Alcator C-Mod Weekly Highlights July 23, 2001 Plasma operations continued at Alcator C-Mod last week. Five run days were scheduled and four and a half were completed. A total of 64 plasma shots were produced with a startup reliability of 80%. Runtime was devoted to ITB and Double Barrier studies, non-dimensional similarity experiments to be coodinated with ASDEX-Upgrade, and turbulence imaging; half a run was also devoted to MSE calibration shots requiring DNB pulses into gas with controlled toroidal and vertical fields, but no plasma. Plasma operations are scheduled to continue this week. We are also considering extending the campaign into next week, which had been designated as a contingency option. Physics ------- The first three run days last week were in support of MP#289A, "Double Barrier Studies". This MP is a multi-run experiment to investigate the physics of the internal barrier formation previously reported in EDA H-mode plasmas with off-axis ICRF heating. While previous results were reproduced, and some additional data on the evolution and possibility of control of the core barrier by on-axis heating and field ramping were obtained, these experiments were limited by the high hydrogen fraction and by control circuit problems withe the 70MHz ICRF system. Additional experiments in support of this MiniProposal are planned for this week. Thursday's run was in support of MP287, "ASDEX-Upgrade Similarity Experiments". The goal was to make high density, high power EDA H-modes in a shape identical to that in ASDEX-Upgrade 'Type II ELM' discharges. This regime, which was only recently obtained on AUG, appears rather similar to that obtained on C-Mod when power is increased into EDA H-modes. At reduced powers, AUG sometimes sees a 'quasi-coherent' mode. We would like to see if the same behavior is observed on the two machines with the same dimensionless parameters. We will obtain edge profiles both at threshold and in the steady H-mode period. AUG will then try to match the dimensionless parameters (nu*,beta, rho*) of these discharges. The shape development part of the run went quite smoothly, and we did get shots which were a very close match to ASDEX Upgrade shapes. Compared to our usual operation, these plasmas are quite low triangularity and very close to being double null. As expected, it was more difficult to get steady EDA H-modes in the lower triangularity ASDEX-Upgrade shape, and the reduced absorbed power because of the high H/D ratio was an additional impediment. More run time with more favorable machine conditions would benefit this experiment. The first half of the 7/20/01 run was for edge turbulence imaging measurements using the Princeton Scientific Instruments Ultra-Fast Camera (PSI-3), which can take 12 successive frames up to 1 million frames/sec. Discharge conditions were set to match a baseline case for which edge profiles are available and theoretical simulations are being done by Hallatschek and Rogers. The camera view is essentially tangential to the field near the outboard midplane at the plasma edge, and illumination is provided by a gas puff, either of D2 or He, from a capillary located at a gap in an outboard protection limiter assembly. Excellent images were obtained showing the evolution and motion of turbulent structures ("blobs") in the poloidal plane. Images obtained with D2 puffs and a D-alpha filter showed structures near the separatrix and extending into the SOL, while images formed with a helium puff and a He-II (4686A) filter showed emission further inside the LCFS, with smaller features and apparent rotation in the opposite sense to those at larger radius. In reviewing magnetics fluctuation data it was noted that, under some conditions with small outer gaps <= 1 cm and at relatively low density, the poloidal magnetic field pick-up coils located underneath the tiles on the outboard AB and GH limiters have sufficient signal from the Quasi-Coherent Mode (QCM) in EDA H-mode to be able to make out the toroidal mode number. At least a dozen shots have been found with reasonable signal levels and the resulting toroidal mode numbers of the QCM range from n = 15 - 19 for ICRF heated EDA H-modes. The coils beneath the tiles are typically only 2.5 cm from the LCFS whereas the coils on the sides of the limiters and on the inner wall are typically 5 cm from the LCFS. Because of the high mode numbers of the QCM, the rapid radial decay of the mode amplitude makes the signal too small to be observed on the coils that are 5 cm from the LCFS. These toroidal mode numbers are somewhat smaller than the calculated n~30 from the radial decay of the QCM from the A-side scanning magnetic probe measurements in Ohmic EDA H-mode, but still indicate a rather high toroidal mode number. Operations ---------- The machine ran well last week, but the H/D ratio during high-power RF operation was higher than optimum, rising above 0.1 during typical H-modes, and as high as 0.2 during some shots. As a result, the ICRF minority heating efficiency degraded significantly during the pulse, compromising some planned experiments. Additional glow discharge cleaning (GDC) in helium