Alcator C-Mod Weekly Highlights August 19, 2002 A DoE quarterly review of the Alcator C-Mod program took place last Thursday. Plasma operations continued at Alcator C-Mod last week, with four run days scheduled and three completed. ICRF antenna conditioning continued, and experiments designed to understand the effects of central ICRF on ohmic H-modes with ITBs were begun. Plasma operations will continue this week. Alcator C-Mod Quarterly Review ------------------------------ The review was held on August 15 via teleconference, with viewgraphs presented on Picture-Talk. Physicists and engineers from MIT, PPPL, and UTexas made presentations. DoE was represented by Rostom Dagazian and Warren Marton. After a welcome by Earl Marmar, the following presentations were made: Operations Jim Irby ICRF Steve Wukitch Lower Hybrid Monty Grimes Joel Hosea (PPPL) Toroidal Rotation Profile Evolution John Rice Comparison of Fluctuations on High- and Jim Terry Low-Field Sides of the Torus Pedestal Stability in H-Mode Dmitri Mossessian DNB & Related Diagnostics Bob Granetz University of Texas Collaboration Bill Rowan (UTexas) The viewgraphs from these presentations can be found at: http://www.psfc.mit.edu/cmod/sciprogram/Q_rev_Aug02/ Operations ---------- 79 plasma discharges were produced last week during approximately 3-1/2 days of operation. The startup reliability for the week was 85%, while on Friday the startup reliability approached 97%. The second boronization of the vessel during this run campaign took place from Wednesday evening through Thursday morning. Approximately 2000 Angstroms of boron was deposited. The run on Tuesday ended after only a few shots with a fault in the OH2U commutation bank. A bad cable and capacitor were replaced and operation resumed on Wednesday morning. The runs on Wednesday and Thursday were dedicated primarily to high power rf conditioning of the E and J-port antennas. The J-port antenna has now been conditioned during plasma operation to a record maximum voltage (~35 kV) with solid heating and negligible impurity or density generation. The previous peak maximum voltage was 30 kV. This run marks the first time the J-port antenna has operated into plasma at voltages similar to those obtained during vacuum conditioning (~36 kV). During plasma operation the antenna has now run at 2.9 MW for 0.1 s, and 2.5 MW for 1.0 s. We expect the antenna to continue to condition up to the operational goal of 3 MW of reliable, sustained rf power. Operation on Thursday and Friday was used to investigate rotation and ITB formation in ohmic H-modes as outlined in mini-proposals 313 and 299a. Much of Thursday's run was needed to develop the ohmic H-modes needed to perform the mini-proposals. However, some rotation data was obtained and is being analyzed. On Friday, after more adjustments of the TF field ramp, target density, and plasma current, usable discharges were produced, though H-modes with as long a sustained ITB period as those produced in earlier campaigns were not obtained. On-axis rf power from 200 kW up to 1 MW was applied to these discharges without causing the density peaking to degrade. Physics ------- Martha Redi (PPPL) visited C-MOD August 12-14, discussing gyrokinetic calculations recently completed for the off-axis RF heated ITB case, during the "trigger" time. The new nonlinear calculations show that the linear stability found with GS2 at the ITB location is maintained, with no strong turbulence in the ITG range of frequencies. Relative turbulence levels , as measured by predicted electrostatic potential fluctuations , are 5, 0.1,100. at the 0.25, 0.45 and 0.64 radii. Further sensitivity tests have been completed showing that higher "q" at the ITB location would destabilize the ITG mode and the ITB would then not be likely to form. These tests suggest that the mini-proposal for q-scans should not exhibit ITB with off-axis RF if q is too high, and that a larger core ITB might be produced if RF heating could be carried out at higher r/a, keeping low safety factor. Sensitivity to variations in magnetic shear do not suggest strong stabilization at the ITB location by s-hat. Sensitivity studies presented at the European Physical Society Plasma Physics Meeting previously showed that decrease of grad(Ne) and increase of (Te) would destabilize ITG at the transport barrier at the trigger time. Recently, experiments to investigate the role of turbulent transport near the density limit have been conducted. Previous experiments had shown an increase in the amplitude and time correlation of SOL fluctuations. In these new experiments, this result was extended unambiguously to regions with closed magnetic field line. The scanning probe was inserted up to 3 cm inside the last closed flux surface and "blobs" were imaged at similar locations with the new fast camera. Lower Hybrid MIE Project ------------------------ Modifications to the new body current circuits have been completed, and the boards have been tested with a low-current fault test source. The response time of the TPS circuit using these new sensors was measured to be about 7 microseconds, which was