Alcator C-Mod Weekly Highlights Feb. 17, 1998 Physics operations continued at Alcator C-Mod last week. Four run-days were scheduled and completed. Approximately 95 plasma discharges were produced. A fresh boronization was carried out on Monday night; a total of 133psi of diborne mixture was used. Tuesday's run began with a series of fiducial shots to evaluate wall conditions and establish reproducible operations. As usual, startup reliability after a boronization was not good, but improved through the day. Following a series of 800kA fiducial discharges, with RF power up to 2MW, the current was increased to 1MA and the RF power to 2.7MW. This resulted in two shots with stored energy above 210kJ and neutron rates over 10^14/sec, which were the highest performance shots obtained in the current campaign. Having established satisfactory operational reliability, we proceeded to carry out an experiment (MP#135) to investigate the role of surface voltage on SOL flows and recycling asymmetry. The goal of these experiments was to look for any influence that the surface loop voltage might have on the up/down asymmetry in recycling light on the inner wall during limiter discharges. The approach was to program 0.1 MA steps in 0.8 MA inner-wall limited plasmas, thus imposing approximately a 2.5V peak-to-peak modulation on the loop voltage. The symmetry of the recycling light on the inner wall was monitored using the B-side and F-side D-alpha reticon arrays. Also, D-Balmer series emission from the inner wall was monitored with the Chromex spectrometer to determine the relative importance of volume versus surface recombination. The symmetry of the inner-wall recycling light did appear to be affected by the perturbations. For moderate density discharges a large up-down asymmetry was generally observed when the loop voltage was high. This asymmetry went away when the loop voltage dropped to zero. Since we were not able to reverse the loop voltage (minimum loop voltage was ~ 0) we could not determine whether the asymmetry actually followed the sign of the voltage, which would have strengthened the evidence for the proposed effect. Nevertheless, we have some interesting data to sift through. Wednesday's run was a continuation of our investigations of reverse shear operation. Current ramp rates up to 5.7MA/sec were tried. Toroidal field was also varied between 4.9 and 5.7T. RF power in the current ramp was varied, with up to 2.7MW injected before 0.1 sec. Electron temperatures approaching 5 keV were obtained at densities around 1.1e20/m3. We found that a ramp rate of > 5.5 MA/sec resulted in MHD modes which were related to impurity injections. Scanning the magnetic field to move the heating off-axis was not beneficial; the time to onset of the first sawtooth crash was reduced considerably even though the central temperatures were similar. Li pellets were injected both in the flat top and ramp up. The plasma accepted three big pellets readily even at 0.07 sec into the discharge. We found no enhanced confinement with the pellets early in the ramp up (0.07 and 0.12 sec), but PEP mode behavior was observed in discharges where the pellet was injected at 0.15 or 0.18 sec. This suggests that deeply reversed shear profiles do not help in obtaining PEP mode. The sawtooth onset was delayed in the latter (PEP) cases to ~.26sec. These shots also had significant increases in toroidal rotation after the pellet injection. PEP modes were readily obtained in the flat top. Thursday's run was devoted to studies of D(He3) ICRF heating at Bt~8 Tesla (MP#150A). This run was significantly better than last week's. A few H-modes were obtained, but did not develop steady state character nor particularly good quality. From the RF perspective, the optimum He3 gas puff for good central ion and electron heating via He3 minority heating was identified. Central ion temperatures reached 4 keV and central Te at the top of the sawteeth reached 5 keV. The multiple H-mode transitions during the discharge make determining the total absorbed power difficult; however, absorbed power appears to vary strongly with He3 concentration with a maximum ~75% . By varying the He3 concentration, the split between minority and mode conversion heating was explored. Lowering the toroidal field to move the D resonance and Shear Alfven resonance out of the plasma did not significantly improve total absorbed power. A more detailed analysis will be required to confirm this. Over the last few discharges, the density, current, and RF waveform were varied to explore their effects on obtaining H-modes. Lowering the density improved H-mode accessibility, but not the duration of the H-modes. Friday's run was in support of MP#163 "High q|| dissipative divertor". This proposal is an attempt to achieve simultaneously a good confinement H-mode with low Zeff and a detached divertor. A previous run was successful in obtaining a detached divertor H-mode with good confinement. However, the Zeff was rather high (although the change in Zeff due to impurity puffing was acceptable). Friday's run was an attempt to improve the plasma cleanliness and to diagnose well the divertor plasma. The goal of simultaneously achieving