was carried out from Friday evening to Sunday afternoon (7/20-7/22), followed by a fresh boronization on Sunday evening. It is hoped this procedure will reduce the H/D to acceptable levels for this week's experiments. Wednesday's run was delayed in order to remove a shunt from the OH2L buss. The shunt had been observed to be giving high readings during Tuesday's run, indicating that its resistance had increased by about a factor of two from its nominal value. The shunt was replaced by a length of busswork, and is being shipped back to the manufacturer for tests and analysis. The buss shunts are not presently used for any control or feedback functions, and primarily provide redundant information with the buss rogowski loops. For several weeks we have noted signs of damage in snubber resistors in the bypass circuit of the chopper supply used for fast vertical position control; we have been replacing these resistors during maintenance, and the problem has not resulted in any loss of run-time. Analysis by the resistor manufacturer indicates that the resistors suffered from overheating as a result of too much power dissipation, but our initial calculations indicate that they were being operated within their specification. We are now testing a new set of resistors, with higher power/energy dissipation rating and better temperature capability, in the snubber circuit. Comparison of waveforms from this campaign with earlier run periods does indicate we have been drawing somewhat higher average currents from this supply than in the past. It is not known whether this is related to the resistor damage. Last week the programming for this supply was changed to reduce the average current back to its historical value, which has the benefit of increasing the dynamic range available for feedback stabilization of the vertical instability. ICRF Systems ------------ The J-port antenna operation at 70 MHz made good progress towards reliable operation. By the end of the week, 1 MW was coupled to the plasma without faulting. The decoupling stub was optimized and the vacuum and plasma operation became more stable. A reflected power signal critical to the accurate prediction of antenna match was found to be malfunctioning; this was repaired, allowing efficient matching to plasma loads. The antenna phase determined from the voltage probes was found to be unreliable and resulted in false phase balance faults. From current probes at the same location, the phase is as expected, 0 deg, and stable. We concluded that these probes are located near a voltage minimum at 70 MHz and the voltage phase measurement is difficult due to the small signal level. The current probes will be used for the phase balance fault arc detectors for the remaining 70 MHz operation. Diagnostic Neutral Beam Systems ------------------------------- The beam operated at full voltage and current throughout the week. We continued with diagnostic development. The arc power supply was modified to allow the arc pulse to be increased from a maximum of approximately 300 ms to a maximum that is at least 700 ms. This is to facilitate conditioning of the arc. On Friday, we completed a beam into gas experiment with various toroidal and vertical magnetic fields to calibrate the MSE diagnostic. Lower Hybrid MIE Project ------------------------ The reflected power probes of the lower hybrid launcher have been tested successfully at PPPL by Stefano Bernabei. At MIT, work continued on the TPS chassis wiring and timing circuitry. Inner Divertor Fabrication Project ---------------------------------- The vendor responsible for fabrication of the Rear Girdle Plates has informed us that they will be unable to meet their previously agreed schedule, and proposed a new delivery date two and a half months in the future. In view of the delays already experienced at this vendor, we have decided to transfer the remaining work on these components to the shop which previously produced and delivered the C-plates; the latter vendor has agreed to complete the work on a schedule compatible with our requirements. In-house work on the probe box and mock-up parts is proceeding. All short studs have been welded on the mock-up wall. Diagnostics were mounted on the wall as well. A plastic Rear Girdle Plate, Inconel C-Plate, Tiles, Transition keys were mounted on the Mock-up wall. Two limiter backing plates with tiles were placed directly above the new Inner Divertor and found to fit perfectly, confirming that there should be no installation difficulties provided the Rear Girdls Plates are within specs. Furthermore, there was very little work required to align the Girdle Plates with the Limiter Backing plates and interlock them with a transition key. Travel and Visitors ------------------- Stewart Zweben was at MIT for the week, continuing to obtain high-speed, high sampling rate "movie" images of plasma edge turbulence with the gas puff imaging diagnostic. A quarterly review was held on Monday, July 16. Rostom Dagazian attended from OFES. The viewgraphs from the presentations can be found at www.psfc.mit.edu/cmod/sciprogram/Q_rev_Jul01/C-Mod_Quarterly_Rev_July_2001.pdf