Alcator C-Mod Quarterly
Progress Report – February 2002
The primary activities at Alcator C-Mod during the first quarter of FY02 were: disassembly/inspection of the tokamak.
Science
Results
Core
Confinement and Transport
Gyrokinetic Calculations of Microinstabilities and Transport for
Double Transport Barrier Plasmas (M. Redi, D. Ernst, G. Hammett)
Gyrokinetic simulations of plasma turbulence with the GS2
code [1,2] are being carried out to examine H-mode, ICRF heated experiments,
which exhibit internal transport barriers.
Early in such an experiment, as the internal transport barrier (ITB) is
being established via off-axis RF heating, linear electromagnetic (and electrostatic) microinstability
analysis shows that suppression of the toroidal ion temperature gradient mode
(ITG) occurs at and within the ITB, in the presence of an increased density
gradient. TRANSP analysis is used to
set initial conditions, with ion temperatures not assumed equal to measured
electron temperatures, but rather consistent with
ci µ cChang-Hinton, matching the observed neutron data.
The simulations, which solve the gyrokinetic Vlasov-Maxwell system, are
run out to 2000 time steps for zones within, at and outside the ITB. The
microinstability growth rates, g, and real
frequencies, w, are verified to have converged and the usual
measure of the electrostatic potential, f 2, is verified to be increasing in
cases designated unstable, and decreasing for cases designated stable.
Before the transport barrier is established we find that
in the barrier region all modes are marginally stable for 0.2 < k^ ri <
0.8 This is of particular interest in comparing to the heat pulse measurements
which show ceff reduced in the barrier region. Outside the internal transport barrier the
ITG mode is strongly growing.
[1] M. Kotschenreuther, et al., Comp. Phys. Comm. 88,128 (1995).
[2] W. Dorland, et. al., Phys. Rev. Lett. 85, 5579 (2000).
Edge/Divertor
Physics
Particle Inventory
The reliability of fluctuation-induced particle flux profiles computed from the A-port scanning probe has been analyzed for a series of Ohmic discharges. The measured flux profiles and the cross-field heat convection profiles that they imply have been compared to separate estimates of the cross-field particle and heat flux profiles. These separate estimates are based on global particle balance (obtained from measurements of ionization profiles and flows towards the divertor), sound speed flow towards divertor surfaces, and heat fluxes arising from parallel electron conduction in the scrape-off layer (SOL). Similar to results reported from other experiments, it is found that the fluctuation-induced particle fluxes significantly exceed the flux estimates from particle balance models. The implied heat loss from the fluctuation-induced convection also appears to be too high; it would exceed the total input power in most discharges. Also, in discharges where the probe could be inserted well inside the LCFS, the radial-outward fluctuation-induced particle flux was found to continue to increase in magnitude, in apparent contradiction to satisfying particle continuity. These observations and others suggest that the fluctuation-induced particle flux estimates made by the C-Mod probe should not be interpreted as indicative of the transport in the unperturbed plasma. An alternative explanation is suggested: the measured fluctuation-induced particle flux is simply the cross-field plasma flow that fills-in the presheath zone formed by the probe. Two separate estimates of effective cross-field particle diffusivities in the presheath zone have been made. The magnitude of these diffusivities is found to be in the range of typical diffusivities seen in the far scrape-off layer (based on global particle balance estimates), lending some support to this new interpretation of the measurement. However, the presheath diffusivity does not appear to be directly related to the diffusivities found in the unperturbed plasma. Thus the fluctuation-induced particle flux 'measurement' made by the probe appears to provide no direct information on the transport level in the unperturbed plasma.
Operations and
Diagnostics
Magnet Inspection
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 reanalysis and simulation of the coax foot behavior, 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.
The "big lift", during which the machine, consisting of the vacuum vessel, PF coil packages, OH stack, lower wedge plate, and TF core and lower arms, is moved from the center to the southeast corner of the cell, occurred on Friday, 10/12/01.