almost two orders of magnitude faster than previously measured. The body current modifications have therefore been determined to be acceptable. Parts came in for the SMA/RG-174 cabling which will be used for the low-level rf signals. We have begun making the cables for the RF input switch, control board, and solid-state amplifier. There are twelve of these assemblies. ICRF ---- Recovery from the second boronization of this campaign was similar to previous campaigns. E-port vacuum conditioned to 40 kV in ~30 one second long pulses and J-port conditioned to 35 kV in ~60 pulses. Comparison of these results to the first boronization of the campaign where ~500 vacuum conditioning shots were needed to reach 40 kV on D and E antennas, and ~1000 shots for the J-port antenna, suggests that significant plasma conditioning should be completed before the first boronization after a vent/maintenance period. During plasma operation, the J-port antenna conditioned to 2.9 MW for 0.1 s pulse widths, and 2.5 MW for up to one second long pulses. The J-port phase control has been modified to eliminate noise sources that resulted in a slow phase lock. A limiter amplifier on the feedback signal produced a -8 dbm noise signal when there was no feedback signal. This resulted in the VCO starting with a frequency 30 kHz off the 30 MHz free-running frequency and out of the capture range of the PLL. The limiter was by-passed resulting in ~3 msec locking time compared to 15 msec. This time was further reduced by trimming the phase of the feedback signal to approximately the phase of the carrier leak signal. The resulting locking time was reduced to less than 1 msec. The transmitter power supplies (30 kV, 180 A transformer rectifiers) were tested recently as part of our routine maintenance procedure. The oil was tested for dissolved gases that could indicate possible arcing problems. D-port's supply indicated high levels of acetylene and hydrogen suggesting some arcing or severe overheating had occurred. Although the supply appeared to be working properly, an engineer from the manufacturer felt it was prudent to do an inspection before continuing operation. On inspection a broken resistor and suspect capacitor on the secondary side filter network was found. Replacement components are scheduled to be delivered on Monday and repairs should begin on Wednesday. The supply could be operational as early as Friday. RFX DNB ------- The DNB continued reliable operation into plasma this week during all four Alcator run days. Beam performance has been good, with efforts continuing to optimize beam current. Anode gas pressure has a strong effect on beam current: pressures below 5 PSI will cause instability faults in the arc; pressures above 7 PSI will allow very reliable operation with low (~3.5A) beam currents. The ideal pressure seems to be around 5.5 to 6 PSI at the anode. An additional optical input to the beam interlock will be installed early next week. The Russian design assumed that the machine gatevalve would be open in preparation for beam firing; a closed gatevalve latches an interlock fault. In typical operation at C-Mod, the gatevalve is opened about a second before the beam fires. Once the gatevalve is open, the latched fault needs to be manually reset. An additional isolated optical input to the BINP interlock module will allow the latched fault to be automatically reset by a Camac output after the machine gatevalve is open. Progress continues on the LN2 line installation. A hole has been drilled in the brick wall separating the set-up lab and the Thomson Scattering Diagnostic lab to allow the LN2 line to pass through the wall. A support shelf for the LN2 line vacuum pump has been constructed and mounted in the set-up lab. A written procedure for the work required in the set-up lab has been approved by the Thomson scattering group. Installation of the LN2 line is scheduled for this coming Monday. The BES optics were used to determine the width of the DNB at the plasma. The beam width was found to have a full width at 1/e points of approximately 8.4 cm. For the current and voltage of the beam during the measurement, this is consistent with expectation. With that measurement in hand, the BES system was converted for a radially resolved measurement of fluctuations. Travel and Visits ----------------- Ron Bravenec (UTexas) spent the week at MIT continuing work on the BES diagnostic. Martin Greenwald was at ORNL last week to attend a meeting of the FESAC panel on integrated simulation. Three national undergraduate fellows have been working with scientists in the C-Mod group this summer. Ross Hays, supervised by Earl Marmar, has performed research on imaging of lithium pellet ablation trails to measure internal magnetic field profiles. Abby Oelker, supervised by Amanda Hubbard, has investigated third harmonic electron cyclotron emission. Derek Kopon, supervised by Jim Terry, is working on comparisons of fluctuations at the high- and low-field sides of the plasma edge. All three NUF students have submitted abstracts for contributed posters on their work to the upcoming APS-DPP meeting in Orlando.