Alcator C-Mod Weekly Highlights September 22, 2003 Reassembly of the MIT alternator began last week. Work continued on Lower Hybrid and ICRF systems. All vacuum work was completed, and preparations are being made for plasma operations. Physics ------- Catherine Fiore is working to determine the position of the ITB foot by two methods: by fitting a function to the bremsstrahlung data profile (the sum of a Gaussian and a quadratic form) and taking derivatives to obtain the inflection point and second by calculating rho*p (defined as the ration of the Larmor radius at the ion sound speed to the pressure gradient scale length) and determining the point where this exceeds 0.014, a value established for JET ITB plasmas and verified on other tokamaks. Both methods determine the foot position to be at the same location. This location, when plotted as a function of magnetic parameters of the plasma, shows that the ITB foot location moves inward with increasing magnetic field and increasing q95. It possibly decreases with increasing plasma current. The ITB foot location occurs at a q value (as determined by EFIT) between 1.1 and 1.3. Run Planning ------------- The first follow-up meeting to the C-Mod 2004 Ideas Forum was held on Friday to prioritize Burning Plasma Support experiments to be carried out during the Fall-Winter 2003 campaign. In addition to on-site personnel, collaborators from PPPL and U. Texas participated in the discussion by video and audio links. The group reached agreement on the six highest priority experiments (out of 20 considered), and designated individuals who will be principally reponsible for writing the detailed MiniProposals for these. The other task forces and topical science groups will be holding similar prioritization meetings during the next two weeks. Operations ---------- On Monday and Tuesday tooling was developed and then used with a borescope and video cameras to remove a thermocouple cable that had come free from the outer vessel wall during the last run campaign. Following repair of two gate valves and installation of new diagnostic flanges and gate valves for the boron injector and a new particle detector, the vessel was pumped down on Wednesday. The vessel heater and cryogenic systems were brought back on-line on Thursday and Friday. A vessel bake is planned for next week. The Hybrid Computer has been tested and initial tests of the data highways have begun. A new set of 14 linux workstations are being installed in the C-Mod control room and old VMS systems are being retired. Alternator ---------- GE returned last Monday and began reassembly of the alternator while working double shift days. By Friday all major work was complete with just a few minor tasks remaining to be completed early this week. Lower Hybrid Systems -------------------- Considerable progress was made on the lower hybrid launcher this week. Emphasis was placed on coupler repair, i.e. removal, cleanup, and re-brazing of windows, and elimination of ceramic fractures. The couplers are undergoing a braze etching process to remove the alumina windows. Currently, all visible alloy has been removed, but the bricks are not yet free from the titanium coupler walls and etching is continuing. This process typically takes over 48 hours. Both MIT and PPPL engineers have agreed to modifications to the coupler design that will reduce the titanium wall thickness on the outside of the outer-most coupler windows. This modification will reduce mechanical stresses on the ceramic windows induced by the brazing process. In order to reuse the present titanium coupler grill, another modification will move the ceramic bricks away from the sidewall scalloping that resulted from cleanup of the original braze joints. The first six of a completely new set of alumina windows have arrived at PPPL from the vendor for dielectric constant measurement at 4.6 GHz. These measurements will determine the specification for grinding the windows to the proper thickness. The vendor for window re brazing has performed 18 braze coupon tests to investigate the choice of brazing alloy, brazing temperature, and the capability to wet or restrict wetting of the ceramic. Measurements of the coefficient of thermal expansion of the ceramic at PPPL agrees within 5% of the measurement performed for MIT by an outside firm, and is close to the published value. Measurements of the titanium CTE disagrees with MIT's outside measurement by about 15%, with both measurements being at least 30% higher than the published value. These discrepancies are continuing to be investigated. Model calculations performed by Bill Beck of MIT using the measured CTE values will guide changes to the coupler design. Nevell Greenough, PPPL, spent the week at MIT, performing high-power RF tests of one of the two forward wave guide stacks with Monty Grimes and Dexter Beals of MIT. The nominal power level through one channel is calculated to be ~15 kW when the lower hybrid waves are launched into the C-Mod plasma. Tests were performed on 25 of the 48 channels, with the achievement of 25 kW for 2.5 sec, 70 kW for 0.5 sec, and 100 kW for 10 msec. The latter values tested voltage holding with a strong mismatch. The wave guide test was highly successful, and the unit will be returned to PPPL on 9/23 for reassembly completion. ICRF Systems ------------ The J-Port transmission line has been modified for 50 MHz operation. The system has good decoupling and will allow co- and anti-current drive and symmetric phasing. In addition, the dummy load transmission line has been modified to allow for 50 MHz operation. Travel and Visits ----------------- George Tynan, UCSD, spent a day at the PSFC to discuss data analysis techniques for C-Mod fluctuation data. The goal is to study non-linear wave coupling dynamics in the plasma edge and its relation to the L/H transition. He also delivered a seminar titled "Inverse Energy Transfer and Shear Flow Formation from Collisional Drift Turbulence" Nevell Greenough, PPPL, visited all week working on the high power tests of the forward wave guide stack. Prof. Riccardo Betti of the University of Rochester visited the Plasma Science and Fusion Center on Friday 19 September to give a seminar on the "Hydrodynamics of ICF Implosions". He also visited with a number of C-Mod staff to discuss theories and their comparisons with experimental results on plasma rotation.