The primary activities at Alcator C-Mod during the second quarter of FY98 were: completion of a successful run campaign which began October 30, 1997, continued work on major systems upgrades (the RF transmitters and the Diagnostic Neutral Beam), continued participation in the critical assessment of fusion science, diagnostic removal, and machine disassembly for magnet repair and maintenance. A more detailed account of these activities follows.

Scientific Results

At the end of February the Alcator C-Mod team completed a productive run campaign which has yielded a number of new results.

Core Confinement Results

As has been mentioned in previous quarterly reports, a very promising confinement regime, which we have called EDA or Enhanced D_a mode after its salient characteristic, is under investigation on Alcator C-Mod. This H-mode regime, illustrated by the traces in Fig. 1, can evolve out of ELMfree discharges and is characterized by good energy confinement, moderate particle confinement, and no type I ELMs., though at b_N > 1.3, it does exhibit some form of small ELMs. The edge temperature can reach 600-800 eV, well above the threshold for type III ELMs, and the pressure gradient in the transport barrier seems to challenge stability limits. Unlike ELMfree discharges, which have long impurity confinement time and often end with a radiative collapse, this regime seems to have true steady-state potential.

Energy confinement in H-modes is seen to be affected by quantities not included in the global scaling laws. For core radiated power more than 50% of the input power, confinement begins to decrease, approaching L-mode levels for radiated fractions over 0.9. High neutral pressures are also associated with degraded confinement. Plasmas with divertor pressures over 40 mTorr begin to show a drop in confinement, a trend which continues to the highest pressures of 150-200 mTorr, where t_E is close to L-mode levels. It is worth noting that for both trends, the drop in confinement is continuous and that even in the extreme cases, discrete transitions can be seen on edge diagnostics when the plasma returns to L-mode. Limiting consideration to discharges with core radiated power fraction < 0.5 and divertor neutral pressure < 40 mTorr, energy confinement enhancement relative to the ITER89 L-mode scaling is found to be 1.8 for EDA discharges and 2.1 for ELMfree. Confinement is about 15% higher than the latest ITER97 H-mode scaling. In C-Mod, the density rise that accompanies the L/H transition is large enough to keep the plasma near the threshold even at the highest powers available - the data are all taken in the range P/P_threshold ~ 0.9-1.6, where P_threshold is taken to be .02nBS. There is no obvious trend for discharges nearer the threshold to have lower confinement.

Impurity particle confinement, t_imp, in EDA plasmas is in the range 0.15-0.2 seconds, roughly 2-3 x t_E, unlike ELMfree plasmas where t_imp > 0.5 seconds. Analysis of the impurity profile evolution shows higher diffusivity and a weaker pinch for EDA discharges than for ELMfree. This difference may be the result of additional turbulence which is seen during EDA. The EDA is characterized by increased density fluctuations in the barrier region seen by reflectometry. These are broadband, extending to at least 400 kHz usually accompanied by a quasi-coherent feature which can appear anywhere from 60 - 130 kHz. Unlike the D_a signal, which suggests that the ELMfree to EDA transition is gradual, the fluctuations are seen to turn on abruptly.

The conditions under which either ELMfree or EDA H-modes are obtained are beginning to be understood. EDA plasmas are more likely at the highest plasma density ( > 3.5 ×10²⁰ m^-3) and high divertor pressures, though there is significant overlap at moderate densities. Higher plasma currents favor ELMfree behavior, with EDA H-modes rare at 1.2 MA and above. The EDA regime is more likely to occur at higher power, though the correlation may be through the plasma density, which also tends to be higher in those cases. Plasma shape seems to play an important role in determining the type of H-mode. For otherwise identical conditions, discharges with low triangularity were consistently ELMfree while those with moderate to high triangularity were EDA. Experiments were performed in which the upper triangularity was scanned continuously from 0.37 to 0.18 and the lower triangularity changed from 0.64 to 0.4. During this scan the discharge gradually lost its EDA character, eventually becoming ELMfree. Density profiles measured by reflectometry and electron temperature profiles by ECE show pedestal widths less than 1 cm, at the limit of the diagnostic resolution. An edge soft X-ray array shows the steepest pedestal, with widths as short as 2 mm in high current ELM-free H-modes. X-ray widths also show an inverse dependence on plasma current and a positive correlation with triangularity. Parameters which cause the X-ray width to drop are precisely those which cause transitions from EDA to ELMfree behavior. Temperature pedestal heights show relatively little dependence on the type of H mode, which may suggest that the EDA regime mainly affects particle and/or impurity confinement. This would be consistent with the observed weak effect on global energy confinement.

