Alcator C-Mod Quarterly Progress Report - May 2001

The primary activities at Alcator C-Mod during the second quarter of FY01 were:

assessment and rectification of the J-port antenna problems, improvement of the DNB

and diagnostics, and data analysis of results from the previous run campaign. Plasma

operation resumed on May 23.

Science Results

Core Confinement and Transport

Double Transport Barrier Plasmas

Double transport barrier plasmas comprised of an edge enhanced Da (EDA) H-mode
pedestal and an internal transport barrier (ITB) have been observed in Alcator C-Mod.
The ITB can be routinely produced in ICRF (80 MHz) heated plasmas by locating
the wave resonance on the high toroidal magnetic field side at r/a = - 0.5 (by operating

with a central field below 4.5 T), and can also develop spontaneously in some Ohmic

H-mode discharges. The formation of the ITB appears in conjunction with a decrease

in the central (impurity) toroidal rotation velocity. The ITB foot is located near

r/a = 0.5, regardless of how the barrier was produced. The ITBs persist for 15 energy

confinement times (tE), but exhibit a continuous increase of the central electron density, up to values near 1x1021/m3 (in the absence of an internal particle source), followed by

collapse of the barrier. This ITB is also evident in the electron and ion temperature profiles, and a drop of the core thermal conductivity, c eff , is confirmed by modeling.

Application of additional on-axis ICRF heating arrests the density and impurity peaking;

this occurs along with an increase (co-current) in the core rotation velocity, and steady

state double barrier plasmas have been maintained for 8 tE or longer, with a current

density bootstrap fraction of 0.13 near the ITB foot. The trigger for the ITB formation is unknown.

Shown in Fig.1 are the time histories of several parameters of interest for a 4.5 T plasma,

which had an EDA H-mode edge pedestal and a core ITB generated by off-axis ICRF

waves. 2 MW of ICRF power at 80 MHz were delivered for the time interval between

0.7 and 1.5 s, as seen by the green curve in the 4th panel of the figure. This deuterium

majority discharge had a hydrogen fraction of 4%. The plasma entered H-mode at 0.76 s,

as evidenced by the drop in the Da signal, and the subsequent rises in the plasma stored

energy, electron density, ion temperature and central toroidal rotation velocity. The EDA

H-mode was well established when around 0.85 s the rotation velocity began to drop in

conjunction with the formation of the ITB, as seen in the continuous rise of the core

electron density.

Fig. 1

The ITB formation may be visualized by examination of the evolution of the electron density profiles:

www.psfc.mit.edu/people/marmar/denmovie2.mpg

By adding a modest amount of on-axis heating power to an ITB discharge, the increase

of the electron density can be stopped and the barrier can be maintained in steady state.

This effect is demonstrated in Fig.1; at 1.25 s additional heating power at 70 MHz was

injected, reaching 600 kW by 1.35 s, as shown in red in the 4th panel. (The total ICRF power is shown in blue.) Not surprisingly, there was an increase in the plasma stored energy and the central ion temperature after this time. Of particular interest is the return

of the co-current toroidal rotation during this interval, along with a halt to the electron density increase. Besides the arrest of the electron density peaking and the increase in the core ion temperature, there are other beneficial effects from the additional on-axis heating, as is demonstrated in Fig.2. Between 1.35 and 1.5 s, the ambient level of central soft x-ray emission was held constant. Note that throughout the ITB phase of this

discharge, sawtooth oscillations were present, and that the sawtooth inversion radius was near R = 0.73 m, well inside of the ITB foot location. Similarly, the additional on-axis heating stemmed the increase of the total radiated power, holding it at a tolerable level of about 50% of the total input power. Likewise, Zeff was maintained at a constant value

of 1.8 during this time. In Fig.2 is also shown the ratio of the central electron density to the value at R=0.83 m, well outside of the ITB foot, reiterating the arrest of the density peaking.

