Alcator C-Mod Weekly Highlights Dec 26, 2000 Plasma operation continued at Alcator C-Mod last week. Three run days were scheduled and completed, although the final run was delayed do to a power outage from the local utility. A total of 45 plasma shots were produced with a startup reliability of 90%. The principal experiments for these runs included a test of critical gradient transport, continuing investigation of core transport barrier formation with off-axis ICRF heating, and an investigation of ICRF mode conversion. Plasma operations for calendar year 2000 are now complete. The system is now warming up in preparation for a manned-access vent in January, 2001, during which modifications will be made to ICRF antennas and in-vessel diagnostic hardware. Physics -------- During ITB experiments carried out earlier this month, a comparison was made between the density peaking factor (ne0/nebar) and the width of the core x-ray brightness profiles. The intent was to determine whether the x-ray profile peaks up in concert with the density profile, thus implying an impurity ITB as well as a plasma particle ITB. It was shown that although the x-ray profiles consistently peaked during the brief enhanced-neutron (EN) modes at the H-L back transition, they were not always consistent with measured density peaking during the off-axis ICRF ITB experiments. The reasons for this are not understood, and need to be investigated further. A study of temperature profile stiffness using modulated heating was begun in the run on Tuesday. The amplitude and phase response can be used to infer an incremental chi, which is usually much higher than the chi computed from power balance. The difference might be attributed to very strong transport that turns on when the normalized temperature gradient is above a threshold. ITG theory, for example, predicts a big increase in transport when R/L_T goes above a R/L_Tcritical - forcing the plasma to marginal stability. These experiments are intended to provide a quantitative measurement of how sharp the transport turn-on is - that is - how strongly the marginal stability condition is enforced. In order to facilitate the data analysis, we tried to synchronize the sawteeth to the modulation of the central heating (at 80 MHz). This was successful. The sawtooth heat pulses were easy to see on all three ECE systems. Clear, progressive phase delays could be seen on the outer channels which means we should have good data for calculation of the incremental chi. The amplitude was increased with the addition of modulated RF and the phase delay decreased. These experiments, with the addition of simultaneous off-axis heating (at a different frequency) will be continued in the next phase of the campaign. The two-frequency experiments on core transport barriers formed with off-axis ICRF heating continued last week. Use of a higher target density, nebar>1.5e20/m^3, resulted in sustained EDA H-modes with pronounced density peaking with off-axis (80MHz) heating alone. Central densities in these cases were up to ne_0=8e20/m3. We were also able to centrally heat with the 70 MHz J-port power during the density barrier phase, at different rf power levels and at different times. Increases in the neutron rate and central T_i were produced by injecting J-port power later in the discharge (1.25 - 1.5 sec) after the density barrier forms. We were able to scan the on-axis 70 MHz power from 0.8 - 1.2 MW in these plasmas. The increase in neutron rate was found to increase with increasing on-axis heating power (as expected). The density increase during barrier formation was found to be arrested by the injection of J-port power on some shots. However, on other shots the density increase was found to continue during J-port heating. A scan of the radial position of the HIREX diagnostic was also carried out to obtain T_i and plasma rotation profiles during on-axis heating of the density barrier mode. The data obtained during this day of running should be adequate to carry out a detailed transport analysis, in order to determine if heating inside a thermal energy barrier was occurring in these plasmas. Friday's run was devoted to an investigation of ICRF Mode Conversion using the Phase Contrast Interferometer (PCI) diagnostic. Using the PCI in high-resolution mode, we ran plasmas consisting of hydrogen, deuterium, and helium-3 to look for mode conversion near the center of the plasma. Since the mode conversion layer becomes more of a vertical line as the poloidal field is reduced, these experiments were carried out at Ip=0.4MA; other parameters were Btor=5.8 T, nl_04=0.5e20 m-2, lower single null, L-mode, Prf=1 MW. Ion Bernstein Wave (and possibly fast magnetosonic wave) signals were seen on the PCI, and there were indications of off-axis direct electron heating from the RF modulation later in the shot. These shots helped confirm some of the TORIC code modelling that has been done since the June 23rd run when these IBW signals were first seen on the PCI. This experiment is part of the thesis research of a C-Mod doctoral student. ICRF System ----------- We were able to improve the reliability and operating power of the 70 MHz, J-port antenna, running in the dipole configuration. We found that the operating power the previous week was limited by arcing in the 9" line near the phase shifter. We suppressed the arcs by pressurizing with SF6 and using a match that maximized the phase shifter length. This allowed up to 1.2 MW to be injected. DNB System ---------- The DNB was not operated due to Christmas vacations. Hardware for permanent repair of the fueling system was completed at UT and shipped to MIT. Vacuum fixtures which will allow operation of the beamline during a C-Mod vent are in the shop. There was an initial attempt to benchmark the beam component mix measurement against the BES system which will be completed following vacations. This week will be used for hardware installation in preparation for operation of the beam during the C-Mod vent and for continued baking of the beamline. Visitors and Travel ------------------- Prof. Ken Gentle (U. Texas) visited C-Mod last week, collecting and analyzing data from the ECE diagnostics during the transport runs.