Toroidal Rotation Measurements

Since plasma rotation plays an important role in the transition from L- to H-mode, and since most observations of toroidal rotation have been made in plasmas with an external momentum source, usually provided by neutral beams, it is important to study toroidal rotation in plasmas with no direct momentum input. C-Mod ICRF-only plasmas provide this opportunity, and co-current rotation in these discharges has been documented previously in these reports and in other publications. The mechanism for this rotation remains obscure, although radial electric fields generated by strong nonambipolar diffusion may be present. When the plasma current direction is reversed, the rotation during ICRF heating also switches, maintaining the co-current direction. The magnitude of the rotation is largest ( ~ 1.3 × 10⁷ cm/s, 200 kRad/s, 30 kHz, background ion Mach number ~ .4) during the best H-mode discharges and the rotation velocity during ICRF heating increases with the stored energy increase, regardless of input power or electron density, over a range of two orders of magnitude. The rotation is the same in D(He³) heated plasmas at 8 T as in H minority heated plasmas at 5 T with otherwise similar plasma parameters. The toroidal rotation is peaked at the magnetic axis and falls off quickly with minor radius. There is no evidence for a poloidal component of the rotation velocity inside of 6 cm. Values of E_r up to 300 V/cm at r/a = .3 have been inferred.

Strong increases in the central toroidal rotation have also been seen in ERS/PEP plasmas. Shown in Fig.2 are the time histories of several parameters of interest for a discharge with lithium pellet injections in conjunction with the RF pulses. After the first pellet, during the current rise at .15 s, the plasma begins to rotate rapidly (8 x 10⁶ cm/s) during the period with no sawteeth, between .2 and .25 s, much higher than expected for this stored energy. (The ECE was cut off after the pellets for some time before .2 and .63s.) The highest toroidal rotation velocities are seen during the current rise in discharges with the longest periods of sawtooth suppression, often achieved by the largest or multiple pellet injections. Plasmas with sawteeth during the current rise, even with small pellets, have very small toroidal rotation. The rotation after the second pellet (in Fig.2), which was just before the RF pulse during the steady current phase, is slightly higher than expected without a pellet. Pellet injection causes the rotation to decrease if it occurs after the RF pulse.

H-mode Threshold Scaling

Analysis of the global H-mode threshold scaling for some of the recent high field (6 - 8 T) data suggests that the toroidal field scaling may be less than linear. Additional data at 8 T fell on top of or slightly lower than previous 8 T data adding more statistical significance to a less than linear toroidal field dependence of the H-mode threshold. The data still fall between P/(nBS) of 0.015 and 0.03, when the absorbed power fraction is included based on a break-in-slope analysis of the plasma stored energy. However, a square root toroidal field dependence also falls within the error bars, suggesting that the H-mode threshold may not be as high as previously thought at high toroidal field. Nonetheless, it is clearly more difficult to get into H-mode at high field, so there is a positive toroidal field scaling. Further analysis of the toroidal field dependence of the H-mode threshold is required for scaling to ITER or for scaling to a future high field ignition machine.

Density Limits

Density limits were also investigated by using deuterium pellet injection this quarter. Ohmic and RF L-mode discharges were studied at 0.6, 0.8 and 1.0MA currents. Starting from a gas-fueled target density of about 1.5×10²⁰ m^-3, the density was raised by injecting a series of D₂ pellets. From 6 to 12 pellets were injected on each shot. This turns out to be a rather difficult way of studying the density limit. Convincing looking scenarios were obtained at 0.6 and 0.8 MA. We didn't quite have enough time to optimize at 1 MA. If the pellets are injected too aggressively, we get extremely high densities > 10²¹, highly peaked, P-mode/PEP mode type discharges. Additional pellets then cause disruptions. Impurity accumulation in the enhanced confinement mode may be partly responsible. If we back off too much, the plasma is too hot and pellet penetration is too shallow. We see good mass accountability, but the density pumps out in as little as 5 msec. The RF coupling is also sensitive; aggressive pellet fueling can cause trips in the RF. If these trips last too long, the plasma will cool and subsequent pellets will penetrate through the axis. Typically, the plasma can take at most one of these; additional pellets usually cause disruptions

It seems always possible to exceed the empirical (Greenwald) limit transiently with one or two big pellets. Exceeding it in quasi-steady state was not possible in this experiment. On several shots, the pellet fueled density buildup allowed us to gradually probe the limit. At 0.6 and 0.8 MA the plasma disrupted about 10% below the empirical limit.