Fig. 2

EDA/QC Mode

The EDA looks like a promising regime - with good energy confinement, no accumulation of impurities and no giant elms. To extrapolate EDA, we need to understand the physics. The observed QC mode is apparently responsible forEDA; they are always seen together. Probes show a strong particle flux from correlated n and E fluctuations. Deff calculated from density profiles and the measured particle source is roughly proportional to the mode amplitude as measured by PCI. The mode is localized to the outer portion of the density pedestal, apparently quite narrow in radial extent - maybe no more than 1-2 mm.

A large amplitude coherent mode is found in non-linear gyro-fluid simulations [Rogers and Drake, U MD], which shows that turbulence is not inevitable. Linear gyro-kinetic stability calculations using the GS2 code find an unstable mode - tentatively identified as resistive ballooning [Dorland, UT]. Jim Hastie has been working with J. Ramos and F. Porcelli on an analytic theory of RB stability.Initial results agree with parametric

dependences seen in experiments.Xu and Nevins [LLNL] using the BOUT code to simulate the edge (including separatrix) find a coherent mode - identified as a resistive

X-point mode with some similar features to QC mode.They predict a higher frequency mode at 4-6 MHz and reversal of propagation for reversed BT.

DIII-D Edge Dimensionless Similarity Experiments

The idea of the experiment was to study the similarity of the H-mode pedestal on two different tokamaks in plasmas with matched local edge dimensionless parameters (such as beta, normalized gyroradius, collisionality, aspect ratio) and matched shape (same elongation, triangularity and safety factor). This should allow an assessment of the relative importance of so-called plasma physics versus atomic physics in controlling the pedestal widths, amplitudes, and gradients which have been shown to impact critically the quality of H-mode confinement. The experiment was highly sucessful and we were able to produce the discharges at DIII-D with the shape parameters matching the C-Mod shape very closely. When input power applied to DIII-D discharges was scaled appropriately, edge plasmas with matching scaled Te and ne were produced. It was found that under these conditions the whole pedestal profiles in H-modeare matched when scaled according to the plasma size.The H-mode regime with grassy ELMs, very similar to the regime seen on C-Mod under the same conditions, was observed in the DIII-D
discharges. Stability analysis of the discharges (both DIII-D and similar C-Mod plasmas), analysis of fluctuation measurements results and of the physics of neutrals at the edge is under way.

Edge/Divertor Physics

Neutral Transport

Analysis of neutral particle transport in the main-chamber scrape-off layer (SOL) at the outer midplane has been performed for a small set of discharges using a newly developed neutral transport code, KN1D. This new code is a 1-D space, 2-D velocity kinetic code that treats atomic and molecular transport including molecular dissociation, charge exchange, and elastic collisions between neutral and plasma species. The code takes as input the measured plasma density and temperature profiles in the main-chamber
scrape-off layer and the measured molecular neutral pressure near the wall at the midplane. A unity recycling condition on main-chamber limiter and wall surfaces is imposed, consistent with that expected in experiment. The overall goals of the KN1D code work are to determine neutral fluxes, ionization source profiles, and charge-exchange heat fluxes for a wide variety of C-Mod discharges, allowing direct estimates of: (1) fueling of plasma in the SOL and inside the last closed flux surface; (2) cross-field plasma transport; (3) charge-exchange heat transport; (4) Lyman-alpha emissivity profiles near the midplane (which can be used to cross-check measured profiles). Initial
analysis has focused on a set of Ohmic L-mode discharges covering a wide range of plasma densities and corresponding midplane neutral pressures. These discharges exhibit a non-linear relationship between the midplane neutral pressure and the plasma flux crossing into the shadow of the main-chamber limiter (as inferred from both Lyman-alpha derived ionization profiles and Langmuir probes mounted on the limiter sides). Results from the KN1D code show that this non-linearity is clearly not the consequence of neutral-neutral collisions or neutral collisions with structures in the plasma-free region
surrounding the plasma, as had been previously speculated. A possible explanation is that the plasma e-folding lengths in the shadow of the main-chamber limiters increase systematically with plasma density (or midplane pressure). The midplane scanning probe diagnostic (which is used to measure the density profiles in the main-chamber limiter shadow) is presently at a toroidal location where the magnetic connection length between limiters is short (0.8 meters). At other locations in the vessel, the connection lengths

can be much longer (> 5 meters) allowing plasma to ‘fill-in’ the limiter shadow both by transport and by ionization. The importance of these effects will be investigated with further analysis using the KN1D code. As a more direct test of this hypothesis, measurements of the neutral pressure in the shadow of the limiter near the horizontal scanning probe will be performed during the next run campaign using new in-vessel Penning gauges.

