From:
irby@PSFC.MIT.EDU
Reply-To: IRBY@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups:
sci.physics.plasma
Organization: MIT
Alcator C-Mod Weekly
Highlights
February
15, 2002
We continue reassembly of Alcator C-Mod. Work also continues on the
new DNB
installation and the Lower Hybrid MIE Project.
Physics
-------
The
PAC presentations are now on the WEB, with links at:
http://www.psfc.mit.edu/cmod/sciprogram/Cmod_PAC_2002.html
We
report here the completion of level 1 Alcator C-Mod program milestone
75.
The experimental work was completed
during the Summer 2001 campaign,
which ended on August 3, 2001. We have also now completed the required
analysis,
and a short report is included here.
The milestone
as extracted from the March 2001 Alcator Project Work
Proposal is:
75.0
Evaluate density control (Target date AUG 2001)
Evaluate potential methods to control wall-gas inventory
and
target plasma density in
anticipation of long pulse operation.
Wall
reservoirs for particles will be characterized and potential
conditioning techniques and fueling
options will be evaluated.
Both active and passive density control
options have been evaluated in
detail.
In passive density control, the wall is conditioned such that it
acts
as a 'pump' during a discharge. Thus,
it is important to establish
the capacity for the wall to pump and methods
to condition the wall between
discharges. In active density control, a cryopump or other activated surface
(e.g.,
getter) is employed. In order for these systems to work they must have
both
sufficient pumping speed and capacity to handle the gas-load during a
discharge.
The gas load is set by the plasma/neutral inventory in the chamber
and the
capacity of the wall to hold neutral gas (since it may evolve
from the
wall during a discharge).
A) Wall Condition and Wall Pumping
Capacity Experiments:
Wall condition and wall pumping capacity
experiments were performed to assess
helium glow discharge cleaning as a
technique to condition the walls for passive
density control (MP273,
"Effects of Glow Discharge Cleaning on Boronization
and Wall
Conditions"). Helium glow discharge cleaning was performed over a
weekend
and between discharges for 1/2 hour intervals. If was found that even
with
the most extensive use of helium glow, the wall could only be depleted
by
about 30 torr-liters of deuterium gas. A single shot
following helium
glow was sufficient to reload the wall. Thus, the idea of
conditioning the
wall for pumping in C-Mod is not a viable option. Similar
conclusions were
drawn from experiments that assessed the rate of change-over
from
deuterium -> hydrogen -> deuterium (MP274,"D/H Changeover").
B) Active Pumping Options:
We have found that the only
pumping techniques that have sufficient pumping
speed and capacity for
C-Mod are cryo-panels and liquid lithium surfaces.
B.1) Liquid
Lithium Surfaces:
On Tuesday April 3, 2001, a 1-day meeting was held to
discuss ideas for
using liquid surfaces for pumping hydrogen in Alcator
C-Mod. The participants
included: Mike Ulrickson (SNLA), Dennis Whyte
(UCSD), Dick Majeski (PPPL),
Jeff Brooks (ANL). It was concluded that
liquid lithium surfaces may provide
a large pumping speed and capacity.
However, the technology is in the
development stage. The risks associated
with the use of liquid lithium
in C-Mod were found to be unacceptable at
the present time.
B.2) Cryopump:
A prototype cryopump has
been successfully operated in C-Mod, yielding ~1000
liters/second pumping
speed for a 1/10 size test unit. This technology is
well established and
utilized in many tokamak experiments. It is desirable
to increase the
surface area of the cryopump and to simplify its design.
Towards this end,
we have investigated the possibility of locating a full
toroidal loop
cryopump in the upper divertor region. Recent analysis of
unbalanced
double-null experiments shows that the pressures in the upper
divertor
can be sufficiently high to enable a pump to be operated there.
Follow-up
experiments are planned during the 2002 run campaign to scope the
neutral
pressures in the upper divertor for a range of upper x-point locations.
B.3)
Long Pulse Experiments:
We have operated C-Mod with pulse lengths
extended to 3.4 seconds, in part to
assess gas evolution from the walls
(results presented at APS, Long Beach,
2001,
www.psfc.mit.edu/cmod/sciprogram/Aps/Aps2001/Wolfe_poster.pdf).
It was
found that the wall maintained a moderate level of pumping throughout
the
3 second pulse. Target densities did not exceed the programmed densities.
However,
owing to volt-second limitations, these long-pulse discharges were
performed
at low to medium densities. Wall saturation & degassing effects,
if
they are to be seen at all, might occur in higher density and higher
power
discharges. In any case, from the wall capacity experiments as
described above, we have a good estimate on the maximum wall inventory
that
could be degassed during a discharge, ~ 30 torr-liters. On this
basis, we have
the information we
need to start a conceptual design for a full-scale
cryopump in
C-Mod.