Several weak H-modes were achieved during multiple deuterium pellet injection with target densities above 4 ×10²⁰ m^-3 even with just 1 MW of ICRF heating. Although they were weak, clear edge pedestals were observed on both the edge soft x ray arrays and on the edge ECE electron temperature measurements. In addition, large amplitude (several Gauss), low frequency (1 - 2 kHz), n=1 MHD oscillations driven unstable by the pellets were observed during both H and L-mode phases of these discharges. During the L-mode phases, the oscillations rotate in the electron diamagnetic direction, but during the H-mode phases, the oscillations rotate in the ion direction.

Dimensionless Identity Experiments

Preliminary analysis of the C-Mod/JET dimensionless identity experiments has shown rather good dimensionless agreement in the transport parameters. However, it now appears that the shapes run on JET in the shots most similar to C-Mod were somewhat different from those previously planned. Therefore more analysis is needed to investigate the extent to which shape differences have compromised the experiment or may explain the observed differences in ELM behavior.

Reverse Shear Experiments

Our investigations of reverse shear operation continued. Current ramp rates up to 5.7 MA/sec were employed. Toroidal field was varied between 4.9 and 5.7 T. RF power in the current ramp was varied, with up to 2.7 MW injected before 0.1 sec. Electron temperatures approaching 5 keV were obtained at densities around 1.1×10²⁰ m^-3. 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 ~ 0.26 sec. These shots also had significant increases in toroidal rotation after the pellet injection. PEP modes were readily obtained in the flat top.

Divertor and Edge Results

A number of runs this quarter were devoted to simultaneous achievement of a good confinement H-mode with low Z_eff and a detached divertor under extremely high SOL parallel heat flux. This goal was in fact reached. During the previous campaign we had been successful in obtaining a detached divertor H-mode with good confinement. However, the Z_eff was rather high (although the change in Z_eff due to impurity puffing was acceptable). The edge bolometer signal was used to feed back on the impurity gas puff, in this case N₂, and this feed back control allowed us 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 varied 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 stuck to some surfaces, causing a buildup throughout the day. The Z_eff early in the run was near 1.0, but increased to 1.3 at the end of the run (during the H-mode, before the impurity puff). On the best shot the H-factor was 1.8 and the Z_eff 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 Z_eff 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. At this time, t_imp was ~ 70 msec, at the lower end of the range for t_imp for EDA H-modes. The x-ray pedestal width and location did not change upon divertor detachment and were typical of EDA H-mode. Measurements of the heat flux using fast thermocouples embedded in the plates showed that high heat fluxes ( > 400 MW-m^-2) were lowered to unmeasurable levels by detaching the divertor. The heat deposition profile was very narrow in the attached H-mode compared to that observed in high power L-mode.

As reported previously, we have developed analysis techniques for determining the local recombination rates in detached regions, using the deuterium Balmer and Lyman series intensities. Opacities for the Lyman beta line are measured along some lines of sight, and opacity effects reduce the overall recombination rates. High spatial resolution CCD images of the divertor D_g emission have also been obtained. Inversions of these images yield the 2D profile of the D_g emissivity. An example of the D_g emissivity profile when the outer divertor is detached is shown in Fig. 3. This diagnostic is one of a number being used in the recombination rate evaluations. The results show that, while recombination is still significant, it is insufficient to explain the magnitude of the divertor plate current reduction seen during detachment. Instead, this appears to be consistent with a reduction in the ionization source, along with plasma pressure loss along field lines by ion-neutral friction. The importance of ion-neutral friction has been verified from parallel flow measurements of ionized and neutral species in the divertor using spectroscopic techniques.