With the installation of the OH2L coax on Jan. 7, reassembly of Alcator C-Mod was begun. Torque checks were completed on the coax and installation of cover plates, the lower wedge plate, and lower TF arms have been staged. Hi-Pot and ringing tests of all the EF magnets were successfully completed, as were similar tests on the OH2L magnet. The OH1 and OH2U magnets were tested after installation of their respective coaxes was completed. Hi-Pot turn-to-turn tests of the TF core were also completed successfully.
Active MHD Antenna
The Active MHD antennas have been redesigned to move them further from the plasma, to make the design more robust mechanically and electrically, and to cover them with boron nitride tiles to withstand high plasma heat loads. The new design employs a stainless steel Bitter-type plate per turn insulated with ceramic coated stainless steel washers and firmly bolted together with ceramic coated bolts. This overcomes potential cracking of ceramic tube insulators in the old design and makes a stronger antenna to better withstand disruption forces. The new design also permits taking apart the antenna should maintenance ever become necessary. Due to the high cost of the boron nitride protection tiles and the limited budget, only two Active MHD antennas will be built this up-to-air and installed at GH port. The remaining two antennas will be built during FY2003 to be installed at EF port next to the E-port ICRF antenna. Prototypes of the amplifiers to drive the antennas from 1 kHz to nearly 1 MHz have been built and higher power versions capable of driving 20 A of current in the antennas are being designed. A prototype matching network was also built that permits 20 A of current to be driven by the existing 400 W amplifier that operates from 80 kHz - 2.7 MHz. These antennas will be used to drive stable modes in the plasma to determine the proximity of the plasma to different types of unstable modes such as lower frequency tearing modes or high frequency Toroidal Alfven Eigenmodes in the presence of an ICRF driven fast iontail.
Computing
Data Infrastructure Upgrade
---------------------------
Three unix workstations have been installed in the C-mod control room. These machines are prototype client workstations for the planned migration from OpenVMS to unix. Users have begun porting their applications, and we plan to move a significant portion of our intershot computing to these platforms before the start of the next campaign.
Diagnostic Neutral
Beam
Consortium RFX has generously offered to loan their DNB injector to Alcator C-Mod for a period of 2 years, as part of a joint scientific collaboration. Agreement has been reached with the Budker Institute in Novosibirsk, where the injector was built, to deliver the RFX DNBI directly to MIT, and to assemble and commission the beam on C-Mod during the present shutdown. This new 50 kV DNBI utilizes a cold-cathode source and according to specifications will produce a 3 amp-equivalent hydrogen atom beam with a full-energy fraction of at least 90%. This should increase the MSE signals by more than an order of magnitude.
Major repairs and improvements are also being made to the MSE/BES in-vessel optics assembly, which should increase overall signal levels by an additional factor of 2-3, thus making it possible to measure B-field angles with a resolution of 0.1 degrees in the core of a wide range of plasmas. A new high-throughput spectrometer and detector is being installed for the CXRS diagnostics. In addition, the high-resolution edge CXRS is being expanded from 6 to 25 spatial channels to cover the entire H-mode pedestal region.
Lower Hybrid
Project
(see separate report)
Collaborations/Participation
in the Fusion Science Community
Ideas Forum
The Alcator C-Mod Ideas Forum was held on December 12 and 13. The purpose of the forum was to provide an opportunity for presentation of specific ideas appropriate for experiments during C-Mod's 2002 run campaign. As usual, the Forum was open to all interested parties, including current and potential future collaborators. Ideas were solicited in six "Task-force-areas":
Advanced Tokamak Issues
Burning Plasma Support Issues
Transport Physics
Divertor/Edge Physics
MHD Physics
RF Physics
A total of seventy ideas were presented. Ideas came from nine different institutions. In addition to the Plasma Science and Fusion Center at MIT, the other eight contributing institutions were:
General Atomics
FRC at the Univ. of Texas
PPPL
UCSD
LLNL
Univ. of Toronto
Univ. of Maryland
Colorado School of Mines
This was the first C-Mod Ideas forum for which all presentations were electronic, with full PictureTalk, audio, and video feeds. This accommodated a number of remote presentations from GA, PPPL, and U of Texas. All of the presentations are available for download as pdf files on the WEB at
http://www.psfc.mit.edu/people/terry/Forum_agenda.html
Additional information about the Forum can be found at
http://www.psfc.mit.edu/people/terry/Ideas_Forum.html
ASDEX Upgrade
We continued our remote collaboration with ASDEX Upgrade on matching pedestal conditions. Dedicated C-Mod experiments were run in July 2001. AUG has resumed operation after a maintenance period. On Friday, Jan 11, they began the first part of their comparison experiments, focused mainly on shape development. They obtained a good match to the dedicated C-Mod shots and also, unexpectedly, were able to closely match the shape of our more typical EDA discharges. Both NBI and RF heating were used. Further experiments aimed at matching dimensionless plasma parameters and comparing ELM behavior will continue later in their winter campaign.