Fig. 3

Another important result from the KN1D simulations bears on the role of charge

exchange heat transport in these discharges: charge exchange is clearly a negligible

player relative to the total heat flux crossing the last closed flux surface. In most cases, neutrals contribute to an inward-going heat flux ‘pinch’ across the separatrix. A comparison with the magnitude of cross-field plasma heat convection in the near and far SOL (see Fig. 3) also show that neutrals play only a minor role in the total heat transported across the scrape-off layer. For reference, an estimate of the cross-field heat flux from plasma convection is shown in the top panel ofFig. 3 for a variety of plasma densities. The corresponding heat flux due to atomic hydrogen (computed by KN1D) is shown in the bottom panel. The heat flux from atomic neutrals does not significantly impact the power losses in the SOL. In fact, at high densities, the atomic fluxes contribute and inward-going heat flux ‘pinch’ over much of the SOL.

B2-Eirene

The neutral transport code B2-Eirene is now operational for Alcator C-Mod geometry, for modeling neutral particle transport in the edge regions.

RF Research

Mode-converted ion Bernstein waves (IBW) near the ion-ion hybrid layer between Hydrogen and Helium-3 were again observed with the Phase Contrast Imaging (PCI) system. Plasmas were run with high field (5.9 T), low density (ne0 = 1x1020/m3)
and low current (400 kA), using a comparable mix of H, 3He and D in order to have
mode conversion occur near the plasma center. Strong signals (5 to 6 times background) were seen for both the D and E port antenna launch (the PCI is directly in front of the E port antenna). RF wave code results from TORIC suggest that the mode-conversion layer and associated electron Landau damping should be more aligned along a vertical chord
at lower plasma current, hopefully simplifying the PCI observation, which vertically integrates along 12 channels. Preliminary results indicate that there is still IBW signal in several distinct major radial locations, but possibly more localized than in the high current case.

Operations and Diagnostics

Operations

All C-Mod invessel work was completed on Friday evening, 4/27.Following inspections, documentation, completion of the invessel checklist, and a final wipe down of the chamber, one of the two access port flanges was installed. On Saturday, 4/28, the final access port was sealed and Alcator C-Mod was pumped down. The cryostat was purged with nitrogen gas in preparation for liquid nitrogen cooldown of the magnets.Approximately the next three weeks were devoted to baking the vacuum vessel, ECDC wall conditioning, vacuum conditioning of the rf antennas, and bringing power systems and diagnostics back online. The C-Mod heater system was brought back online in
preparation for the 120C.The CRYO system for controlling liquid nitrogen to the magnets was also brought online.Themagnets were cooled down to -25C to protect them during the bake.The ECDC system was brought back into operation briefly to check the gas control system and the magnetron source.ECDC wall conditioning was also initiated.The pulsed gas system was baked out and ready for CAMAC cycle tests on May 7. Plasma operations resumed on Alcator C-Mod May 23-25. Two run days were
scheduled and completed, with the primary focus being machine clean-up and conditioning in preparation for physics operation. A total of 20 plasma shots were produced, most lasting over 1 second; startup reliability was nearly 75%, which is very high for this early in the campaign.

Recent success in commissioning a 4-strap (J-port) antenna has allowed the maximum injected power to reach 2.5 MW from this antenna.From analysis of the antenna performance, a RF-plasma edge interaction at power levels > 2.5 MW resulted in injections and hot spots on the antenna top and side protection tiles (sometimes on the Faraday screen).These hot spots appeared to be connected on a field line with the longest connection length, typically a corner tile and a side tile.The corresponding side

tile hot spot would change in accordance with the edge plasma q. Furthermore, arc damage was concentrated in areas on the antenna strip line where the E-field was parallel to the ambient B-field.From the typical operation, several arc events were identified with an energy content consistent with the observed melt damage.The empirical electric field was 15-17 kV/cm.This is consistent with an empirical observation made at JET

where 1.5 kV/mm for E parallel to B and > 2.5 kV/mm for E perpendicular to B.