Scrape-off Layer Transport Analysis

Significant progress has been made in characterizing the particle transport physics of C-Mod's scrape-off layer. Simple estimates of the total ionization source strengths in the main chamber (from measurements of D_a emission and midplane neutral pressures) yield numbers that clearly exceed measurements of the total ion flow through the divertor throat (measured by doppler spectroscopy and the scanning Mach probe). The implication is that, for most discharges, plasma in the main chamber recycles primarily on the main chamber walls (including the `flat-plate' portion of the divertor) rather than flowing into the divertor and recycling on the divertor plates. This observation is consistent with the original design philosophy for Alcator C-Mod's divertor: the divertor is optimized for high heat flux handling, with a throat width that accommodates approximately one power e-folding length. This optimization has been quite successful, allowing access to dissipative divertor regimes including the detached divertor regime. However, since the particle flux e-folding width is necessarily longer, particle fluxes from the core are not accommodated by the divertor. This effect is probably not unique to Alcator C-Mod and will need to be confronted in a reactor.

Modeling with the UEDGE transport code shows that in order to fit the measured scrape-off layer density profiles, some combination of a diffusion coefficient that rapidly grows with distance from the last-closed flux surface, or an inward particle pinch effect, is required. This result arises as a consequence of enforcing a realistic boundary condition for Alcator C-Mod that all wall surfaces have a unity recycling coefficient. These results, in conjunction with the above observations, have important implications for a reactor: (1) the neutral pressure at the midplane may be controlled more by the magnitude of the anomalous cross-field plasma transport rather than the geometry of the divertor or main chamber wall structures; (2) high midplane neutral pressures may be unavoidable in a long-pulse reactor where one can not employ the transient wall conditioning/pumping techniques that are used in most tokamaks operating today (with the exception of Alcator C-Mod). The latter effect may be the most serious with regard to the charge-exchange sputtering fluxes on the main chamber walls and the effect on the L-H power threshold.

RF Research

Radio frequency heating experiments have been concentrated on studying fixed frequency operation at 80 MHz, measuring heating efficiencies for two heating scenarios, hydrogen minority {D(H)} at 5.3 Tesla and ³He minority {D(³He)} at 7.9 Tesla. Recent D(³He) experiments have shown good total power absorption (up to 80%) when the ³He concentration was optimized (between 2% and 4%). A weaker dependence on minority fraction is seen for the D(H) case.

Experiments were performed in which the toroidal field was increased shot-to-shot from 5.6 T, thus moving the H cyclotron resonance as far as 16.7 cm off-axis. Nonetheless H-modes were obtained for D(H) minority heating even up to the highest field. The plasma current was also increased such that the scan was done at constant q. However, the H-factor (calculated assuming 90% absorbed RF power injected) progressively decreased as the resonance was moved. This result differs from JET's experience and reconfirms earlier C-Mod results. Furthermore, the inverse sawtooth phenomena observed on the inner wall dB/dt probes vanished approximately when the H cyclotron resonance passed beyond the sawtooth inversion radius. The speculation is that the sawteeth redistribute the minority ions, which decreases the absorption. Another result is the bursting neutron phenomenon often observed after an H-L transition. The H and 2nd harmonic D resonance is well outside the inversion radius, so 2nd harmonic D absorption cannot be the cause of the increased neutron rate.

For fields above 7.1 T, where the H cyclotron resonance is approximately at the antenna position, He³ was added to systematically investigate the minority and mode conversion regimes. All of these high-field plasmas were in L-mode. The physics results are encouraging. A scan of He³ concentration was performed at B_t ~ 7.9T. The RF was square-wave modulated at frequencies from 55 Hz up to 200 Hz to permit spatial localization of the electron (mode-conversion) heating as the He³ concentration was varied. At low concentrations of He³, the bulk ion temperature increases by ~ 1.2 keV while the electron temperature increases by ~ 0.7 keV. For higher He³ concentrations, significant mode conversion electron heating was observed. This electron heating was observed to move further off-axis as the He³ fraction increased, in agreement with theoretical expectations. In a single shot, as the He³ concentration increases, the RF can go from predominantly minority to mode conversion (ion heater to electron heater).

Continued studies of D(He³) ICRF heating at B_t ~ 8 Tesla showed that H-modes were obtained, but they did not develop steady state character nor were they of 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 made determining the total absorbed power difficult. However, absorbed power appeared 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. Lowering the density improved H-mode accessibility, but not the duration of the H-modes.

Analysis of data from the inner-wall array of RF loop probes has begun. This array is directly opposite one of the fast-wave antennas, and the loop probe signals often show a large transient when the RF is turned on. A series of three consecutive shots has been identified in which the decay time of the turn-on transient increases as the minority cyclotron resonance is moved outward from the center to the edge of the plasma. This decay is thought to be caused by heating of the bulk plasma and the generation of a minority ``tail''. Comparison is underway between those observations and the predictions of the Fokker-Planck code FPPRF.

