Alcator C-Mod Weekly Highlights August 12, 2002 Plasma operations continued at Alcator C-Mod last week, with four run days scheduled and completed. Physics experiments were carried out in the Transport and Edge/Divertor Topical Areas. ICRF antenna conditioning continued. Progress continued in DNB Systems, Diagnostics and the Lower Hybrid Project, Plasma operations are scheduled to continue next week. Operations ---------- Plasma operations were carried out Tuesday through Friday last week, with a total of 71 plasma discharges produced with a startup reliability of ~70%. Improvements in the cryogenics system and cooling cycle have led to significant increase in the attainable shot rate, with up to 37 machine pulses produced in an eight-and-a-half hour run day. Wednesday's run was primarily devoted to MP#306, "Density Limit Physics and Turbulence Measurements". The experiments extended our previous work in two ways. They utilized low power Ohmic discharges run on the outer limiter. This configuration allowed unambiguous determination of the last-closed flux surface and allowed maximum penetration of the scanning probe into the closed flux surface region, to best follow the evolution of the profiles as the collapse is approached. The experiment followed the approach used in previous runs where, after establishing a high density discharge, the plasma current is ramped down to reach the density limit at a predictable and reproducible time. In addition to the scanning probes, we also monitored edge turbulence with the new 28 frame PSI fast camera. The scenario we outlined resulted in strong marfes appearing at about 70% of the usual "Greenwald" limit followed by a sharp density rise and a disruption. We successively scanned the A-side probe, reaching past the last closed flux surface by almost 3 cm. ISat was bursty for all cases, though the correlation time shortened from 30 usec to about 10 usec as the probe was inserted more deeply. At the same time, we got some good data with the psi camera. Blobs generated from within the lcfs were observed. Also on Wednesday, in a piggyback experiment, MSE measurements were taken early in the discharge while the q-profile was still evolving, and when the density was still modest. Good MSE data was acquired over times 0.05 - 0.60 sec, during which the engineering qedge as deduced by EFIT ranged from >7 to about 4.7. This data will be useful as benchmark/calibration for the MSE shakedown. In another piggyback experiment on Wednesday, the new Inner Wall Scanning Probe (ISP) was successfully operated into the SOL for the first time. This probe assembly, which has a rest position in a recessed section of the inner wall limiter, is activated by pulsing current through a solenoid so as to produce an IxB torque which rotates the probe on a pivot, bringing the Langmuir probes on the tip past the limiter surface and into the SOL. Initial results with the ISP on Wednesday were highly promising, and the probe was then employed as one of the key diagnostics on Friday's experiment. Friday's run was dedicated to MP#311, "Spatial Dependence of Edge Turbulence Statistics". The primary purpose of these experiments is to explore the radial and poloidal variation in the statistics of edge plasma fluctuations. In particular, plasma fluctuations were recorded in the `near' and `far SOL' locations and on the high and low-field sides of the torus in a series of ohmic L-mode discharges. The experiments were also designed to accomplish a number of secondary goals: to compare the SOL density, Te, and parallel flow profiles on the high-field and low-field sides, and to explore the influence (if any) of the new inner divertor configuration on the conditions in the SOL and divertor. The experimental plan called for a density scan with Ip=0.8MA, Bt=5.3T, in a standard lower x-point equilibrium. For each density point, three shots with different probe configurations were planned. All three scanning probes, the ASP which scans radially near the midplane, the FSP which scans vertically, and the new ISP which scans the inboard SOL from the inner wall, were used, as well as the fixed probes in the divertor tiles. In addition, fast diodes and the new PSI fast framing camera viewed gas puffs from capilaries located at the inboard and outboard midplane. The first part of the run was spent optimizing the equilibrium and probe timing and insertion depths. In the course of determining the maximum "safe" depth at which the ASP could be operated in dwell mode, i.e. located at a fixed position rather than in a rapid scan, the "north" probe incurred disruption damage, preventing us from inferring poloidal fluctuation phase velocity or fluxes from the ASP data for this run; the other three probe tips remained operational. The ASP will be removed from the machine for repairs on Monday, and should be back online in one to two weeks. The first part of the density scan, 1.0< nebar <2e20 m^-3, was substantially completed, although some long-time data samples taken by the ASP were corrupted. The ISP was operated successfully and recorded interesting data concerning the profiles and turbulence behavior in the inboard SOL. The probe was operated as far in as the nominal separatrix. The maximum probe angle was kept under 70 degrees in these experiments; future experiments in which the ISP is rotated past 90 degrees may provide data on parallel flows. Swept I-V characteristics from the ISP are interesting. Fluctuation levels on the inboard side are clearly lower than outboard. The plasma profiles seem quite flat after the probe penetrates through a short (~2mm) scrape-off layer. These data are being analysed. Additional runtime is being requested for the completion of this experiment after repairs to the ASP are accomplished. ICRF Systems ------------ The antenna operation improved this week during conditioning runs on Tuesday and Thursday. The maximum injected power was ~3.8MW (1.5 MW from D and E-port and 0.8 MW from J-port). After obtaining 1.5 MW on D and E, we concentrated on J-port and