APS
At the 2001 APS meeting in Long Beach, CA, Martin Greenwald presented a review talk entitled, ’Density Limits in Toroidal Plasmas’. There were three invited orals reporting on C-Mod research: ‘An Experimental Study of ICRF Wave propagation and Mode Conversion to IBW by Phase Contrast Imaging in Alcator C-Mod’ by Alex Mazurenko, ‘Double Transport Barrier Experiments on Alcator C-Mod’ by Steve Wukitch and ‘Edge Turbulence Imaging in Alcator C-Mod’ by Stewart Zweben. There were an additional ten contributed oral presentations and 24 posters.
Domestic Travel
Amanda Hubbard went to State University of NY at Albany on Nov. 2, and gave a Physics Department seminar on "Understanding and Improving Energy Transport in Fusion Plasmas."
Ian Hutchinson and Robert Granetz attended a post-APS workshop on stabilization of MHD, which was held at GA the week of Nov. 12. The primary focus was on using rotation and active magnetic feedback to reduce the amplitude of resistive wall modes (RWM) and neoclassical tearing modes (NTM), which currently limit the maximum beta achievable in a number of tokamaks. Other topics included the use of flowing lithium walls to stabilize the RWM, and theoretical work on RWM's and NTM's. In addition, Ian gave an invited talk on measurements of torque and mode locking in Alcator C-Mod.
Martin Greenwald traveled to PPPL on November 14-15 to participate in a meeting of the NCSX program advisory committee. He took the opportunity to discuss plans for the national fusion collaboratory with NSTX staff.
Miklos Porkolab attended the Snowmass planning meeting at Livermore on Nov. 27,28, and presented plans for the RF subgroup activities, together with Don Batchelor of ORNL. Miklos and Don are the Wave-Particle Interaction Subgroup conveners for Snowmass.
Earl Marmar was in Livermore Nov. 27-28, for a meeting of the Next Step Options Program Advisory Committee, which advises the Director of the VLT on efforts to design next-step magnetic-confinement fusion facilities for the U.S. program
Steve Wolfe and Ron Parker participated in the DIII-D Physics Advisory Committee (PAC) meeting in San Diego the week of Nov. 31.
Steve Wukitch, Jim Zaks, Dave Terry, and Montgomery Grimes attended SOFE the week of Jan. 29 in Atlantic City, NJ. Steve presented an invited talk on C-Mod with an accompanying paper. Jim, Dave, and Montgomery all presented posters, also with accompanying papers. Peter Titus, PSFC Fusion Engineering and Technology Division, also discussed analysis results of our divertor and antennas.
International
Travel
Montgomery Grimes and Peter Koert visited Backnang, Germany the week of Nov. 31 to discuss two major RF components with a vendor. The circulators for the Lower Hybrid Project were reviewed, and plans for the loan to MIT of a fast ferrite tuner for our ICRF program were discussed.
Near Term Plans
Following the Ideas Forum, the list of experiments was distilled down to 32 run days. The list of these runs may be found at:
http://www.psfc.mit.edu/people/terry/task-force_priorities.pdf
Nine run days dedicated to AT support have been allocated, which include ITB studies and density control. Six run days for ICRF optimization of heating and current drive are planned. There are five run days for BP support, which include high performance plasmas, and support for FIRE and Ignitor. The remaining nine days will include transport, MHD and divertor studies, with an additional three contingency run days.
Schedule
The short term schedule may be found at:
http://www.psfc.mit.edu/cmod/operations/short_term.pdf
This calls for vacuum pumpdown at the end of March.