Modeling of possible advanced tokamak operation on C-Mod has been undertaken with the ACCOME current drive and equilibrium code, combined with the PEST-II stability code. Starting with ICRF heated target plasmas characterized by inductively driven current profiles (q₀ < 1), it is found that reverse shear current density profiles can be created and maintained using off-axis Lower Hybrid Current Drive (LHCD). Reversed shear plasmas at the b limit could be produced in C-Mod at B₀ ~ 4.5 Tesla, I_p ~ 0.8 MA, and án_eñ in the range from 1×10²⁰ to 2×10²⁰ m^-3, with pulse length of about 10 skin times and 2 L/R times. These plasmas are sustained by a high bootstrap current fraction ( ³ 0.7). The shaped C-Mod geometry (k ~ 1.7, d ~ 0.7) results in high ideal MHD b-limits, even in the absence of a conducting shell (b_N £ 3.5).

Operations and Diagnostics

During this quarter toroidal fields up to 8 T were produced, although certainly not for the first time. Voltage regulation on the alternator has been improved, resulting in higher voltage from the TF supplies, and a consequent reduction in the time required to reach high field, and in the magnet heating and between-shot cool-down times. Power systems behaved well during the high-field operation. On February 24, early in the ramp-up phase, a short occurred in the TF magnet. The run was stopped, and the run period was terminated about one week earlier than scheduled. The Alcator C-Mod maintenance period was advanced by one week as well. Removal of diagnostics, igloo blocks, gas systems, thermocouple and heater cabling, and RF transmission lines and resonant loops began in preparation for machine disassembly and maintenance. The bus tunnel was also removed. The cryostat covers were removed. The drawbars holding the domes to the cylinder were released, and the top and bottom domes and the cylinder were removed. The PF magnets were removed and the TF magnet disassembled.

The TF arc started on the TF vertical leg at the first-to-last turn location and exhausted out radially between the leg and the cylinder. The damage was localized to the leg and upper arm, which can readily be replaced because of the jointed design. Fabrication of a new TF leg is already well underway. New copper plates are being machined and are in-house. Tooling for assembly is being set up, and processing for the new feltmetal is moving along rapidly. Complete documentation of the fault site has been carried out to assist diagnosis and correction. Additional inspection and measurements of the rest of the TF were undertaken. Inspections of the other upper arms indicated the need to replace feltmetal in many locations, particularly on the inner-most pads. We are currently reviewing the early analysis of the finger joints in this location and also extending this analysis with both analytic models and the better numeric tools now available. After an extensive set of measurements were made to document finger joint locations, spring plate tension, and arm-to-core clearances, the lower TF arms were removed. The lower arms and core fingers were found to be in very good condition, with little feltmetal wear. Understanding this upper-lower asymmetry has of course become a very important task for our analysis group. A review of the TF fault is planned for May 6th at MIT.

Progress continued on the DNB, the power supplies for the DNB, and on the upgrades of the RF transmitters/antennas/power delivery systems. The technical details of these upgrades can be found in the weekly reports. Delivery of the PPPL 4-strap antenna has been delayed because the antenna assembly had a problem with the TZM Faraday shield rods. Several of the braze joints and threaded rod sections have cracked. Further investigation by PPPL entailed pull- and shock-testing the rods. A single Faraday rod is made of three pieces: one straight piece with a threaded end (center post) and two bent rods with threaded ends. The three pieces are brazed together. All rods passed the pull test, but the bent rods failed the shock test. The failures show no signs of ductile deformation before breaking which suggests the material is brittle at room temperature. The proposed short term solution is to remove the threaded TZM and replace it with Inconel. In parallel, PPPL is also investigating replacement of the TZM rods with copper coated inconel. PPPL has continued to prepare the RF test stand for antenna electrical characterization and testing. Gerd Schilling (PPPL) has continued to develop a resonant loop model for the new antenna.

Analysis of data from our new phase contrast interferometer (PCI) shows a rise in 75-150 kHz turbulence during H-modes. The turbulence frequency changes as plasma density, temperature and other parameters change. Frequency vs wave number plots allow measurement of the dispersion relation for this kind of turbulence. Also, a very strong PCI signal is obtained when a pellet is injected into the plasma. Future plans include upgrades for a wider beam, a new detector, and heterodyne measurements of RF waves.

