From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
June 2, 2003

Plasma operations continued at Alcator C-Mod last week, with three run days
scheduled and completed. Runs were carried out in support of MP#326, "High
Power ICRF Operation", MP#318, "High Performance Operation at 5 tesla", and
MP#327, "Developing low density EDA H-mode targets for LHCD".

Plasma runs are scheduled to continue this week.


Operations
----------

A total of 85 plasma discharges were produced over three run days last
week. All tokamak systems performed well, with overall startup reliability of
about 95%. The ICRF system operated at high power (>4mW) all three days, using
all three antennas, with peak power exceeding 6MW for short pulses. High
radiative power fraction and elevated H/(H+D) ratio were observed, and may
have contributed to the limitations on the pulse length of the highest power
operation.

A fresh boronization was carried out beginning Friday evening through Saturday
afternoon.

Physics
-------

Similar high frequency modes in the core of RF ITB plasmas have been
identified by Martha Redi, with the GS2 code, and by Alan Lynn (U. Texas) from
ECE measurements. Nonlinear gyrokinetic calculations predict an apparent
geodesic acoustic mode at 77 kHz in a low field case at the trigger time. The
ECE heterodyne measurements of 80 kHz were made during ITB in a high field
case.

Operation on Friday continued experiments begun last year on development of
low-density EDA H-mode target plasmas for Lower Hybrid current drive, in
support of the future Advanced Tokamak program. H-mode plasmas were produced
with Ip=600kA and sustained RF power at the 4MW level. EDA, as evidenced by
observation of the Quasi-coherent mode on reflectometry, BES, and fast
ECE and magnetics, was obtained with target densities down to ~9e19/m^3, while
ELM-free H-modes, with no QC mode, were produced at a lower target density,
~6e19/m^3. At lower RF power the QC-mode was absent even with the 9e19/m^3
target. The performance of these low-density H-modes was good, with H-factors
around 2 or above. ACCOME modeling of these plasmas will be carried out; we
anticipate they will show good LH current drive efficiency.

ICRF Systems
------------

The primary focus of the RF operations was to provide high power into plasma.
Short pulses (~0.12 sec) at 5 MW into 1MA H-mode discharges were achieved, and
peak powers over 6MW were briefly obtained with plasma currents up to
1.2MA. The systems operated well into low current, higher toroidal field
discharges that were difficult to match to during the last run campaign.

Improvements to the demodulator circuits that detect rf power levels
and drive fault detection circuitry resulted in better reliability of
the rf systems this week. Up to 2.7 MW has now been coupled through
the j-port antenna, although more conditioning is required for full
performance operation.


Lower Hybrid System
-------------------

High power testing of the prototype circulator was begun this week.
Apparent waveguide faults limited peak power to approximately 100 kW.
The fault location has not been identified and may not be in the
circulator. We are in the process of tracking down the source of this
fault and also a gas leak in the circulator that may eventually limit
performance.

The PXI Interface chassis, SFOL chassis, Logic chassis and patch panels were
installed into the LH Active Controller racks and a temporary ethernet was run
to the rack area. Fiber optic cables were pulled into place for
CPS and Low Power Microwave Rack; termination is still required.

The lower hybrid launcher forward stacked waveguide completed its 150C bake
cycle last week. New locations with blistering and peeling were found,
resulting in a decision to disassemble the guide and replate.

The lower hybrid grill couplers have been successfully stripped and are now
being prepared for plating. An electrode assembly has been designed and
fabricated at PPPL in cooperation with MIT and the vendor.



DNB System and Diagnostics
---------------------------

DNB performance was very good last week, with beam currents averaging 4.5 Amps
at voltages of 49.5 kV. No gas system problems have re-appeared. The computer
CRT in the cell was changed out for a spare flat panel display. Work
continues on fiber links between the local Russian beam camac and the C-Mod DNB
camac (which is recorded in MdsPlus). The fiber links are being made up from
spare and used AFOL boards.

Diagnostics
------------

The 132 GHz reflectometer channel has become operational. Iniital data look
very good. The auto power spectra show the expected specular reflection, MHD
activity, and a broad background of density fluctuations. The coherent
scattered power, which is an indication for the density fluctuation level
varies when the plasma evolves and at times there is even a weak cross
correlation visible with the 88 GHz channel.

The NSTX X-ray imaging crystal spectrometer was installed on C-Mod
5/24. Everything went very well. Tom Holoman and Doug Lebrie moved
the equipment from NSTX to MIT on Friday morning and put it on the
C-Mod platform in the afternoon. On Saturday, John Rice, Ken Hill, Doug
Lebrie and Manfred Bitter installed the spectrometer on the horizontal
(lower-hybrid) port and aligned the crystal. The next steps are to install
the detector and connect the Korean PC-based data acquisition system to
MDSplus. The spectrometer will then be ready to take data.



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

Ken Hill, Doug Lebrie, Brent Stratton, and Manfred Bitter (PPPL) visited to
work on the installation of the NSTX X-ray imaging crystal spectrometer on
C-Mod.

Bob Childs and Rui Vieira were at PPPL on 5/30 for ongoing evaluation of the
LH launcher following the 150C bake cycle tests carried out last week.