| Quarterly Progress Report on Alcator C-Mod - September 1999 |
The primary activities at Alcator C-Mod during the third quarter of FY99 were: repair, upgrade, and maintenance of the ICRF auxiliary heating systems; resumption of the C-Mod run campaign; analysis of data from the run campaign; and continued participation in the critical assessment of fusion science. A more detailed account of these activities follows.
Scientific Results
Core Confinement and Edge Pedestal Results
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| Figure 1. Density profiles and time histories from an Ohmic H-mode discharge showing |
| an enhanced neutron rate and a peaked density profile. |
Investigations of the so-called ``Enhanced Neutron Mode'' have continued. In these discharges, just after the H-L transition, the neutron rates increase by up to a factor of 3 over the H-mode level. This increase is attributed to spontaneous formation of an internal transport barrier (ITB) triggered by the profile changes following the H-L transition. The density profile evolution and the time history of the neutron rate are shown in Figure 1. The increased neutron rate can be well fitted using the measured central electron temperature from the ECE GPC system and the central density calculated from the visible bremsstrahlung array. Most of the increase in neutron rate comes from the temperature increase in these discharges; the central density is in fact falling at the time of the increase in neutron rate. Large amplitude, low frequency (2 - 10 kHz) MHD modes appear just before the peak in the neutron rate on most of these discharges. They appear to be m=1, n=1 in the core and m=2, n=1 on the edge magnetic pick-up coils. These modes become so large that they lock to the wall, but so far, they have not led to disruptions. The large MHD can be present even when there is not a large increase in neutron rate. While there are also cases in which there is no clear MHD mode on the magnetics and there is a large increase in the neutron rate, the central soft x-rays do see an m=1 mode. Calculations of the profile of the density from the visible bremsstrahlung indicate that the large m=1 mode begins when a steep gradient in the density and pressure arrives at the sawtooth inversion radius.
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Ohmic H-modes with a large electron density gradient in the core of the plasma on top of the edge density pedestal have been observed. The plasma core and edge density profiles are measured using Thomson scattering diagnostics. These H-modes lasted more than 1 second and developed a sharp core density gradient in addition to the edge pedestal. This is in contrast to ``normal'' H-modes (both ohmic and RF induced) in which the profile is characterized by steep edge density gradients and more or less flat core profiles. The core density gradient develops gradually during the H-mode and is not a continuation of the edge pedestal but starts well inside the separatrix at Rmajor of about 0.87 m while the edge pedestal is located around the separatrix at Rmajor of about 0.89 m. The resulting core density profile can be strongly peaked, with the central density as high as 6×1020 m-3 and the pedestal density around 2×1020 m-3. Further investigations of these phenomena are under way.
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A quasi-coherent mode in the frequency range from 50 kHz up to 300 kHz is often observed by both reflectometry and phase contrast imaging (PCI) during Enhanced Da (EDA) H-modes. Time histories of the frequency spectra from the reflectometer and the PCI, illustrating this mode, are shown in Figure 2. This mode was believed to be located on the EDA H-mode pedestal; however, no direct experimental evidence had been reported due to the limitation of both diagnostics. After configuring the reflectometer to take fluctuation data in four channels, (the channel with highest frequency, 110 GHz, has the critical density of 1.5×1020 m-3), the location of the mode has been roughly identified. It is observed in some shots with low H-mode pedestal height that the mode appears [disappears] on the 110 GHz channel depending on whether the critical density is on [inside of] the pedestal. (See Figure 2.) The density profile used for the comparison is from the visible bremsstrahlung measurement. Comparison among different reflectometry channels also indicates that the mode starts to appear at the top of the pedestal and soon fills the whole barrier region.