Analysis from the 88 GHz reflectometry channel, modified as part of the PPPL collaboration for high resolution fluctuation measurements, has begun. Fluctuations in the 100 kHz range have been measured during H-mode operation. The narrowband components have frequency shifts that can be correlated with sawteeth and D_a activity. A novel aspect of this new fluctuation diagnostic is that both the upper and lower sidebands of the AM modulated signal are monitored. These sidebands are separated by only 265 MHz and should therefore usually be well correlated. However, we have found that during H-mode formation, soon after application of RF power, the correlation can be poor. We are investigating both instrumental and plasma-physics-related explanations for this effect.

DNB diagnosticians completed their optical designs for MSE and CXRS. The optics were developed using commercial optical design packages. The diagnosticians are now involved in arbitrating the use of space on the flange and in recommending modification of structures near the machine to provide adequate access for the diagnostics.

Collaborations and Participation in the Fusion Science Community

At the end of January a mini-review of the MSE optical system was held. Norton Bretz and Bob Parsells of PPPL presented an outline of the current design to members of the MIT and UT-FRC staffs.

Dr. 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. Dr. Maqueda was also present during March to work on the IR imaging periscope. It was found that small chips had broken off the edges of the ZnSe optical elements and coated them. This was the reason for the observed decrease in the transmission of the periscope. Modifications to the periscope are being planned to fix this problem for the next campaign. We also plan to modify the view of the periscope to image the divertor surface below the outer nose. Finally, the Kodak fast framing camera hardware was removed from the C-Mod cell and shipped to Los Alamos so that the intensified imager can be serviced.

Christopher Watts from Auburn University visited during February to work on the Temperature fluctuation measurements using the Heterodyne Ordinary-mode radiometer. A different configuration of the Auburn ECE ``T_e fluctuation'' system yielded preliminary correlation power spectra consistent with earlier results on TEXT.

Gary Taylor of PPPL visited in February and continued commissioning and operation of the new 19-channel ECE diagnostic (GPC2). This was successful; 18 channels had good signal to noise. He was able to measure T_e response during the D(H) modulation experiments and will be able to carry out analysis of RF deposition with more detail than was previously possible. Some further optimization of filters will be necessary for edge measurements.

Chris Rost completed his PhD. His thesis dealt with edge absorption of ICRF power, ion tail formation and impurity generation during ICRF. He will be joining the Phase Contrast Imaging (PCI) diagnostic effort at GA, as an MIT Post Doc.

Along with the PSFC edge theory group, we have entered into a collaboration with Howard Scott and Alan Wan from LLNL. The collaboration involves the use of the CRETIN code for modelling of radiation transport in dense plasmas. We are interested in using this code for modelling of the transport of deuterium radiation (particularly the Lyman series) and the effects of radiation transfer on the ionization/recombination balance in our edge plasmas. Xavier Bonnin, a theory post-doc, along with a graduate student will be working with Jim Terry and Bruce Lipschultz on this code.

Our University of Toronto collaborators, Professor Peter Stangeby and his grad student, Steve Lisgo, visited Alcator at the end of March. The primary purpose of the visit was to discuss the current state of modelling that the U. Toronto group is doing using DIVIMP and EIRENNE codes to simulate C-Mod edge and divertor plasmas. Each collaborator gave a seminar on this and related work concerning perpendicular transport in the SOL. Work is progressing well in using the combined DIVIMP-EIRENNE model to understand the neutral dynamics and flows in the SOL. Steve Lisgo has been concentrating on analyzing D_a brightness data from the inner wall. He finds the best match to experiment when the emitting region near the divertor entrance has an electron temperature near 1 eV. We discussed other data that could be included in the modelling and thus improve the constraints on the model.

Seven C-Mod papers (one together with authors from DIII-D) were selected by the US panel reviewing the US contributions to the 1998 IAEA conference on Plasma Physics and Controlled Nuclear Fusion Research. These synopses have been submitted for consideration by the International Committee.

A DOE Quarterly Review was held as a video-conference on April 22. Rostom Dagazian represented DOE in Gaithersburg. The video-conference approach appears to be a viable and economical alternative for selected future reviews.

Selected Domestic Travel

Dr. Steve Wukitch visited Tokamak de Varennes (Canadian Center for Magnetic Fusion) in January for two days. The purpose was to examine a working LHRF system and discuss their operational experience.

