Alcator C-Mod Weekly Highlights July 30, 2001 Plasma operations continued at Alcator C-Mod last week. Five run days were scheduled and four and a half were completed. On what was locally reported as the hottest day of the year so far, a water chiller failure caused temperatures in the cell and power room to rise to levels that affected data acquisition hardware and caused shutdown of the RF transmitters, forcing Wednesday's run to be stopped early. Runs on Thursday and Friday were extended to 7:00PM. A total of 73 plasma shots were produced last week, with a startup reliability of about 75%. Experiments carried out included a study of neutral particle control and recycling in Helium discharges, non-dimensional similarity studies for comparison with ASDEX-Upgrade, L-H and H-L transition dynamics, and continued studies of the double transport barrier mode with off-axis ICRF. Plasma operations will continue this week, which is scheduled to conclude the 2001 C-Mod Experimental Campaign. Physics -------- Following GDC in helium, boronization, and ECDC in helium carried out over the July 20-22 weekend, Monday's run was devoted to a study of neutral particle control, fueling, and gas inventory experiments using helium plasmas (MP297). The goals of these experiments were to quantify the relative contributions to the midplane neutral pressure of divertor neutral leakage and main-chamber plasma recycling; and to perform full accounting of the fuel particle inventory in the vessel including gas and plasma phases in the lower divertor, main-chamber, and upper chamber regions. The experiment consisted of opening and closing sets of the divertor bypass "flappers" with a period of 0.2sec during the discharge, while monitoring the effect on plasma density and local neutral pressures. Which sets of flappers were operated was varied from shot to shot. There is a narrow operating window in helium plasmas where there is sufficient helium pressure in the divertor (> 40 mtorr) to allow the flappers to 'fuel' the plasma yet the divertor is not detached, and some time was spent optimizing conditions. It appears that the response of helium plasmas to the flapper opening is very similar to deuterium plasma, suggesting that wall pumping does not affect the gas-fueling transient that occurs when the flapper is opened. As in the deuterium plasma, the midplane pressure is more tied to the line-averaged density than the magnitude of the local divertor bypass conductance. While most of the shots were run with normal feedback on the line-averaged density, we also did two shots with a fixed pulsed gas programming. This resulted in a noticeably smaller perturbation in the line-averaged density in response to the flapper modulation, which suggests that lag in the pulsed gas affecting the plasma combined with the chosen 200 ms flap period might have amplified the influence of the flapper on the line density. This effect will have to be taken into account in the analysis. Runs on Tuesday, Wednesday, and Friday were devoted to additional studies of the double barrier mode plasmas with off-axis and on-axis ICRF heating (MP#289A). These experiments have been a major focus of this phase of the current campaign, making use of the J-port four-strap antenna operating at 70MHz and the older dipole antennas at D- and E-port at 80 and 80.5MHz. This week's runs investigated the effect of the relative timing and power levels of the off- and on-axis heating in forming the internal transport barrier, arresting the central density and impurity peaking, and heating within the ITB. Two experiments were carried out during an extended run on Thursday. The goal of MP287 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. AUG will then try to match the dimensionless parameters (nu*,beta, rho*) of these discharges. We completed the first three parts of the experiment. The machine conditions were much better than on 7/19, with H/D typically about 6-8%. Once again, we found that C-Mod does not get steady, EDA-type discharges at this shape, and also that RF coupling is much more difficult (the two effects may be related). On some shots a weak quasi-coherent mode was seen on PCI, but it was not strong enough to cause large particle transport. The optimum density seemed to be nel=9.5e19 m-2; at higher targets the gas pressure increased substantially. H-mode density became steadier, but seemed to exhibit bursts of higher transport, reminiscent of the typical behavior with Type III ELMs and a cold edge, though the ELMs were not immediately obvious. An H-mode shot which was not steady-state but had moderate and steady increases in density and radiation was obtained, which should be suitable for ASDEX-Upgrade to attempt to match. We completed this phase of the experiment by taking a slow power ramp to get the threshold power and edge parameters at identical target conditons. This will be suitable for AUG threshold comparisons. The second part of Thursday's run was devoted to L-H-L hysteresis studies. Plasmas were 800 kA and of a fairly standard C-Mod shape. All discharges gave very steady EDAs. The RF was programmed with a triangular waveform from 0.4 to 2MW, and then back to zero. Plasmas stayed in H-mode down to very low RF power; there was typically a short period of type III ELMs before the back-transition. Good edge TS data were obtained, which we will compare to previous results with the ECE and visible bremsstrahlung. The goal is to examine the hysteresis curve of input flux vs various gradients, such as grad T and grad P/n. This is being compared to predictions of various H-mode theories, in collaboration with Ben Carreras of ORNL. Results will be presented at the Transport Barrier workshop in September. More results have been obtained in our study of edge turbulence using the ultra-fast framing camera. We have observed the evolution of edge/SOL turbulence on a 4 microsecond timescale. Twelve sequential images, each with high radial and poloidal resolution of the emission from a gas puff at the plasma