to moving this system forward.
The electrical design for the klystron test stand is already finished, and
fabrication of components and system wiring are proceeding. The conceptual
design of the low-level RF system has also begun. This system will eventually
monitor the forward and reflected power levels, control relative phases,
and record several hundred channels of data relating to the operation of
the klystrons, waveguides, and launchers. In addition to the task of
testing the klystrons, the klystron test stand will be used as a breadboard
for the control and protection components of the Lower Hybrid system.

Work continues on the ICRF systems primarily on the coupling and protection
circuits. Arc protection for D and E have been set for the directional
coupler at the FPA output. The threshold has been set at ~ 50 kW and the
ratio of reflected to forward power is ~0.46. Testing of the antenna
protection systems will begin this week. The J-port 78 MHz loop assembly for
[0,pi,0,pi] phasing has been assembled for the upcoming run. The natural
frequency of the loops is 77.6 MHz. The decoupler stub has been set and the
decoupling is of order -60 dB at 78 MHz. Changes to improve the D- and
E-port loop assembly and make the system more robust to variations in
plasma loading are also underway.


Physics:

Impurity transport coefficients in weak (H-factor = 1.15) EDA H-mode plasmas
have been determined from x-ray and VUV spectroscopic measurements, in
conjunction with x-ray and edge bolometer array observations, following
calcium fluoride injections. Overall impurity confinement times were of
order 70 ms, and transport coefficients were found to be close to
the neoclassical values in the vicinity of the edge transport barrier.

Cynthia Phillips, PPPL, visited MIT from February 29, 2000 to March 2, 2000
to work on ICRF related problems in Alcator C-Mod with Paul Bonoli.
Recently, a version of FPPRF was incorporated into the TRANSP
analysis code which allows selection of the full-wave ICRF code TORIC
as an option in the bounce averaged Fokker Planck module FPPRF. This new
option was tested by running TRANSP and FPPRF with the following
rf options:

(i) The old SPRUCE wave code (written by D.N. Smithe)
(ii) A version of SPRUCE modified by the Belgian rf group
to account for higher order FLR effects in the
dielectric tensor elements.
(iii) The TORIC rf module, written by M. Brambilla, which
solves explicitly for the rf electric field of both
the fast wave and ion Bernstein wave.

Comparisons were carried out on a TFTR Test Case with D(H) minority
heating. The results are summarized below:

(i) The SPRUCE code as modified by the Belgian rf group
exhibited unphysical minority absorption off-axis
similar to test cases on Alcator C-Mod.
(ii) The old SPRUCE code gave reasonable results for
minority absorption with power deposition profiles
peaked on-axis at the (H) resonance layer.
(iii) The TORIC rf code gave absorption profiles quite
similar to the old SPRUCE code, but were slightly
narrower and more peaked on-axis.

Based on these test cases it was concluded that the FPPRF module
using TORIC as an rf option is working properly. We then proceeded
to set up a test case for Alcator C-Mod with the TORIC rf code
selected as the rf module. This C-Mod case is now being executed
remotely at PPPL.

Paul Bonoli has been exploring high poloidal mode number simulations
for D(He3) mode conversion scenarios. Previously, 15 poloidal modes
simulations found excessive minority damping of the mode converted ion
Bernstein wave. Using 127 poloidal modes, the IBW power to electrons
increased from 25% to 45% and is within experimental error bars of the
measured electron power deposition. The higher poloidal mode number
runs allow better resolution of the IBW wave which is a short
wavelength mode. A 255 mode simulation is currently underway.


Travel and Visits:

Miklos Porkolab, Ian Hutchinson, and Earl Marmar visited OFES
on Feb. 29. Earl Marmar and Ian Hutchinson made a presentations to OFES
staff in Germantown on the results from C-Mod from the past campaign
and future plans.

Raffi Nazikian visited from PPPL last week to discuss upgrades to the
C-Mod/PPPL Reflectometer. He also gave a talk on his plans at the
C-Mod Physics Meeting.

Norton Bretz spent the greater part of last week, 2/29 - 3/3/00,
working with MIT collaborators on the MSE control and data
acquisition software. The MDSplus tree has been planted, programming on
the control and monitor functions begun, and uncertainties about
CAMAC module support have been resolved. Also, schedule arrangements
for work on the image dissector, fiber optics, and shutter near F-Port
were worked out.

Amanda Hubbard was at PPPL on Wednesday, as a member of the National
Undergraduate Fellowship selection committee. Four students have been
offered summer projects at the PSFC (3 on C-Mod, 1 on VTF).