Operations and Power Systems
----------------------------
Reassembly
of Alcator C-Mod continued this week with the installation of
nearly all
the lower TF springplates. Following
completion of the
springplate installation, we expect the machine to be
craned back into
position on the lower dome mid next week. Extensive measurements of the
TF joint
resistances must also be made before the machine is moved.
A picture of
the machine in the Southeast corner of the cell can be found
at:
http://www.psfc.mit.edu/cmod/operations/EngImages/Inspection_2001-2002/Disassembly_Assembly/Machine_assembly_A/DCP_0143.JPG
Machining
of the cylinder to provide better tangential access for
diagnostics began
this week. Estimates of
time-to-completion for this
process are being made.
A careful
inspection of the Alcator C-Mod TF magnet, OH coax,
and most other machine
components was carried out between Aug
2001 and Jan 2002. The TF magnet horizontal arms and vertical
legs
were found to be in excellent condition. After the TF fault in 1998,
changes to the springplates and
springplate pressure, plating of the
sliding joint surfaces, and the
addition of a graphite coating
to the feltmetal, have resulted in very
much improved performance
of the TF magnet. Several feltmetal pad locations
indicated that more graphite
would be useful, and we are therefore
increasing the coating thickness at
all joint locations during
reassembly.
These locations showed a somewhat frosted appearance
indicating
more interaction with the opposing TF finger than expected.
Inspection
of the OH coax connections indicated a reduction in the
preload on several
of the bolts; by in one case as much as 33%.
Damage to the bolt threads
was also found as well as to the helicoil
inserts into which they
engage. After re-analysis and
simulation
of the coax foot behaviour, and extensive testing and
measurements
of the coax components, several changes were made to the
coax connection. High strength
bolts are now being used and
the engagement of the bolts with the inserts
was increased up to a
factor of two.
The Belleville stacks that maintain the preload have
been changed
to provide more compliance, and wedges have been added
to restrain
movement of the top of the coax foot.
These changes will
improve the reliability of these crucial
components.
More generally, heater and thermocouple leads, TF
instrumentation
cabling, LN2 cooling lines, and bus components are all
being
refurbished as part of the the inspection/reassembly effort.
Lower Hybrid MIE Project
------------------------
PPPL
continues to direct fabrication by outside vendors of LHCD launcher
components. Shop visits are being made as needed to
resolve issues or
check status. A
second three-waveguide launcher prototype has been
fabricated, and an
integrated splitter-coupler system test is being
assembled. This test will check out the E-taper, 3 dB
divider,
H-taper, phase compensation, reflected power probes, possible
RF
leakage from the bolted plates, and performance of the gold seal
at
the coupler nose under vacuum.
The same setup will be used to test a
preliminary design of the
reflected power water load presently under
construction. The setup will then be tested at high power
at MIT.
All twelve klystrons needed for Phase 1 of the LH MIE
Project are now
installed in the Cell.
A photo of one of the carts installed in the
cell can be found
at:
http://www.psfc.mit.edu/cmod/operations/EngImages/RF/lower_hybrid/DCP_0142.JPG
Wiring
continues for the transmitters in the Cell. Transmitter #3 wiring
is
approximately 75% complete. HVPS
commissioning continues.
A full voltage (50kV) pulse lasting nearly 5
seconds was applied
to the dummy load.
Preparations are underway for the HVPS wire tests.
A Ross relay is
being prepared to short the supply with a short piece of
small diameter
wire. This process will, among other
things, test the fault
protection of the supply. Design of the input RF switch assembly was
completed this
week. The parts to assemble this switch assembly were ordered.
Cabling for
the transmitter fiber-optic connections was ordered.
ICRF
Systems
------------
Machining of the D and E-Port Faraday
shields continued this week, and is
nearing completion. Work on the J-Port antenna current straps
also continued.
DNB
and Related Diagnostics
---------------------------
The RFX
beam components (29 crates of equipment) were received this week and
were
stored in anticipation of arrival of the installation and commissioning
team
from Novosibirsk. Visa problems have delayed
their arrival by a few days
from the original date of February 17. Testing of PLC software for
the vacuum
system continued. Output signals from
the PLC were tested.
Progress was made on the fueling system
hardware. Matt Sampsell of UT-FRC
visited
for several days to work on mirror calibration for the toroidal CXRS.
Travel
and Visits
-----------------
Cynthia Phillips attended the ITPA
meeting in Naka, Japan, February
6-8. At the meeting, she presented
results from the C-Mod 2-frequency
ITB experiments and plans for the LHCD
program.
Miklos Porkolab was in San Diego this week where he served
on the Program
Advisory Committee for the Virtual Technology Lab.