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
Dec 1, 2003

Research operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed. Experiments were carried out in support of
Divertor/Edge, Transport, and Burning Plasma Research efforts. Progress was
also made on the Lower Hybrid project.

Plasma operations are scheduled to continue this week.

Operations
-----------

A total of 62 plasma discharges were produced during three run days (M-W),
with a startup reliability of 70%. All tokamak subsystems were performing
well.

The first half of Monday's run was devoted to completion of MP#335, "Edge
plasma flows in upper versus lower x-point discharges". The primary focus of
this last set of discharges was on double-null and near-double-null equilibria
using SSEP control to determine the balance between upper and lower
x-points. Excellent data were obtained from the three scanning probes (ASP,
FSP, ISP) as well as from HIREX (core rotation) and Chromex (He+1 Doppler in
inner and outer SOL). The flow measured by the inner-wall scannning probe
(ISP) is found to be a very sensitive indication of the magnetic balance.

The experiments on Monday afternoon and Tuesday were in support of MP#363
"Investigation of L-H Threshold conditions in LSN, DN, USN and limiter
configurations". Good data, including measurements of edge profiles and flows,
were obtained in all three diverted configurations. The inner-wall limited
experiments will be completed on a later run day. Both ramping power and
steady RF power were used to identify the power threshold. Typically, the
L-H transition appeared at lower power in the steady RF cases. Local electron
temperature at r/a~0.94 was observed to undergo a gradual increase before the
transition, suggesting gradual improvement in local confinement. For LSN
discharges, the threshold RF power was ~0.85MW, with Te(edge)~130eV; for USN
the threshold was ~2.9MW, and Te(edge) at the transition~300eV. The threshold
in DN discharges is found to be extremely sensitive to the magnetic balance,
with variations noted corresponding to changes in SSEP of the order of
1-2mm. Similar sensitivity is observed in core and edge flows, as noted in the
MP#335 results, and again in these discharges. For well-balanced cases, the
threshold power was ~1.6MW and Te(edge) at the transition time ~230eV.

Both MP#335 and MP#363 contribute to the C-Mod Level 1 Milestone SC6-1b,
"Compare energy confinement, H-mode threshold and divertor particle dynamics
in SN, DN and inner-wall limited discharges in Alcator C-Mod".

Wednesday's run was devoted to MP#362, which calls for development of high
normalized current (I_N=I/aB=1.4) equilibria in the ITER shape for use in
future Burning Plasma Support experiments. In particular, the goal of these
experiments was to develop LSN discharges with kappa=1.85, delta_x=0.49,
q95=3, at the ITER field of 5.3T and Ip=1.6MA. This elongation matches the
highest obtained in C-Mod SN plasmas, but at higher current and smaller
triangularity than previously achieved. As expected, the non-axisymmetric
(vertical) instability resulted in high disruptivity during the discharge
development process. Halo currents were monitored and found to be well within
the allowable range. Modifications to the fast vertical position control
feedback gains were successful in suppressing the instability, though these
were not fully optimized. The target parameters were achieved by the end of
the day without vertical instability, although full length non-disruptive
discharges were not obtained. Additional development work is required to
complete this MP.

ICRF System
-----------

High power (up to 5.4 MW), sustained, ICRF was coupled in lower single,
double, and upper single null discharges during operation on Monday and
Tuesday.

In preparation for testing a load tolerant matching network on E-port,
modifications were begun on the transmission lines to accomodate additional
directional couplers.


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

A second 3-window prototype coupler was brazed by our vendor. UT tests
indicated there were no brazing voids, and visual inspections indicated very
good filling of the braze around the edges of the window. The new brazing
material was also easily cleaned off the window and titanium surfaces, a
process that had proved very difficult and time consuming with the previous
braze material. Instrumentation is now being developed to monitor the
coupler temperature during the braze process, and a simulation of the
24-window coupler is being developed to interpret the instrumentation data
and help set limits on the thermal cycle used to braze the couplers. At
least one additional 3-window prototype will be tested before a 24-window
coupler braze is attempted.

The Lower Hybrid launcher Forward Wave Guide assembly machining has been
completed and is being inspected at the vendor.

We continue to develop the LH control and protection system hardware and
software.

One refurbished klystron, which had been found to have a problem with the
filament, has been returned to the vendor for inspection and evaluation.

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

Martha Redi (PPPL) visited Alcator C-Mod November 25 and 26th, to meet with
MIT physicists Catherine Fiore and Paul Bonoli about recent papers they have
submitted to Physics of Plasmas, as well as on upcoming conference
presentations on C-Mod Internal Transport Barrier Plasmas.

Earl Marmar was in Naka, Japan, Nov 22-25, for two meetings: an ITPA/IEA
workshop on planning of joint experiments; a workshop on "Physics needs for
high beta steady state tokamak". At the second, he gave a presentation
entitled "Approach to Quasi-Steady-State AT Plasmas on Alcator C-Mod".



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