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One of the attractive features of the EDA H-mode confinement is a reduced impurity confinement as compared with ELM-free H-mode. Nonetheless, EDA impurity confinement is significantly longer than in L-mode. For these reasons it is important to document the impurities and their concentrations in discharges exhibiting EDA confinement. This was done in a run that occurred immediately after a boronization. The plasmas were extremely clean relative to pre-boronization levels. The concentrations were measured using passive x-ray and VUV spectroscopy. Zeff was 1.2-1.25. The E and D antennas were used for auxiliary heating, and RF powers varied from 0.9 to 2.2 MW. EDA H-modes were routinely achieved with the H-factors (ITER-89P) of 1.6. During the EDA H-mode phase, the radiated power as measured by the core foil bolometer typically increased from 400 to 950 kW, while the power as measured by the core diode (radiation only) bolometer increased from 50 to 250 kW. (This was for ICRF plus Ohmic input powers of ~ 2.6 MW.) Ar was puffed for diagnostic purposes on all but two shots, and was found to account for ~ 20% of the radiated power increase during H-mode. Taking the foil and diode bolometers at face value, it appears that ~ 300 kW (55%) of the power loss increase brought about by EDA is from neutrals, while 80-100 kW (20%) is from Ar. The rest (100-150 kW) is probably accounted for by B and F. Although this requires further quantitative analysis. Ar (puffed), B, F, C, O, Mo, and Fe impurities were measured. C, O, Mo, and Fe concentrations were all much lower after boronization.
Recent experiments, using discharges in which the molybdenum impurity levels are not as low as they are just after boronization, have improved our understanding of the Mo source in ICRF-heated discharges. In the past, the inner wall has been identified spectroscopically as the source of the molybdenum found in the plasma core during current ramp up, when the plasma is limited by the inner wall. However, until the beginning of the current experimental campaign, there had not been a consistent correlation between the spectroscopically measured molybdenum sources and molybdenum core densities during the steady state portion of the plasma discharges and especially during the RF heating pulse. This was the case despite systematically monitoring a number of surfaces including the outer divertor (biggest source previously observed), the inner divertor, the inner wall, the antenna protection limiters, and the ICRF antennas at the midplane. During the last internal inspection, significant erosion had been found along the top protection tiles of both (D & E) antennas, which prompted redirection of two spectroscopic views to look at the top of the two antennas instead at the midplane. Measurements since then have shown that indeed the top protection tiles seem to be the main source of molybdenum during RF heating. In L-mode, clear correlations between the antenna Mo sources and the core Mo levels are found. The antenna source rates (and the core Mo density) correlate with the RF power levels. The correlations can be mainly attributed to a local effect (e.g. local sheath acceleration of ions) rather than to a global effect through the heating of the plasma, although the latter has been also observed. Other sources have been excluded from being the main contributors to the observed Mo core levels because of lack of correlation between Mo sources and Mo core density. Specifically, it has been observed that the outer divertor and inner wall source levels decrease significantly during ELM-free H-modes, in contrast to the antenna Mo source, which is unchanged, and to the increasing (due to transport) molybdenum core density.
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The Beam Emission Spectroscopy receiver is being used to look at fluctuations in the edge Da emission. The views are essentially toroidal. Highly correlated fluctuations (for frequencies below ~ 50 kHz) between radially separated views are seen. In contrast, correlations between poloidally separated views drop off rapidly with separation of the views. In both cases there is little (poloidal) or no (radial) indication of propagation. These results are interpreted as being due to contamination by Da emission from the near- and far-fields of the views.
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An IDL routine has been written which automatically determines the confinement mode (L-mode or H-mode) of the plasma by looking at the edge soft x-ray brightness profile from the outboard midplane. The routine takes advantage of the fact that the soft x-ray emissivity develops a pedestal shape in H-mode, and that the emissivity at the top of this pedestal typically is 10-40 times higher than the emissivity in L-mode. Initial testing indicates that the routine is very reliable, but not fool-proof. The routine may become a valuable tool for statistical analysis of large amounts of plasma data where manual determination of the confinement mode is not feasible.
Divertor and Edge Results
As has been reported previously, Alcator C-Mod has ten sets of mechanical flaps which, when open, allow gas conduction from the private flux region of the C-Mod divertor to the region outside the main plasma. The open-or-closed state of these bypass flaps can be changed as desired in ~ 50 ms. Previous experiments have found no discernable effect of the divertor bypass (within an accuracy of ~ 10%) on energy confinement. This includes Ohmic, L-mode and EDA H-mode discharges. Further experiments using these bypass flaps have now been done. The state of the bypass flaps has no clear effect on the H-mode threshold power over a range of densities. Because the ``flaps open'' case (improved conductance from the divertor to the main chamber) reduces the divertor neutral pressure by about a factor of two, we conclude that the divertor neutral pressure does not play a role in the H-mode threshold.
