Alcator C-Mod Weekly Highlights Dec 18, 2000 Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled and completed. A total of 69 plasmas were produced with a startup reliability of 85%. All the runs this week employed ICRF at two frequencies (70 and ~80MHz) to study the ITB plasmas obtained with off-axis heating. Plasma Operations will continue this week. Physics ------- Results from a gap scan run conducted last September have been analyzed. The midplane pressure is insensitive to variations in the outer gap. This result is in contrast to two other factors - density and confinement mode on which the midplane pressure depends more strongly. We compared the effect of changing the outer gap on three different sets of probes - the A scanning probe (ASP) which is inserted between two outer poloidal limiters, the F scanning probe (FSP) which measures the SOL profile near the divertor entrance and the Outer divertor probes located in the outer divertor surface. We found that varying the gap has a clear effect only on the ASP SOL profiles. As the gap is decreased the width of the SOL at the ASP location decreases accordingly. The same is not true for the other probe measurements. We found that they were relatively unaffected by gap changes. This result implies that the SOL 'fills in' in regions where the field line length is long enough (3-5 m) between obstacles, but not for short connections lengths as is the case for the ASP (.8 m). This result was further emphasized for the case of varying the inner gap. When that gap was reduced to zero, all the probes indicated that the SOL was 'filling in'. This 'filling in' effect is a clear demonstration of what has been inferred from LaBombard's analysis of SOL profiles and ionization sources, namely,that the perpendicular transport in the SOL is very large, leading to large interactions with the wall. In fact, as the outer gap is reduced the interaction with the far wall stays the same as the plasma 'fills in' the SOL, leading to the independence of midplane pressure on gap. The major focus of last week's runs was an investigation of the internal transport barriers obtained in Alcator C-Mod with off-axis ICRF heating. These plasmas feature a combined H-mode edge and an ITB, as indicated by formation of peaked density profiles. The regime is accessed in C-Mod using ICRF heating at 80 MHz with a central toroidal field <4.5 tesla, which places the minority hydrogen resonance well off-axis to the high-field side. The mechanism for generation of the barrier is unknown, but previous experiments have indicated a correlation with changes in core rotation, and a relationship to other core barrier regimes including PEP mode and the spontaneous formation of an ITB at the H->L transition (EN mode). The immediate goal of last week's experiments was to study the conditions leading to the formation of the ITB, and to employ simultaneous application of off-axis (80 MHz) and central (70 MHz) ICRF minority heating to heat inside the barrier region. By using a central source of auxiliary heating the experiments would determine if in addition to the density barrier, an energy barrier (i.e. temperature) could also be created. A second goal of this series of experiments is to demonstrate control of the ITB by changing the local temperature and density gradients by varying the on-axis and off-axis ICRF heating powers. The first stage of the experiment was to reproduce the density barrier mode at 4.5T with off-axis ICRF heating only (80 MHz system). A sequence of shots was then taken in which high resolution xray spectroscopy (HIREX) data were obtained at different radial locations out to r/a of 0.5; T_i and plasma rotation profiles are derived from these data. Application of central heating, using the 70MHz ICRF, was hampered by the power limitation described below, and also by irreproducibility of the L-H transition on the second and third day of the experiment. Long delays between application of the 80MHz ICRF and the establishment of the H-mode were observed on a number of shots; this effect may have been related to a locked mode, but this has not been confirmed. Some heating inside the barrier was observed, but systematic studies were not completed. Additional run time will be devoted to these experiments. ICRF Systems ------------ The J-port antenna was converted to a two strap configuration using straps 2 and 3, driven by Transmitter #4, which was retuned to 70MHz. Initial two frequncy heating in support of the ITB experiments at 4.5 T had been limited to 0.7 MW by arcs in the transmission line and phase shifter. The transmission line arc has been repaired, and we are currently working to improve operation of the phase shifter. Further analysis of the 4-strap antenna data suggests that an electric field is developing along a field line connecting the corner of the antenna box, side plate and Faraday screen. Installation of an insulating limiter between straps 2 and 3 and covering the front Mo tiles with BN could alleviate this problem by lowering the density on the field lines and preventing field lines connecting between metal surfaces. In addition, installation of current probes to measure the antenna current distribution will help clarify antenna operation with plasma loading. These modifications could be accomplished with a manned-access vacuum break. DNB Systems ------------ CXRS, BES, and MSE signals were observed during plasma operation last week. Simultaneous spectroscopic measurement of the beam component mix will allow simulation of the signals, a valuable benchmark for future improvements. Spectrally resolved measurements of the region observed by the BES/MSE filter systems were made to help verify the BES and MSE measurements. Analysis of existing data will continue through this week, although the beam will not be operated due to vacations. Travel and Visitors -------------------- Earl Marmar, Joe Snipes, Spencer Pitcher, and Miklos Porkolab participated in the Burning Plasma Science Workshop in Austin, Texas from 11 - 13 December 2000. Dr. Rick Goulding (ORNL) visited to discuss RF physics and engineering issues.