lowered the toroidal field to 4.6 T. We obtained 1.8 MW for ~0.1 sec from J-port. We also confirmed that the improved J-port arc detection functioned properly and that the phase feedback appears to require 15 msec to lock. J-port appears to require more conditioning to become functional for physics operations. During operation, we identified a malfunctioning J-port DC1 arc detector module. We successfully changed to the new demodulator/arc modules for DC1 arc monitoring to remedy this problem. After changing these units, we did vacuum conditioning on all three antennas. When we finished, the pressure rise associated with D and E was below detection limits but J-port was indicating a measureable gas puff. Each antenna was reconditioned to ~35 kV and the process was much easier than it has been in the recent past. To further improve operations, we plan to vacuum condition before plasma operation next week. In particular, we will condition the antennas to 35-40kV for repeated 1 sec pulses (up to 25 pulses). This is to assess the reliability of operating at high voltage. Furthermore, we will investigate the density increase associated with D and E-port in single null plasmas. We have observed that the density rise with 2 MW into single null plasmas was negligible, but there is up to ~10% increase in density for limited plasma discharges. The Spinner load (borrowed from GA) has been sited in the power room between the J-port stub tuners and phase shifters. This unit will allow long pulse testing of the transmitters at high power in preparation for long pulse plasma operations. We have begun to make preparations for water and electrical service to this unit. The coax for connecting the load to the coaxial switch will be specified in the next week. DNB Systems ----------- The DNB continued to operate into C-Mod plasmas this week with very reliable performance. In addition to our standard operation, we were able to modulate the beam at 14ms on 4ms off for improved MSE background emmission information. A "roof" has been installed over the DNB power supply racks to protect against any condensate drips from the air handler. We have a greater understanding of the time delay problems that we experienced last week. During commissioning, the BINP software engineer installed software buttons to allow us to trigger the beam via software or via hardware trigger. If the beam is fired using the on-screen software trigger and then switched to hardware trigger mode, the next shot will experience a time delay. However, all subsequent shots will have no time delay. We are able to work around this problem by firing conditioning shots via remote trigger only. The BINP software engineer is aware of this behavior, and should give us some feedback shortly. Work continues on cryogenic line installation. We should be able to complete the cell and diagnostic lab portion of the installation within the next two Mondays. The DNB injector is now operating very reliably into nearly all plasma discharges. This week we have started modulating the beam in order to better measure the background emission during the DNB pulse. BES measurements during beam-into-gas (after disruptions, for example) show a nearly Gaussian beam profile with a 1/e full width of 7.4 cm (spec is 6 cm). BES measurements at the plasma edge are dominated by high background plasma emission, so next week the BES views will be changed to look farther into the plasma. During the past week, the MSE has been optimizing its filter settings for peak performance, and preliminary pitch angle profiles have been measured with a relative resolution of <0.1 degrees. We took the opportunity this week to take MSE measurements at a series of times early in the discharge startup when the q-profile was still evolving and when the density was still modest. These data are being used for benchmarking the MSE. Lower Hybrid MIE Project ------------------------ Wiring was completed on the new 24VDC power supplies for the flow meters on transmitter #3, and DC testing of the supplies was completed successfully. The flow meters were tested and calibrated on Friday. Wiring of the added current sensor is complete. Testing of the fast and slow body current fault circuits is planned for Monday. After researching cost and availability issues, the semi-rigid cable RG-402/U was chosen for use on low-power RF circuits. Some sample connectors have been ordered to evaluate fabrication and tooling issues. A meeting with the contractors concerning out-of-spec water pump parameters took place on Thursday. The main topic of discussion was the high stall pressure of the smaller pump which was more than anticipated. The issue is that a condition might arise, due to operator error, that could result in pressures high enough to potentially damage high-value components. MIT proposed a pressure relief valve installed on the main bypass circuit rather than having the pumps removed and modified. The issue is still open. Diagnostics ----------- On Monday, the magnetics data acquisition underwent a massive reconfiguration to incorporate the new compact PCI-based digitizers (CPCI) and to finally retire the troublesome CAMAC digitizers. There were literally hundreds of signal cables changed in the magnetics racks, many data acquisition modules changed or removed or added, and hundreds of corresponding software changes that had to be made in the magnetics MDSplus tree. During the run on Tuesday, the debugging of this new system required just 4 test shots. The new magnetics data acquisition operated extremely reliably on all shots last week. Travel and Visitors ------------------- Miklos Porkolab, Ron Parker, Earl Marmar and Amanda Hubbard attended the FESAC Panel meeting in Austin, Texas, Aug. 6-8. Randy Wilson (PPPL) visited this week to participate in RF testing and conditioning experiments. Ron Bravenec (U. Texas) was at MIT making BES measurements of the DNB profile. Stewart Zweben (PPPL) visited this week to participate in turbulence experiments using the new PSI fast framing camera.