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.