Alcator C-Mod Weekly Highlights Sept 30, 2002 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Experiments were carried out in support of the RF Physics topic and Advanced Tokamak research. Progress continued in the DNB and Lower Hybrid Project. Operation is scheduled to continue this week. Operations ---------- Plasma runs were carried out Tuesday-Friday last week. A total of 98 discharges were produced with a reliability of 85%. Tokamak performance continued to be good. Tuesday's run was primarily devoted to continuation of the evaluation of ICRF minority heating and antenna performance, employing the D- and E-port dipole antennas at 80 and 80.5MHz. Minority concentrations varied from 3% early in the run up to 6-8% . Changes in the lower triangularity from 0.6 to <0.5 appeared to significantly increase the H-mode threshold. The remainder of the week's runs were in support of two experiments involving Internal Transport Barrier (ITB) physics. Runs on Wednesday and Friday were devoted to MP#301, which is designed to determine the dependence on plasma current of the location of the ITB obtained with off-axis ICRF heating. Measurements of the rotation profiles before and after ITB formation show a strong reduction in toroidal rotation inside the ITB "foot" location, and a more modest reduction outside. The rotation profile appears to peak near the minority resonance location. Preliminary results at lower currents (600kA) suggest that the ITB foot location may have moved to smaller minor radius, but these results need to be confirmed. Additional experiments, including higher current operation. are required to complete this experiment. The run on Thursday was in support of MP#299, an experiment to study the effect of central ICRF heating on ITB discharges which evolve spontaneously in some ohmic H-modes. These ohmic "double-barrier" discharges were observed and partially documented during the 2000 Campaign. In some ohmic H-modes, produced by reducing the toroidal field to induce an L-H transition and subsequently raising the field back to an intermediate value, the density profile is observed to peak and a clear "foot" appears, indicating a barrier at least to particle transport. Typically the density and impurity profiles continue to peak and the H-mode usually terminates either due to radiative collapse or MHD activity. The present experiments were designed to heat inside the ITB using ICRF minority heating at 70MHz, from the 4-strap J-port antenna. The expectation was that the density (and impurity) peaking would be arrested by the additional on-axis heating, and a quasi-steady profile would be obtained. Obtaining the ohmic target with an internal barrier proved very sensitive to the equilibrium shape. Most of the ohmic H-modes produced early in the run were ELM free and highly radiative, with no clear ITB formation. Ohmic ITB's were eventually obtained in a configuration with increased triangularity and larger SSEP (the distance between the primary and secondary separatrices), which seemed to favor development of an EDA-type ohmic H-mode. Once a suitable target was obtained, we determined that the density peaking was indeed arrested with RF power above 400kW, and at 600kW the central density was clearly clamped while the central temperature and global stored energy increased. Lower Hybrid MIE Project ------------------------ System tests of all three transmitters and cooling systems were performed. These were the first closed-loop tests with all three transmitters. Several minor issues were uncovered, and are being remedied. DNB Systems ----------- The DNB continued to operate throughout the week. Significant progress was made on the automatic LN2 fill system, which has been successfully put into operation. Travel and Visitors ------------------- David Mikkelsen (PPPL) spent the week at MIT discussing last week's experiment to measure the electron temperature gradient scale length in ohmically heated C-Mod plasmas. Gerd Schilling was at MIT for the week to participate in the ICRF operation, ITB experiments, and discussion on further antenna modifications.