It has also been found that the bypass flaps do not directly affect the density threshold for divertor detachment. The plasma density at which detachment is observed ([`(ne)] = 2.2×1020 m-3) does not depend on the state of the bypass flaps, although this threshold density is reached earlier when the flaps are open, since open bypass flaps do provide for a faster density increase (for the same gas puff) than closed flaps. This result is consistent with the model that the detachment threshold depends more upon upstream conditions, e.g. PSOL, and that there are more than enough divertor neutrals to remove the SOL momentum.
Studies of the effects of the bypass flaps on divertor impurity compression have continued. In contrast to previous work, a non-recycling impurity (N2) was used for these experiments. Experiments were performed at three different discharge densities (1.1, 1.6, and 3×1020 m-3). The N2 gas puffing position was varied from the outside midplane, to the outer divertor, to the private flux region, to the inside midplane. Discharges were run with either the flappers open or closed for the entire discharge. No major differences were observed in any discharges comparing flaps closed with flaps open. These results were based on spectroscopic measurements of nitrogen, both in the divertor and the main chamber. Nitrogen signals were also obtained on the divertor RGA and the omegatron. This null result was obtained despite the usual factor of 2 effect of the flaps on the divertor neutral pressure, and suggests that nitrogen does not pass through the bypass (as recycling gases do, such as argon). This is consistent with nitrogen being a non-recycling species.
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Nitrogen impurity-puffing into high density (ne0 > 4 ×1020 m-3) discharges showed that the detachment threshold and depth of detachment were modulated by the amount of nitrogen puffed. This is not a new result. However, new images of the detachment in Da and Dg light show the detachment modulation clearly. A region of volume recombination is observed to move from the outer strike point up to the x-point along the outer leg as the detachment becomes deeper. After the nitrogen puff, the recombination region retreats back to near the outer strike point. In the cases of deepest detachment, the Dg emission reaches closed field lines inside the separatrix.
RF Research
During the conditioning experiments of the J-port antenna (5.2 T plasmas with the H resonance is on-axis at 78 MHz), it was observed that the best heating was observed with a relative antenna strap current phasing of [0,p,p,0]. Other phase relations did not result in H-mode and had significantly more impurity influx, particularly for the impurities of the Faraday screen, Ti and C, and for Mo.
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First data from the RF phase contrast interferometer (RFPCI) were obtained. The RFPCI measures the density fluctuations associated with the launched RF wave. This diagnostic will allow measurement of core RF waves. Probes will continue to provide information about the edge. Initial analysis suggests that the amplitude scales with injected RF power. Further testing and experiments are required to verify the operational and measurement reliability.
Operations and Diagnostics
Much of the operational time on the tokamak this quarter was devoted to commissioning of the ICRF system and conditioning of the antennas. The problems experienced during the last quarter with the ICRF transmitters (poor reliability and two tube failures) have for the most part been solved after a significant reworking of the RF control and protection systems. This rework was conducted with the help of PPPL scientists and engineers. In addition one of the high power tubes now in use is on-loan from PPPL. During this quarter another high-power RF amplifier tube failed. It was clear upon investigation that the failure was due to inadequate cooling of the filament connection which resulted in a vacuum leak. This 8 to 9 year old tube was installed at MIT in 1996. A replacement tube was borrowed from GA. Four high power (each 2.5 MW nominal) tubes are now installed and operational. Launched RF powers of 4.4 MW have been achieved, and work on improving the system reliability is continuing.
On July 27 plasma operations on C-Mod were interrupted to repair an internal gas feed line. The run on the 27th had experienced a high incidence of current-rise disruptions, and unusual levels of iron influx were determined by the spectroscopic diagnostics. The run was halted, and visual inspection revealed that a stainless steel gas line had broken and was sagging into the discharge region. On the 28th the vacuum vessel was vented to helium and the broken line was removed, using special tooling introduced through a gate valve at K-port. Following a boroscope inspection to verify that no other internal components were compromised, the machine was pumped down. A low temperature bake (55 deg. C) was done, and Electron Cyclotron Discharge Cleaning (ECDC) was run for ~ four days. The machine was boronized after the H/D ratios showed that the walls were reasonably free of water. Good H-mode plasmas were obtained nine days after the machine was vented.
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Most of the personnel formerly working on the DNB were shifted during this quarter to the ICRF commissioning effort. