Alcator C-Mod Weekly Highlights August 26, 2002 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and three were completed. Experiments included investigations of Internal Transport Barrier formation with off-axis ICRF heating and detailed characterization of the four-strap ICRF antenna. Plasma operations are scheduled to continue next week. Operations ---------- Plasma runs were carried out on Tuesday, Thursday and Friday of last week. A total of 55 plasma discharges were produced, with a startup reliability of 75%. The run on Tuesday was devoted to Interal Transport Barrier (ITB) formation using off-axis ICRF heating at a field of 5.4 tesla with 70MHz ICRF (resonant on the low-field side). The principal experiment was MP#309, "5.4 T ITB Thermal Transport Investigation", with concurrent discharge development for MP#301, "ITB Foot Location Movement via Ip Scan" and MP#307, "High Bootstrap Fraction Regimes". MP#309 called for small (1-2%) Bt ramps to allow small changes to the heterodyne ECE system channel location. This was for interchannel calibration and to attempt the measure the Te gradient with and without the ITB's. The Bt ramps worked as desired. Changes were visible in the Te time traces over the ~ 1 cm the channels were shifted. In the outer channels changes were easy to see, less so as one looked further in. It should be possible to estimate the Te gradients from these. There were also plenty of sawteeth for heat pulse propagation analysis. The ITB scenario development was less successful. While some ITB shots were produced, the strong density rises and peaking similar to the reference discharge from last year were not reproduced. Instead, impurity accumulation and high radiated power fractions appeared to limit the performance. Several hours were lost due to a problem with the OH2L commutation switch which resulted in several blown fuses. The miniproposal(s) were not completed, and additional runtime is scheduled. The run on Wednesday was stopped after only three shots when a contactor for one phase of the alternator drive motor secondary overheated. The switch was repaired and re-installed in time for operation on Thursday. Thursday's run was devoted to characterization of the J-port four-strap antenna performance as a function of plasma parameters and equilibrium (plasma-to-antenna gap). The gap scan showed that the antenna can couple out to 2 cm without significantly lowering the loading in EDA H-modes (5.8->5.25 ohms). The current scan, from 0.75 to 1.1MA, suggested an even weaker dependence. We then did an abbreviated field scan and coupled 2.8 MW into a 5 T discharge. The antenna operated well into both low field and high field side resonance positions. Further tests at 3.8 T and 5.4 T will be necessary to complete this scan. Some shots at the beginning of the run had to be devoted to power conditioning in order to recover reliable operation at high power. J-port quickly conditioned to 1.8 MW but took 7 shots to condition to 2.4 MW. We conclude that vacuum conditioning is no longer improving the antenna performance in plasma operation. Apparently, approaching high power operation requires starting at moderate power levels and working up. The run on Friday was in support of MP#301, "ITB Foot Location during Ip scan". For this run, operation was at 4.5 tesla, using 80MHz ICRF from D- and E-port antennas for off-axis heating (on the high field side). The purpose of the experiment was to determine whether the location of the ITB generated under these conditions, as indicated by the foot of the steep density gradient region, is sensitive to the plasma current or q profile. While ITB generation was obtained on several shots, additional runtime will be required and scheduled to continue this work. ICRF Systems ------------ ICRF Antenna plasma operation had mixed results this week. All three antennas appear to have poor high power handling characteristics until they are power conditioned on a particular day. Thursday's antenna characterization run allowed a more controlled investigation of J-port. We found that the antenna required about 7 discharges to condition to 2.4 MW. For E-port, the required conditioning is less clear. On Thursday, it was about 5 shots, but on Friday the E-port did not appear to condition. Vacuum conditioning appears to have limited effectiveness but may prepare the antennas for ~ 1 MW operation. The Uptegraff power supply for transmitter #1 was repaired this week and the antenna system was placed back in service. After vacuum conditioning, the antenna operated reasonably well into plasma. Additional effort was made to reduce the noise associated with RF operation. Running low power RF, the shielding on the MHD antenna feedthrough was found to be the largest source. This was improved and several diagnostics showed siginificantly less RF noise on Thursday's run. Diagnostic Neutral Beam Systems ------------------------------- The DNB continued to operate without significant problems this week. The anode pressure optimization mentioned last week was continued, and beam currents (unneutralized) near 5 amps were generated for a narrow range of anode pressure, between 5.5 and 6psi. This indicated that improved pressure regulation may lead to improved beam reliability in producing these high currents. Operation of the beam was greatly simplified with the addition of electronics to reset gate valve protection just prior to the shot. This has been done manually, and was a significant distraction to the operator. We made high resolution measurements of spectra observed by both the MSE and BES filter systems to test the predictive models and check for background line spectra. The BES system is seeing some impurity lines which are likely carbon, and which were expected to appear in the spectrum. The LN2 vacuum line to provide automatic refilling of the DNB cryopumps has been installed and evacuated. This line runs from the cell, through diagnostic lab 156A, and through the set-up lab to the LN2 fill stations. A nitrogen vapor vent was installed in the diagnostic lab that will remove and vent gas that evolves in the line, ensuring that the line always contains liquid nitrogen. The vacuum pump was installed in the set-up lab, and the line is currently evacuated to a pressure less than 1e-3 Torr. The line will be connected to the LN2 supply and checked for leaks in this coming week. Tubes to the DNB cryo pumps from the distribution panel in the cell are being fabricated and should be installed next week as well. Lower Hybrid MIE Project ------------------------ Power-up tests of transmitter #3 continued. Operation of the low-flow pump shutoff was verified. The accuracy of the displayed filament and solenoid metering was confirmed after changing scaling. Two analog input channels for solenoid current and voltage were repaired. A complete start cycle was performed using remote control, with no faults. Correct operation of the HV enable command was verified. Transmitter #3 is nearly ready for continuation of HV testing. The remote screen display was updated to give more and better information to the operator. Travel and Visitors ------------------- Christopher Watts, New Mexico Institute of Mining and Technology, visited the PSFC. He is interested in using the FRCECE system to explore the electron temperature fluctuation measurement. During this visit, he discussed the measurements with Perry Phillips and Alan Lynn. Christopher will work on the analysis remotely at New Mexico Tech.