Some images may be seen at:

www.psfc.mit.edu/cmod/operations/EngImages/

RF/ICRF/P0001408.JPG

www.psfc.mit.edu/cmod/operations/EngImages/INVESSEL/2001/

Close_Up_Survey/P0001531.JPG

www.psfc.mit.edu/cmod/operations/EngImages/INVESSEL/2001/

Close_Up_Survey/P0001558.JPG

The antenna has been further modified to reduce the RF-plasma edge interaction.The protection tiles have been moved to the same radius as the other antennas.A BN septum has been installed to interrupt field lines, and all metal surfaces nearest the plasma except for the Faraday screen have been removed or covered . The strip lines have been rotated to make E perpendicular to the ambient B-field and increased the strip line spacing to reduce maximum electric fields. An additional arc detection system based upon the relative phase between antenna strap voltage has been implemented.New magnetic probes to measure the current in the antenna straps have been installed.In addition, new voltage probes and directional couplers have been added.A local pressure measurement and optical monitoring system has been implemented for the strip line region behind the antenna back plane. See:

http://www.psfc.mit.edu/people/irby/

jport_antenna_stripline3a.jpg

Diagnostics

ECE system:
We designed a modification to the internal ECE mirror system to improve the spatial resolution.Tests demonstrated that the spatial resolution is now limited only by diffraction (<1 cm) near the half radius.During the last C-Mod vacuum break, this modification was successfully installed.This modification should help in our search
for temperature fluctuations in C-MOD. A new data acquisition system based on the PCI bus is now under construction. This will eliminate the problems that we are having with the CAMAC technology used in our measurement of temperature profiles and provide full profiles for the entire discharge with frequency resolution up to 200kHz. Temperature fluctuation data will still be collected using the high frequency CAMAC system providing sampling times up to 5MHz.

Impurity injection laser:

A laser was installed for laser ablation of impurities for transient transport experiments.

Diagnostic Neutral Beam

CXRS optics:
The CXRS optics were removed, refurbished, and reinstalled during the recent C-Mod opening.The mirrors were repolished, windows were cleaned, shutters were added to the toroidal optics, and the optical systems were realigned.
BES optics:

The BES optics were repaired and recalibrated during the recent C-Mod opening.One in-vessel mirror, which had become detached, was secured. Redesign of the optical system to provide better damage resistance and to improve the fluctuation resolution was begun.

Beam duct monitors:

Diagnostics were installed in the DNB duct to monitor any reionization effects.These included pressure gauges and H-alpha monitors.These have been used and are working satisfactorily.

Progress on beam conditioning:

The best beam pulses were taken during February.During these pulses, the full energy component was as high as 23% and the water component was negligible.After repairs to the beam cryo system, a brief up-to-air for reconnection to C-Mod, and installation of the

beam duct monitors, the beam is being conditioned with the expectation that it will return to those levels.

CXRS calculations:

A calculation of photon emission cross sections from partial charge exchange was developed so that we can compare the emission cross sections for various sets of state selective charge exchange cross sections (partial cross sections).This will also be useful to investigate the effect of beam excitation as it transits the neutral gas in the beam duct.

The ambient emission due to plasma excitation of the charge exchange transitions was calculated for comparison with the above calculation.

MSE:

Repairs of an invessel mirror restored the MSE/BES view locations to proper alignment.Although initial beam into gas shots did not yield sufficient signal for the necessary MSE calibration, further attempts will be made after the beam drift duct has been properly

conditioned.Initial beam into plasma data was obtained, showing modestly improvements in signal to noise ratios compared to the end of the last run campaign.As the beam conditioning continues, a return to the high full energy component fraction achieved in February will yield better signals for MSE.

Lower Hybrid Project

Good progress has been made during the last quarter in all WBS areas. Highlights include: final design review of power supply/modulator; placing of circulator procurement contract; near completion of klystron cart assemblies and initiation of equipment racks assembly; completion of RF transmitter design; prototype testing and evaluation of low level phase and amplitude control system; successful brazing of
RF windows in prototype coupler; and development and testing of novel RF arc detector. The project remains on schedule and within budget.

