Alcator C-Mod Weekly Highlights June 7, 1999 Last week was a maintenance week at Alcator C-Mod. No plasma operations were scheduled. This week will also be a maintenance week. Physics and Analysis --------------------- Impurity screening during recent divertor bypass experiments has been analyzed. A number of views of the plasma were monitored during the bypass opening in both the VUV (Ar-IV) and visible (AR-II) to see if the ionization source due to the bypass opening could be localized poloidally. The evidence shows that a) the ionization source change is primarily in the divertor region; and b) the cross-correlation of different views (and different spectrometers) indicates that the ionization change brought about by the bypass opening occurs either above the x-point or at smaller major radius at the inner divertor near the separatrix. This is interesting because one would expect the increased source to be near the bypass opening - thus near the outer midplane. Further experiments are needed to confirm this preliminary conclusion. Improved ion impurity spectra have been obtained from the Omegatron probe. A combination of data acquisition techniques and signal processing now permit unambiguous identification of resonances with intensities 0.1% of the non-resonant current. Improvement in the signal to noise ratio has revealed resonances with M/Z of 1, 5.5, 8, and 12, and has improved the resolution of the resonances previously reported, with M/Z=2, 4, 6, 10, and 11. Candidate ion species for the newly observed resonances include: doubly charged boron11 (M/Z=5.5) and oxygen16 (M/Z=8), and singly charged carbon12 (M/Z=12). Unlike many other resonances, the resonance with M/Z=1 is non-degenerate and can correspond only to singly charged hydrogen. The intensity of the M/Z=1 resonance is consistent with H/D ratios determined by spectroscopic methods for the same shot, but further analysis of the Omegatron data is required to give a quantitiative estimate of the hydrogen density. High frequency magnetic fluctuations have been observed during ICRF heating in the 1999 campaign at frequencies of 300 kHz and 800 kHz as well as at 440 kHz and 940 kHz. The pairs of modes are observed when both D- and E-port antennas are turned on and they are separated by exactly the 500 kHz difference frequency of the two RF transmitters (80 MHz and 80.5 MHz). These high frequency modes are correlated in time with up to 200 kW of reflected power from the D-port antenna. The modes are observed only in EDA H-mode and not in ELM-free H-mode or in L-mode (or in any Ohmic plasmas). They have nearly constant frequency for up to a couple of hundred msec. When there are particularly large sawteeth, the frequency is modulated by the sawteeth by about 20 - 30 kHz. The frequency rises sharply just after a sawtooth collapse and then slowly returns to the pre-collapse frequency during the ramp of the next sawtooth. These modes are in many ways similar to the modes that were observed in 1997 - 1998 during EDA H-mode at 600 kHz, except that these modes have even larger amplitudes with ~Btheta/Btheta about 5 x 10^-7 and these modes shut off abruptly as soon as the ICRF switches off rather than decaying away after 10 - 15 msec for the previous years' modes. The different frequencies also present another difference that makes it somewhat difficult to associate these new modes with possible TAE modes. In order to have the TAE frequency, omega_TAE = V_A/(2qR) = mode frequency, we would need a resonant q value of about 1.75 in the case of the 300 kHz mode. This would be well outside the sawtooth inversion radius and so at a sawtooth collapse, the density would increase at that radius. Then, the frequency should decrease right after the sawtooth collapse, if it were a TAE mode, but the frequency is observed to increase sharply after the sawtooth collapse. So, it is still unclear what type of modes these are and whether or not they are related to the high frequency modes observed in 1997 - 1998. One possibility might be that the ICRF transmitters are putting out some power at 80.8 MHz, which is then beating with the power at 80.5 and 80.0 MHz to give the 300 kHz and 800 kHz beat waves. However, the fact that the frequency is modulated by the sawteeth suggests that this is not the case and that the modes are driven by the plasma. While RF coupling changes due to the sawteeth could easily change the amplitude of an RF interaction, it is not easy to see how this could change the frequency. It is proposed to put a spectrum analyzer on the directional coupler that measures the D port reflected power to look at what frequencies are generating the reflected power when these modes are observed. ICRF Systems: ------------- Transmitter #4's DC power crowbar system has now passed the wire test. This completes the testing required to validate the refurbished protection circuitry on this transmitter. Testing has begun on FMIT#3 crowbar. The transmitter cooling system for all four transmitters has been improved. The water system was flushed and cooling channels cleared. An additional screen filter has been installed on the water inlet and a check valve removed. Improvements planned for later in the summer include installation of an additional filter screen and a better check valve. Testing was begun on the RF control system that protects both the antenna and transmitter. Some wiring errors in the primary control board have been corrected in FMIT#4. This system must be operational before RF testing into the dummy load can begin. Visitors and Travel ------------------- Ben Carreras is visiting us from ORNL. He is currently analysing Te fluctuations measured by GPC2 (the PPPL polychromator) in recent ohmic discharges, and will also look at signals from other diagnostics. His visit will continue this week. Several students from the University of Wurzburg are visiting the PSFC for the summer as part of an exchage program. They started June 1. Two students are working at C-Mod. They are Dominique Huebner, working under Bruce Lipschultz's supervision on spectroscopic problems, and Stefan Krotz, working for Steve Wukitch on ICRF problems.