Alcator C-Mod Weekly Highlights Sept 29, 2003 The maintenance period at Alcator C-Mod continued last week. Progress continued on all systems in preparation for resumption of tokamak operations in early October. There was also considerable progress on the Lower Hybrid Project. Operations ---------- The vessel was subjected to a 120 degree C bake beginning on Monday and lasting through the weekend. Electron Cyclotron Discharge Cleaning (ECDC) in deuterium was carried out simultaneously with the high temperature bake. The alternator reassembly has been completed and the unit is expected to return to service this week. The machine was on turning gear over the weekend. Some instrumentation work remains to be completed prior to operation. ICRF System ------------ Work began on tuning transmitter #4 (J-port) to 50 MHz in preparation for plasma operation at that frequency in October. DNB Systems ----------- The diagnostic neutral beam system is being readied for resumed operation. Instrumentation in the LHe dewar was repaired and re-installed. The vacuum system is reassembled and ready for startup. A new turbopump has been installed. A new connector flange that provides additional instrumentation between the gate valve and the pump was installed. The new 2d cxrs detector was installed and initial alignment completed by Bill Rowan (UT-FRC). The detector will allow the number of spatial channels which can be used simultaneously to increase to 40. Lower Hybrid System ------------------- The Lower Hybrid launcher work continued this week. Dielectric constant measurements have been completed for four new ceramic window test samples at the operating frequency of 4.6 GHz. The measurements result in a slight reduction in window thickness, and the new dimension has been passed on to the manufacturer for grinding. Removal of the old windows from the first coupler through acid etching of the braze material has progressed slowly. Seven bricks have been removed, nine bricks are loose but not yet out, and the remaining eight bricks show acid flow past the periphery. We are attempting to speed up removal by purchasing baffling, acid pumps, and acid heaters/controllers in order to direct acid flow better, increase the acid temperature to raise the etching rate, and allow all remaining couplers to be etched in parallel. This is expected to speed up this process considerably. Our brazing vendor has performed eight additional braze coupon tests, six with ceramic and two with bare metal. These tests concentrated on the two alloys which had performed best in the initial investigation, Gapasil and Silver ABA. Preliminary indications from a tapered gap test show good wetting and filling. The Silver ABA seems to wet the titanium metal better, but also wets the alumina better, which might lead to a greater cleanup effort. A test summary report is expected from the vendor by 9/29. Work is continuing with Bill Beck (MIT) on the concept of thinning the coupler outer walls to reduce mechanical stresses on the ceramic. Two thinned-wall concepts will be braze tested starting next week, with a total of six to seven 3-channel samples. The braze alloy will be chosen after receipt of the test summary report. Bill is also investigating measuring post-brazed stresses. The Forward Wave Guide that was successfully power-tested at MIT has been returned to PPPL. Positioning holes are being reamed for final assembly, to be followed by trial fit-up with flanges. A final assembly procedure is being generated, and will be reviewed with MIT. At MIT, work last week centered on testing of the circulators and the reworked water-cooled dummy loads; improvements to these loads made by the vendor to bring performance up to specification include silver plating and increased water flow. The tests were carried out with the assistance of an engineer from the vendor. After some on-site modifications, both loads successfully passed full power, full pulse length tests. The next tests were with the circulator terminated into a matched load. Full power and pulse width operation was achieved. The output was then terminated with a shorting plate (to simulate an arc) with the water load to Port 3 of the circulator. Full power and full pulse width were again achieved. During the last of the 5 second circulator tests, problems were encountered with the klystron V3 of cart 2. The output power was found to be low early in the pulse and then increasing in time. This behavior may be indicative of the tube becoming "gassy". We are considering means to remedy this situation. Meanwhile, the two rebuilt klystrons received earlier from CTL are being perpared for installation and testing. Visitors and Travel ------------------- Garry McCracken visited the PSFC Sept 25 and discussed edge physics with the edge group as well as the prospects for ITER. On Wednesday of last week, Earl Marmar, Martin Greenwald, Bruce Lipschultz and Miklos Porkolab travelled to Germantown to meet with OFES staff. Discussions centered on recent research highlights and plans for C-Mod. Attending from DoE were John Willis, Rostom Dagazian, Erol Oktay, Curt Bolton, T.V. George, Steve Eckstrand and Arnold Kritz. Catherine Fiore, Dimitri Mossessian, Amanda Hubbard, Miklos Porkolab, Darren Ernst and Jerry Hughes (MIT), and Perry Phillips, Alan Lynn, and Matt Sampsell (UT-FRC) attended the 9th Annual IAEA TM on H-Mode Physics and Transport Barriers 9/24-9/26 in San Diego. Work was presented on control of ITBs in Alcator C-Mod (Fiore), and the role of TEMs and neoclassical transport in Alcator C-Mod ITBs (Ernst) . C-Mod/DIIID/ JET dimensionless scaling results for the edge pedestal were reported by D. Mossessian and the role of plasma and neutral transport in the edge pedestal in Alcator C-Mod was explored by J. Hughes. A. Hubbard presented her work on local threshold conditions and the fast transition dynamics of L-H mode transitions in Alcator C-Mod. Phillips, Lynn and Sampsell presented posters on "Observations of Core Modes during RF-Generated Internal Transport Barriers in Alcator C-Mod" and "Measurements of the Quasi-Coherent Mode on Alcator C-Mod"