 

Collaborations/Participation in the Fusion Science Community

DIII-D (see above in Core Confinement and Transport)

LHD/CHS

John Rice spent 6 weeks working at LHD. Recent LHD achievements include 1 MJ of stored energy, 4% beta, and a 2 minute steady state discharge.An analysis of `naturally'
breathing plasmas, and a comparison with those induced by running the plasma
against the stainless steel wall of the LHD vacuum chamber were performed. There is a strong dependence of the breathing frequency on the plasma density. There is also a perturbation on the electron density and radiated power emissivity which modulates at the breathing frequency, located near r/a = 0.5. CHS titanium x-ray data from an imaging camera system were also analyzed. In the plasma periphery, the emission is dominated
by inner shell satellites from B- and C-like titanium, and it can be concluded that the impurity diffusion is very slow in the plasma center, and highly anomalous near the edge. John also attended the large tokamak workshop and review of JT60-SC in Tokai, Feb. 7 and 8, where he presented recent C-Mod results.

JET

Joe Snipes spent the week of 22 - 26 January 2001 at JET to take part in experiments attempting to obtain Enhanced D alpha H-mode on JET.The experiment took place on 24 January.While long ELM-free H-modes were obtained, there was insufficient time to complete the experiment due to maintenance problems.In the final shot of the experiment, gas puffing into an ELM-free H-mode did produce a mode that was visible on the 92 GHz edge reflectometer channel at about 40 kHz during the ELM-free phase.The mode numbers have not yet been determined.However, since the density and radiated power continued to rise in the ELM-free phase, it was clear that the mode did
not significantly reduce edge particle confinement.Contingency time was approved for the experiment and Joe participated remotely in a continuation ofthe experiments on Friday, 2 February. There were seven discharge with -p/2 phasing of the ICRF antennas at 2.8 T, and 2 MA, to drive current near the q=1 surface, in an attempt to destabilize
sawteeth and prevent long 'monster' sawteeth.The first discharges had less than 2 percent hydrogen concentration and drove a substantial fast ion tail that did stabilize the sawteeth such that large n=2 MHD modes were driven after the first sawtooth collapse, which substantially degraded confinement. Raising the hydrogen concentration helped to destabilize the sawteeth and eliminate the large MHD modes.There were, however, problems with the ICRF such that only 5 to 6 MW of RF power was available, which was insufficient to get into H-mode.2.5 MW of NBI was added on top of the available RF power to stay in H-mode.Several long ELM-free H-modes were obtained as the target
density was raised in an attempt to find an operating window in which the EDA H-mode may exist on JET.However, there was insufficient time to complete this density scan.There was no clear evidence of EDA H-mode in the pulses obtained, but more analysis of the reflectometer and magnetic fluctuations needs to be done to see if there were any modes present that could be related to the Quasi-Coherent modes observed in C-Mod EDA H-modes.

Domestic Travel

Martin Greenwald was at UCLA Feb. 6 as part of the FESAC sub-panel reviewing the US fusion science theory and modeling program.
Miklos Porkolab attended the VLT-PAC meeting in San Diego, on Feb 22,23. He also gave a talk at the US-Japan Workshop on Plasma Control and Sustainment Using RF Waves. The title of the talk was ‘Progress in ICRF Experiments on Alcator C-Mod and Future Plans’. This workshop was held at General Atomics on Feb. 22,23 also. Subsequently, Miklos attended the FESAC meeting in Gaithersburg, MD, on Feb 27,28, and participated in the March 1st Fusion Congressional Outreach Day activities.

Earl Marmar went to a JET collaboration discussion at OFES in Germantown on Feb 26.

Earl Marmar, Ron Parker, and Miklos Porkolab attended the DoE Budget and Planning Meeting March 13 to 15 in Gaithersburg.Earl and Ron gave talks on C-Mod and the LH MIE Project.

