Alcator C-Mod Weekly Highlights Sept. 23, 2002 Plasma Operations continued at Alcator C-Mod last week. Five run days were scheduled and completed. Experiments were carried out in support of the RF, Transport, and Divertor/Edge topical areas. Progress continued on the Lower Hybrid fabrication and on the Diagnostic Neutral Beam System. Operations are scheduled to continue this week, after a routine maintenance day on Monday. Operations ---------- Plasma runs were carried out on Monday through Friday last week. A total of 115 plasma discharges were produced, with a startup reliability over 90%. This is the largest number of discharges produced in a week during the present campaign. A fresh boronization was successfully carried out on Thursday night, completing the re-conditioning process following the vent two weeks ago. Runs on Monday and Tuesday combined ICRF antenna conditioning with studies of heating physics and plasma performance as the H/D ratio continued to decrease toward levels consistent with H-minority heating. The D- and E-port dipole antennas were conditioned up to about 1.5MW each. The J-port four-strap antenna was conditioned up to 2.7MW. However, another BN cover tile was broken off this antenna during a rampdown disruption on Monday, and injections from the exposed region limited the useful power from J-port to < 2MW. The broken tile was clearly observed on the wide-angle plasma video view, and over several days has moved from the upper shelf of the outer divertor down into the divertor throat. So far, no obvious deleterious effects have been determined, although the tile is in a location that is subject to significant power flux. Wednesday's run was in support of MP#314, "Evaluate BT sweep method of LTe measurement". This MP was submitted by David Mikkelsen (PPPL), who participated as co-Sesson Leader from his office in Princeton, carrying out between-shot data analysis. Small sweeps of Bo were used to obtain L_Te from each GPC channel in three conditions. Raising q(a) lowered L_Te, as qualitatively expected from ITG theory; lowering the density (to decouple Te and Ti) didn't have much affect on L_Te. Plasma microturbulence simulations with GS2 will be carried out in due course. These experiments also accessed two non-standard operating regimes on Alcator C-Mod: a low-density ohmic regime characterized by poor confinement and atypical central MHD activity; and a high fueling efficiency regime featuring an upper chamber marfe and possible detachment, which appeared at higher values of n/I. The latter effect was also encountered and studied during the subsequent run. Thursday's run was devoted to MP#319, "Experimental/Theoretical Comparisons of SOL Turbulence in Alcator C-Mod", submitted through the Divertor/Edge and Transport Groups. The goals of this set of experiments were to investigate the B scaling of the turbulence at two different fields for shots with good scanning probe profiles; vary the outer gap to see if the gap influences the dynamics of the turbulence; change and measure the characteristic density scrape-off lengths in the near SOL by operating at low (n/n_{GW} ~0.2) and high (n/n_{GW} ~0.6) density and observe any changes in k-spectrum or turbulence dynamics; and to change the characteristic density scrape-off length in the near SOL by operating first in L- and then in H-mode and observe any changes in k-spectrum or turbulence dynamics, using Ohmic H-mode so that scanning probe profiles are possible. Key diagnostics included scanning probes at the outer and inner midplane; gas puff imaging fast diodes at the inner and outer midplanes, as well as additional fast diodes observing the inner and outer SOL; fast imaging cameras; BES and reflectometer; and a variety of edge pedestal diagnostics. Based on preliminary observations, the plasmas at n/n_G=0.2 were less turbulent than those at higher Greenwald parameter, for both high (6T) and low (2.6T) fields. At 6T, 1MA, nebar>1e20/m3, we obtained ohmic H-modes. These were encountered at unusually low values of the threshold parameter P/nBS. No obvious difference was observed in the turbulence characteristics as a function of the outer gap distance, pending more detailed analysis. At 5.4T and n/n_GW>0.45 the discharges were again characterized by an upper chamber marfe and high fueling efficiency; the effect apparently is dependent on Bt, as such behavior was not encountered at the same range of n/n_GW at 2.6T. Visible emission was significantly inside of the nominal separatrix, implying a much colder edge region in these discharges. In a series of standard ohmic ELM-free H-mode discharges, turbulent images were clearly narrower than in the L-mode cases. Nonetheless, blobs were still clearly seen in the fast PSI camera images. Following Thursday's run, a fresh boronization was carried out overnight. Recovery from boronization on Friday was relatively smooth, with startup reliability over 85%, comparable to earlier in the week. Early in the run, the H/D ratio was around 2%, rising to ~7% by shot 19, after which is remained fairly steady. We pursued RF heating and confinement using D and E port antennas. After some antenna conditioning, and some filter changes in the cell to alleviate cross-talk problems, we had several good H-mode discharges, with total RF power approaching 3 MW and H factors around 1.5. The last three plasmas of the day were devoted to ITB scoping, by ramping the TF down to 4.4 Tesla during the H-mode phase. Core rotation was observed to slow, while the far off-axis rotation did not. Several more ohmic H-modes appeared near the end of current flattops in 5.3T discharges, at quite low values of the threshold parameter P/nBS. Speculation is centering on the possible role of the presence of BN in the divertor in facilitating these high-field L-H transitions. ICRF System ----------- ICRF operated into plasma on three C-Mod runs last week, as scheduled. Antenna operation was generally satisfactory, although the D- and E-port antennas had some difficulty in simultaneous operation at powers above 1MW each. At high power levels, the reported D-port power appeared to be high by 300 kW. Calibrations are being checked, and the power monitor data will be re-processed. The engineering group is analyzing the loss of the the BN cover tiles under disruption loads. The likely failure mode is the fastener approach used on these tiles. The fastener effectively introduces a disruption force on the BN tile. We are in the process of designing a cover tile to replace the present set and estimating the upper limit to disruption forces to check the assumptions underlying the design. We have the necessary BN material in house to replace these tiles. We replaced the spark gaps on FMIT#4 FPA after a tube arc caused the grid spark gap to fire. The tube was properly protected by the spark gap and crowbar circuit, and the transmitter was put back into service. DNB Systems ----------- The DNB operated into C-Mod during all five operating days this week. Both continuous and modulated beams were made, and reliability continues to be good. Anode gas pressure regulation is critical for good reliability, and improvements to the present anode gas regulation are currently under consideration. The LN2 fill system is operating and performance improvements continue. Additional pipe insulation has arrived and is being installed on the LN2 piping system. This will allow the automated fill system to be left operating on a continuous basis. The LN2 fill paragon operator interface screen has undergone one revision, and will be fully tested this coming week. Lower Hybrid MIE Project ------------------------ Transmitter #1 has now been brought online. The equipment rack, PLC, and Transmitter Protection System have been powered up. Fiber optic enable signals between the three transmitter racks and the pump controller interface were verified. The semi-rigid cable fabrication effort continued. Visitors and Travel -------------------- Martha Redi visited MIT PSFC September 16-18th, meeting with Catherine Fiore, Paul Bonoli and Amanda Hubbard regarding new linear and nonlinear calculations of the formation stage of the off-axis RF heated ITB. Nonlinear calculations continue to confirm the linear result of suppressed ITG microturbulence in the ITB region before formation, relative to high and moderate microturbulence outside and within the plasma half-radius. Linear sensitivity tests show little effect of variations in magnetic shear on the suppressed ITG mode, but increased plasma safety factor and removal of the Boron impurity are found to destabilize the ITG mode in the ITB region. Martin Greenwald was in Cordoba, Spain to attend the joint EU-US TTF meeting and present an invited talk on density limits and edge turbulence. He also took the opportunity to discuss plans for upcoming density limit experiments on JET.