On Jan. 29-30, Paul Bonoli travelled to PPPL to attend the PhD thesis defense of John Wright. Paul was one of the thesis readers and had suggested the problem which eventually became Wright's thesis topic - formulating the quasilinear diffusion coefficient of Kennel and Engelmann to include poloidal mode coupling of fast ICRF waves in toroidal geometry. Dr. Bonoli also gave a seminar on Advanced Tokamak modelling in Alcator C-Mod.

Miklos Porkolab attended the Advanced Tokamak Workshop at GA the first week in February, and presented the talk: ``Alcator C-Mod Advanced Tokamak Research''. He also attended the DIII-D Advisory Committee meeting, to hear the DIII-D Program Overviews.

Earl Marmar attended two meetings at the San Diego ITER co-center in mid-February: 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''.

On February 19 Earl Marmar attended a meeting of the Fusion Physics Planning Committee, which discusses U.S.-Japan bilateral fusion physics exchanges. The meeting was held at the Princeton Plasma Physics Laboratory.

Earl Marmar attended the ``National Compact Stellarator Concept Design Study Informal Program Advisory Committee'' meeting on March 5-6, held at the Princeton Plasma Physics Laboratory. Earl is a member of the Informal PAC.

Martin Greenwald, Amanda Hubbard, Rejean Boivin and Bob Granetz attended the Transport Task Force workshop in Atlanta in March. Two oral presentations were made, and one poster was presented. Bob Granetz spoke on ``Characterization of Edge Pedestals in Alcator C-Mod with X-ray Imaging'' and Amanda Hubbard discussed ``Comparisons of Edge Parameters on C-mod with Numerical Simulations of Rogers and Drake''. Rejean Boivin presented a poster on ``Investigation of the role of neutrals in Alcator C-Mod Plasmas''. Martin Greenwald was elected chairman of the TTF during this meeting.

March 23-26 Jim Terry attended the AIP Topical Conference on Atomic Processes in Plasmas. He gave an invited talk on ``The Study of Volume Recombination Radiation Opacity Effects in Alcator C-Mod''. An invited paper on the same topic will be published in the conference proceedings.

Paul Bonoli attended the Sherwood theory meeting March 23-27.

Miklos Porkolab attended the Special ISCUS-SWG1 meeting in San Diego on the Advanced Tokamak version of ITER. He gave the presentation and lead discussion on ``Importance of Shaping in AT Regimes''. He also contributed to the discussions on the ISST Tokamak, a new initiative from MIT on an Ïnertially Shielded Superconducting Tokamak".

In March Earl Marmar attended the US committee meeting on IAEA synopsis submission in Washington D.C.

In early April Miklos Porkolab visited the Los Alamos National Lab, both the Magnetic Fusion Group, as well as the laser fusion group, and gave a two hour seminar on the MIT PSFC Programs. He also discussed collaborations, in particular in the MHD modeling area, since LANL is completing a new nonlinear MHD code, called NIMROD, under the leadership of Allen Glasser.

Near Term Plans

Our near term plans are focussed on analysis of data from the last run campaign, the repair of the TF magnet, and machine maintenance and reconfiguration for the next run campaign. The physics staff are preparing presentations for the up-coming ``Next Step'' Forum (April), the PSI meeting (May), the conference on the Diagnostics of High Temperature Plasmas (June), and the EPS meeting (July). An ``Ideas Forum'' for the C-Mod program, open to the community, is tentatively planned for August. Mini-proposals for experiments are being solicited from the community. (Approved MP's can be found at http://www.psfc.mit.edu/server-java/MiniProposals.)

M. Umansky, S. Krasheninnikov, B. Labombard, J.L. Terry, ``Comments on Particle and Energy Balance in the Edge Plasma of Alcator C-Mod'', submitted to Physics of Plasmas.

B. Labombard, ``Coupling between Cross-field Density and Temperature Scale Lengths in High-Recycling Scrape Off Layers'', submitted to Plasma Physics and Controlled Fusion.

J.L. Terry, B. Lipschultz, A.Yu. Pigarov, S.I. Krasheninnikov, et al., ``Study of Volume Recombination and Radiation Opacity Effects in the Alcator C-Mod'', to be published in the Proceedings from the AIP Conference on Atomic Processes in Plasmas, Auburn, AL (1998).

J.W. Conner, ... B. LaBombard, et al., ``Comparison of Theoretical Models for Scrape Off Layer Widths with Data from Compass-D, JET and Alcator C-Mod'', submitted to Nucl. Fus.