Alcator C-Mod Weekly Highlights Sept. 9, 2002 Plasma Operations continued at Alcator C-Mod last week. Four run days were scheduled, but operations were interrupted late in the second day in order to remove a broken tile from the divertor region. Operations are planned to resume next week. Progress continued on the Lower Hybrid fabrication, which also conducted a status review, and on the Diagnostic Neutral Beam System, Operations ----------- Plasma runs were carried out on Tuesday and Wednesday last week. A total of 21 plasma discharges were produced, with a startup reliability of about 55%. Many of the discharges in these runs terminated with full-current (up to 1.2MA) disruptions, which may have contributed to the low startup efficiency. Tuesday's run was in support of MP#318, "High performance operation at 5 Tesla". The goal was to evaluate combined heating at 70 and 80MHz, with the minority hydrogen resonances on either side of the magnetic axis, at high current such that both resonances were inside the sawtooth mixing radius. The goal of this run was to understand the RF heating and confinement at high input power and plasma current. Discharges were run at 1.0 and 1.2MA, with the toroidal field set to 5.1 tesla, corresponding to q95 ~ 3.5 and 3.0, respectively. Total RF power of up to 4.7MW was injected. The performance in terms of H-factor in these plasmas was relatively low, with H_89 < 1.3, based on the stored energy reported by EFIT. The 1 MA performance was better than that at 1.2MA; the 1.0 MA shots were marginally EDA, while the 1.2 MA shots were in the ELM-free regime, with increasing radiation losses throughout the H-mode phase. H-factors in ohmic portions of all of these shots were below 1, and were systematically lower with increasing plasma current. This may simply be the result of inaccuracies in EFIT for very low beta_p discharges. The reasons for the apparent low performance in both the ohmic and RF-heated phases are under investigation. Wednesday's run was in support of MP#315, "Shape Development for JET/C-Mod Dimensionless Similarity Experiments". The target parameters for this experiment were Ip=1.2MA, a/R=0.30, Bt=5.4T, lower triangularity of 0.45 and upper triangularity between 0.3 and 0.5. An additional goal of these experiments was to determine whether C-Mod could access the EDA regime with these parameters. The combined requirements of low a/R and high current, together with high target density and RF power resulted in high disruptivity in these experiments. Typically, the plasma became vertically unstable after termination of the H-mode, as li increased and beta dropped. Only ELM-free H-modes weer obtained in these discharges. Late in Wednesday's run, following a series of high current disruptions, a broken tile was observed in the divertor region on two of the plasma video views. Shortly thereafter, the run was terminated following a minor vacuum incident. The vessel was vented on Thursday in order to remove what on inspection were found to be two broken BN tiles, which were determined to have come from the top of the J-port ICRF antenna. The tiles were retrieved using special tooling inserted through a flange on K-port, without requiring a manned access. The vessel was then pumped down, and a 120C bake, as well as deuterium ECDC, was begun. Plasma operations will resume this week. As the boronization quality and H/D ratio will have been compromised by the vent, experiments calling for ohmically heated plasmas will be scheduled for the first runs, during which we will evaluate the state of the wall conditioning. The engineering group is analyzing the tile failure. It was noted that BN tiles in a similar location experienced a similar failure during the 2001 campaign, resulting in a redesign of the hold-down scheme which was implemented in the present antenna. Based on the experience from the previous occurrence, the performance of the antenna is not expected to be affected by the tile loss. Diagnostic Neutral Beam System ------------------------------ The DNB was back in operation on Wednesday. The leak that stopped operation the previous week was identified and repaired. It occurred in a flange associated with the cryo-pump and may have been due to thermal cycling. The torque of the bolts on these flanges will be a regular maintenance item. As is typical after a vacuum break, the beam performance was compromised. Approximately seventy-five beam shots have been fired since pumpdown, and beam performance is almost back to normal. Thursday and Friday were spent working to complete the automated LN2 fill system. The LN2 vacuum line is in place and has been successfully tested to forty PSI. The solenoid panel in the cell was tested, and one solenoid was found to be unreliable. The solenoid was replaced and now all solenoids function properly. Stainless steel delivery lines were installed from the panel to the DNB cryo pumps. The entire system was pressure tested to forty PSI and used to deliver liquid nitrogen into a dewar at the DNB with good performance. Monday will be spent insulating the stainless delivery tubes, attaching tube clamps, and setting up the PLC controls for the solenoids. ICRF System ----------- The coaxial switch for FMIT #1 (D-port antenna) was disassembled, checked, repaired and reassembled after arcing problems were noticed earlier this week. The FMIT #1 and #2 vacuum breaker houses were cleaned, the mechanisms checked and the vacuum bottles hipotted to 25kV successfully on Thursday and Friday. These have now been put back into service following this routine maintenance procedure. Cable attenuation for forward and reflected power were re-calibrated. The differences between the recorded and new measurements were <1 dB. Lower Hybrid MIE Project ------------------------ A status review of the Lower Hybrid launcher project was held at PPPL on 9/6/2002. Ron Parker, Steve Wukitch, Rui Vieira, and Bill Beck came from MIT. Joel Hosea, Stefano Bernabei, Doug Loesser, Greg Pitonak (DoE), Joe Rushinski, Gerd Schilling, Skip Schoen, and Randy Wilson attended from PPPL. Project costs and schedule were discussed, and visits were made to the shops and labs where launcher fabrication and assembly are taking place. Work is progressing on developing an easy and reliable method of putting connectors on RG-402 semi-rigid coax. Some custom tooling for this purpose has been produced. Travel and Visitors ------------------- Stewart Zweben (PPPL) was at MIT for the week. While here, he held useful discussions with Jim Terry and Steve Scott on possible edge turbulence control experiments.