From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroup:
sci.physics.plasma
Organization: MIT
Alcator C-Mod Weekly
Highlights
May 29,
2002
First pumpdown of Alcator C-Mod occurred on Monday
of last week, followed
by several days of leakchecking. Work has continued on the Lower Hybrid
High
Voltage Power Supply, ICRF systems, and the engineering system
required
for Plasma operation. Diagnostic
calibrations and installations
continued.
Physics
-------
Yijun
Lin presented results at the Physics meeting on his analysis of a
short
wavelength rf wave (SWW) observed by Nelson-Melby during his
PhD
research. The SSW is a low-field
side ion-ion resonance that
occurs
in a multi-species plasma.
This wave also appears in 2-D full wave ICRF
simulations, such as
TORIC and AORSA. The wave has been recently
identified
as a slow warm plasma ion cyclotron electromagnetic wave
of
the 'light' species.
The
SWW can be calculated by solving the full-electromagnetic dispersion
equation
together with a cylindrical wave vector transformation due to the
poloidal
B field. The wave solution can re-produce most features
shown in TORIC,
such as the wavelength and propagation distance . The root
can also be
approximately obtained from the slow wave dispersion equation.
The wave is
damped by both electron Landau damping and ion cyclotron damping.
The
wave propagation distance is highly sensitive to poloidal B field, and
also
depends on the plasma composition, density and temperature. Large
poloidal B field, high
concentration of the 'light' species, low plasma
density and low
temperature favors the SWW. The SWW is
similar to a wave
branch found in D-T plasma with minority tritium shown
in an early paper
[F. W. Perkins, Nuclear Fusion 17(6), 1977]. As shown in the TORIC
simulation, the
SWW carries comparable (or even larger) power as the
mode-converted Ion
Bernstein Wave (MCIBW).
In the conventional ICRF picture, only the
fast magnetosonic wave and MCIBW
are thought to be important, therefore,
the findings and confirmation of this
short wavelength wave have very
significant implication on the ICRF
mode-conversion physics and other RF
applications, such as current drive
and plasma flow drive.
J.
Snipes visited IPP-Garching from 6 - 8 May 2002 to participate in High
Density
H-mode experiments on the W7-AS stellarator and compare their results
with
the EDA H-mode on C-Mod. This is a new
regime recently discovered on the
W7-AS stellarator after divertors were
installed, first called Improved
Confinement and now called HDH, which has
some similarities with the EDA H-mode
on C-Mod. It is an improved energy confinement regime that occurs only at
high
density and is characterized by relatively low particle confinement,
which
keeps impurities from accumulating in the core of the plasma. Dr. Fritz Wagner
requested a visit to
W7-AS by a C-Mod scientist to compare this new regime with
the EDA
H-mode. A clear transition from a low
energy confinement mode to a
higher energy confinement mode was observed
as the target density increased
shot-to-shot above about 1.6 x 10^20
m^-3. The energy confinement
doubled
above this density threshold and the core radiation
decreased. No evidence of
a
Quasi-Coherent Mode (QCM) as found on C-Mod has yet been observed, but
their
diagnostics are not setup to measure very short wavelength
modes. A scanning
magnetic probe
similar to the one we use on C-Mod is not being built and should
be
installed within a couple of weeks to be able to measure short wavelength
modes
in the plasma boundary.
Operations
----------
On
Monday, 5/21, a first pumpdown of Alcator C-Mod was performed
followed by
several days of leak checking. Several
leaks were found and
fixed. On
Thursday the vessel was brought back up to air to complete
MSE/BES
calibrations and to complete other invessel work. All invessel
work was completed by Friday evening. Igloo blocks were installed during
the
week, heater and TC cables were run, and diagnostics continued to be
installed
and aligned. We continue to checkout
and bring on line ACCESS,
ECDC, CRYO, HEAT, and other engineering
systems. Liquid nitrogen has been
added
to the sump for the first time, and the LN2 pump operated
successfully
that supplies nitrogen to the magnets.
On Saturday, 5/25, the
neutron calibration was performed, and on Tuesday,
5/28, following a
re-weld of a leaking tangential port, the machine was
pumped down. Preparations are now being made to bake the
machine.
Lower Hybrid MIE Project
------------------------
Good
progress is being made on the HVPS.
Several full load pulses without
any faults have been
obtained. Wire tests are being
performed to verify
fault protection for the klystrons.
ICRF
----
New
wiring lists for the RF cell racks were completed, and fault system
cabling
in RF Cell Rack #2 is in place and connected.
DFOL chassis were
installed in RF Cell rack, and the wiring was
completed. The CAMAC RF
auto-tune
system wiring was completed.
Cross-connect blocks have been also
installed in the rack and the
wiring has been started.
The RF Cell phase-shifter/stub tuner
control wiring is in progress.
Pressure sensor connectors for the
shifter/tuners are ready for wiring.
Calibrations continued on the
demodulators.
RFX DNB
-------
The DNB gas system
component fabrication is complete and ready for
final assembly and a leakcheck. An isolated ethernet connection
has been
provided for the DNB computer.
Isolation is required since the computer
operates at the DNB ground
potential and proper, safe operation of the DNB
requires single point
grounding.
The data for calibrations for CXRS, MSE, and BES was
collected. The
data for the CXRS
calibrations was completed first, and analysis is
nearing
completion.
Work has started on the LN2 delivery system. The LN2
level sensors that
will provide a high level indication at the cryo pumps
are assembled. The
cryo panel that will mount in the cell behind the DNB
is currently being
designed.
New beam duct bellows supports
have been fabricated and installed.
Diagnostics
-----------
The
FRC high resolution ECE diagnostic is almost ready for plasma
shots. Waveguide was measured and alignment of the
waveguide
supports completed; only cutting of the waveguide remains. CAMAC was
installed, tested, repaired,
re-installed, and is now working.
Signal
conditioning modules, triggers and PC data acquisition were
also
installed.
Travel and Visits
-----------------
Tom
Fredian attended a two day National Fusion Collaboratory workshop held
at
PPPL this week.
Perry Phillips, UT-FRC, was here last week working on the ECE
radiometer.