periphery, are taken. "Blobs" of emission are seen moving poloidally and primarily outward radially. They appear to be generated near the separatrix. Their lifetime is of order 10 microseconds in which time they move of order ~1 cm, which is also their typical size scale. Some of these facts were known previously, e.g. size and lifetime, but what is new and intriguing is the observation of their birth, transport, and disappearance. Similar 'blob' behavior is seen in both L- and H-Mode confinement, at least in the SOL. MPEG movies, each showing total time intervals of 48 microseconds, are available on the WEB at http::/www.psfc.mit.edu/people/terry/MPEG_1010726007.mpg http::/www.psfc.mit.edu/people/terry/MPEG_1010726011.mpg http::/www.psfc.mit.edu/people/terry/MPEG_1010726015.mpg http::/www.psfc.mit.edu/people/terry/MPEG_1010726017.mpg Operations and Engineering --------------------------- A new shunt for the OH2L bus was fabricated in-house, has been installed and tested, and is now opertional. The commercial OH2L shunt which failed last week has been returned to the manufacturer and is in the process of being opened for an inspection. The new bypass snubber resistors in the "EFC" chopper supply are holding up well with no failures to report. Additional resistors to replace the remaining old-style ones have been ordered. ICRF Systems ------------ Post-boronization, the antennas were re-conditioned in He plasmas. The J-port voltage maximum was observed to be ~30 kV for 70 MHz operation compared to ~25 kV at 78 MHz. The J-port antenna coupled 3 MW and appears to be reliable when the maximum voltage is <30 kV. The combined D, E and J-antennas injected 5.5 MW last week. The antenna fault system on J port was changed from voltage probe input to current probe input. Phase limits were decreased from +/-90 degrees back to +/- 65 degrees. Spurious phase balance faults have been virtually eliminated at J since this change. DIagnostic Neutral Beam Systems -------------------------------- The DNB did not run consistently this week. Due to the high heat in the cell, the suppressor control circuitry failed. An intermittent heat sensitivity had been observed, but we were unable to find the source of the problem until this failure. The suppressor is expected to be back in operation on Monday and to continue operation through the final week of the campaign. The beam penetration/reionization model was successfully bench marked against BES data. This will be critical for pursuing beam mods during the shutdown period. Inner Divertor Fabrication Project ---------------------------------- Fabrication of all the probe box and mock-up parts presently designed has been completed. Also, modification of the Tile Support Plates that go into the C-Plate pockets has begun; several mills of material need to be removed from each side of the tile support plate to make plates fit loser into the pockets. Mock-up building is moving forward and we continued to assemble components on the Mock-up wall. Lower Hybrid MIE Project ------------------------ EMI/RFI shielded racks for transmitter carts and CPS arrived and work to install equipment into the racks started. Relocation of the klystron air flow blowers on the carts has begun. One of the three Transmitter Protection System (TPS) optical interface boards was sent out for fabrication. Wiring for one of the TPS chassis was completed, and the chassis was installed in the EMI/RFI rack. Travel and Visitors ------------------- Bruce Lipschultz attended the Int'l meeting on Plasma Edge Issues (July 10-11th) at the the JAERI Naka site. There were 5 sessions at this meeting with a number of talks. The sessions were on high density operation, ELMs, main chamber recycling, SOL flows and high-Z materials experience. Bruce Lipschultz gave a presentation on main chamber recycling work at C-Mod and DIII-D as well as a presentation on high-Z wall operation in C-Mod. He also chaired the main chamber recycling session. He also spent time (July 10th-13th) with collaborators N. Asakura and T. Nakano at JT-60U on US-Japan collaboration FP3-5, "Comparison of main chamber recycling in C-Mod and JT-60U'. The purpose of this work was to compare and contrast JT-60U and C-Mod SOL data and plan towards joint experiments on JT-60U and C-Mod. Diagnostics and experiments were idenstified for the September run period on JT-60U. Data is now being taken on C-Mod. Professor Yuichi Takase from the University of Tokyo visited MIT on Friday (July 20, 2001) and worked with Paul Bonoli on full-wave modelling of high harmonic fast wave (HHFW) heating in the NSTX device. They also discussed internal transport barrier experiments (ITB) in Alcator C-Mod. Paul Bonoli visited the Princeton Plasma Physics Laboratory (PPPL) from Monday - Thursday (July 23-26, 2001). He worked with Dr. Cynthia Phillips and Yuichi Takase on HHFW heating in NSTX and he gave a talk at the weekly NSTX Group Meeting titled "Full-wave modelling of HHFW heating in the NSTX device". Paul Bonoli also attended a 2 1/2 day planning meeting for the RF Sci-Dac Initiative (Scientific Discovery Through Advanced Computing). The meeting was hosted by Cynthia Phillips (PPPL) and was attended by representatives from ORNL, Lodestar, PPPL, Mission Research Corp., Comp-X, and MIT. The meeting was extremely productive. A three year workplan was formulated and physics issues related to our proposed problems were discussed in detail. Martha Redi and David Mikkelsen, PPPL, visited CMOD last week, July 23-27, Martha worked with Catherine Fiore on gyrokinetic modelling of dual RF experiments. David worked with Steve Wolfe to learn more about how EFIT can be used to estimate the q profile inside C-Mod plasmas. Dan Kellman of GA visited Josh Stillerman and Joe Bosco to discuss our CAMAC system and the new CPCI-based system. He also met with Dave Terry and Bill Cochran concerning power systems issues. Dan has worked at GA on their chopper supplies and other power systems and has been a point of contact regarding our own chopper supply.