Amanda Hubbard gave atalk at California State University of Sacramento on Thursday, 3/15, as part of the APS/DPP Distinguished Lecturers in Plasma Physics series,entitled ‘Understanding and Improving Energy Transport in Fusion Plasmas.’

Jim Irby visited PPPL Mar. 27 for discussions with Douglas Loesser, Randy Wilson, Stefano Bernabei, and Gerd Schilling on the Lower Hybrid Launcher.He also met with Raffi Nazikian and attended a PPPL/MIT proposal review of the reflectometry collaboration.

Miklos Porkolab and Paul Bonoli attended the Annual Sherwood Fusion Theory Conference in Santa Fe, NM, on April 2-4.

Bob Childs was in Brooklyn, NY on Apr. 6 supervising copper plating of the ICRF antenna stripline components.

On April 6, Amanda Hubbard gave the Plasma Physics Colloquium at Columbia University,NYC, entitled ‘Edge transport barrier and fluctuations in Alcator C-Mod’.

John Rice was at the Univ. of Wisconsin on Monday, Apr. 9, and gave a seminar on the latest C-Mod results.

Rejean Boivin was at GA Apr.17 and gave a talk on neutrals.

Miklos Porkolab attended the Tom Stix Memorial Service in Princeton on April 23.

Jim Terry reported at the weekly Physics Meeting on his observations at the April meeting of the APS, which took place April 28 - May 2 in Washington D.C. This event is a general meeting of the APS. Jim presented an Alcator C-Mod overview poster entitled‘Experiments in Plasma Physics and Fusion Science on the Alcator C-Mod Tokamak’.It was one of six other fusion posters at the meeting. There was one invited talk from the fusion community on self-organized criticality in plasma transport. The participation at this meeting was an ‘outreach’ by the fusion community to the rest of the physics community.

Ron Parker, Miklos Porkolab, Joe Snipes, and Spencer Pitcher attended the Burning Plasma Workshop II at General Atomics on May 1-3.Options for a future burning plasma experiment - Ignitor, FIRE, and ITER-FEAT, were discussed.

Martin Greenwald travelled to Washington DC May 7 to attend a meeting of the ESnet Steering Committee where he presented a talk on Office of Science programs and their network requirements to a group of DOE program managers.

Miklos Porkolab and Spencer Pitcher attended the FESAC meeting in Germantown,

MD, on May 15,16. During the Public Comment session Miklos gave a brief presentation on the status of graduate students at the PSFC. He emphasized the healthy status of graduate student enrollment at MIT, and the strong job market for recent Ph.D's graduating in plasma and fusion sciences. Spencer discussed his ‘open letter to

FESAC’ advocating a burning plasma tokamak experiment, during the public

comment session.

Jim Terry, Brian LaBombard, John Rice, Martin Greenwald, Catherine Fiore,

Jerry Hughes, and Valerie Censabella attended the 14th Transport Task Force

(TTF) meeting in Fairbanks, Alaska, May 16-19. LaBombard, Greenwald and Terry

also participated in a Meeting on Plasma Turbulence and Transport in Edge/SOL

regions, also held in Fairbanks on May 14-15, prior to the TTF meeting.

International Travel

Rejean Boivin gave a colloquium to the Physics department at the University of Montreal on C-Mod and fusion in general on Mar.4.He also met withrepresentatives who were present at the APS-DPP teachers' day (Quebec city -- October 2000) to discuss feedback from the meeting, and plans for the future. He also made contact with officials from the Montreal Science Museum who wanted to learn more about plasmas and fusion and had heard about the teachers' day.
Steve Wukitch attended the US-Japan RF physics workshop in Japan, March 22-24.

Martin Greenwald travelled to Lausanne Switzerland to participate in the international database meeting held April 2-5.He took the opportunity to meet with Jo Lister, Basil Duval, Richard Pitts and Yves Martin to discuss plans for joint TCV - C-Mod

experiments.

Amanda Hubbard attended the International Workshop on Physics of Internal Transport Barriers, Edge Pedestal, and Steady State Operation in Tokamaks, in Garching, Germany the week of Apr. 28.She gave talks on C-mod pedestal andITB results, and also

made plans for a comparison experiment with ASDEX-Upgrade on Type II elms and EDA modes.