good confinement, low Zeff, and a detached divertor was reached. The algorithm developed in the previous run (980123) to use the edge bolometer signal to feedback on the impurity gas puff was used to achieve detached divertor H-modes with varying scrape-off layer power flows. This technique worked well in that varying degrees of detachment were achieved by varying the requested edge bolometer waveform. The deepest detachments achieved were up to the nose of the outer divertor. The amount of nitrogen required for minimal detachment varies with q-parallel; the higher the q-parallel, the larger the impurity puff needed to see detachment near the strike point. Also, the nitrogen definitely sticks to some surfaces, causing a buildup throughout the day. The Zeff began the day near 1.0 and was up to 1.3 at the end of the day (during the H-mode, before the impurity puff). On the best shot the H-factor was 1.8 and the Zeff was 1.1 as the H-mode developed. As nitrogen was puffed and the divertor detached the H-factor decreased to 1.6 and the Zeff increased to 1.4. The impurity confinement time was measured by injection of a trace amount of niobium into the detached phase of the H-mode. This time is ~70msec, in line with previous measurements for an EDA H-mode. The x-ray pedestal width and location do not change upon divertor detachment and are typical of EDA H-mode. Measurements of the heat flux using fast thermocouples embedded in the plates (Gangadhara and LaBombard) show that high heat fluxes (>400 MW-m^-2) have been lowered to immeasureable levels by detaching the divertor. The heat deposition profile is very narrow in the attached H-mode compared to that observed in high power L-mode. Over the weekend, work on the ICRF transmitter FMIT#3 continued. Preliminary measurements indicate that the transmitter works well below 70 MHz and will become more difficult for 70-80 MHz. Currently moderate power has been obtained at 75 MHz. Dr. Gerd Schilling (PPPL) has indicated that 1.5 MW was obtained at 80 MHz and 2 MW for 76-78 MHz. Retuning of FMIT#3 to 78 MHz will continue this week. Lines from the D_0 Lyman series (n=2,3,...9 to 1) are now routinely observed in the VUV spectral region when observing the divertor plasma or a MARFE in the main chamber. In addition, radiative recombination continuum emission corresponding to photo-recombination directly into the D_0 ground state is observed. The contiuum emission rises sharply around 92 nm (photon energy=13.6 eV) and decreases with increasing photon energy. The fall-off of this continuum emission is a strong function of the local electron temperature when the temperature is less than about 3 eV. In the C-Mod divertor plasmas and in the MARFE, temperatures from 0.7 to 1.3 eV have been measured using these continuum sprecta. Studies are underway to model the spectra, including the Lyman and Balmer series lines and the continuum spectra, for a single temperature/density region. The modeled spectra will be compared with the measurements of these features to see if the single temperature/density assumption is justified, and, if so, to measure the temperature and density. The DNB vacuum system reached its ultimate pressure of 10**-8 Torr early this week and remained there. Additional tests of the breakdown voltage of the oil in the DNB Accel supply indicate that it is holding at an acceptable level and conditioning is finished. Refurbishment of the control system for the Accel supply continued. Dr. Gerd Schilling (PPPL) was visiting last week. He participtated in experiments and RF operations. Perry Phillips, UT-FRC, reported on progress at FRC and at UC-Davis on the high resolution ECE system for C-Mod. Millitech completed construction of the two second harmonic mixers for the system. These mixers cover the frequency band from 234-306gHz, and each has a 40+gHz bandwidth. A prototype IF detector system is under construction at UC-Davis with a projected completion this month. A page on the FRC WWW site was set up to describe the high resolution ECE system and to chronicle progress. Ricky Maqueda from Los Alamos visited C-Mod during the weeks of Feb. 2 and Feb. 9. He installed a set of temperature references on the IR imaging system to aid in the calibration process and added a water chiller to cool down the IR camera and telescopic lens so as to reduce the number of background counts. Unfortunately, the recent boronization campaigns have apparently coated the sapphire vacuum window reducing substantially the throughput and yielding results of only qualitative value. Christopher Watts was up from Auburn University last week to work on the Temperature fluctuation measurements on the Heterodyne Ordinary mode radiometer. A different configuration of the Auburn ECE T~ system yielded preliminary correlation power spectra consistent with earlier results on TEXT. Earl Marmar attended two meetings at the San Diego ITER co-center last week: a progress meeting on the design of optical/spectroscopic diagnostic systems for ITER; and the Eighth workshop and technical meeting of the ITER expert group on diagnostics. At the second meeting, he gave a talk entitled: "Alcator C-Mod: Spectroscopic Diagnostic Development".