Eric Nelson-Melby traveled to the CRPP-EPFL in Lausanne, Switzerland on May 7

and gave a seminar on C-Mod RF PCI results entitled "Phase Contrast Imaging

of Ion Bernstein Waves in Alcator C-Mod."

Reviews

Lower Hybrid

On 2/7 and 2/8 the Lower Hybrid PDR was held at MIT.The review panel consisted of Tim Bigelow (ORNL),Tom Intrator (LANL), Nevell Greenough (PPPL), Steve Knowlton (Auburn), and Claude Gormezano (Frascati).Both T.V. George and Rostom Dagazian represented DOE at the review.The MIT group presented both overall plans and some details of the power, control, timing, and data acquisition systems.They also presented results from the klystron test stand indicating the status of all 16 tubes.The PPPL group presented extensive details of the launcher design.

Diagnostic Neutral Beam

A review of the DNB and related diagnostics was held at MIT on Thursday, 2/15. The panel members were Fred Levinton (PPPL),Ray Fonck (UWM), Larry Grisham (PPPL), and Al Von Halle (PPPL). Chuck Finfgeld organized the review and represented DoE.Many from MIT attended including Bob Granetz, Earl Marmar, David Terry, Howard Yuh, Jim Irby, Bruce Lipschultz, Jim Terry, and John Rice.Gerd Schilling and Garrit Kramer attended from PPPL. Rostom Dagazien (DoE) participated in the review via video conference.
The charges to the panel were:

Please comment on the progress in installation and initial operation on Alcator C-Mod of the University of Texas Diagnostic Neutral Beam (DNB), giving due consideration to the financial, material and human resources that have been available.

Please comment regarding the difficulties that have been encountered in initial DNB operation, and give every suggestion you can for improving DNB performance.

Please provide your best assessment of the potential ultimate performance of the DNB and the resulting prospects for the desired plasma diagnostic measurements, including current density profile (MSE), ion temperature and rotation profiles (CXRS) and internal fluctuations (BES).

Earl Marmar has assigned responsibility for coordinating DNB and related diagnostic activity to Bob Granetz, implementing a suggestion made by the C-Mod Program Advisory Committee that a senior staff member take on this challenge.

Installation of improved duct diagnositics and addition of shutters to the charge exchange optics have been implemented. The remainder of the recommendations will be addressed during the coming campaign.

ICRF

On Wednesday, 3/14, a final J-Port antenna review was held via video conference.Steve Wukitch presented details of the problems we had with the antenna, and the changes we are making to allow full power performance during the next run period.Steve's presentation can be found at
www.psfc.mit/people/wukitch/antreview.pdf

Attending the review were:

ORNL:Phil Ryan, Rick Goulding

PPPL:Gerd Schilling, Bob Ellis

GA:Bob Pinsker

MIT:Steve Wukitch, Rejean Boivin, Jim Irby, Peter Titus, Bruce Lipschultz,

Brian Labombard

UTexas:Perry Phillips

There was general agreement on the antenna modifications.There was also strong agreement that phase measurements with fast time response are essential.These measurements have been implemented. Since the modifications require supporting the antenna at the top and bottom from the center of the antenna, a concern was raised about thermally induced stresses. FEA analysis of the antenna performed following the review indicates that these stresses are in fact small.

Near Term Plans

The immediate goal of this run period is to evaluate the performance of the
J-port antenna, and the DNB. Among the scientific goals for this run period are: H-mode operation with ICRF power in excess of 5 MW; further documentation/understanding of the QC mode and double transport barrier plasmas.

Several unanswered questions regarding the double transport barrier plasmas

will be addressed: Is this ITB an energy barrier as well? Can the ITB be sustained in steady state? Why is the ITB foot location near r/a = 0.5, regardless of ICRF

resonance location, or whether it is formed in Ohmic plasmas or by pellet injection? Why is there an abrupt threshold (in lowering BT) for the ITB formation by ICRF heating? Does the ITB appear only with the ICRF resonance on the high field side, or is there a power threshold? How is impurity transport affected by the ITB?

Schedule

Plasma operations will continue until July 27.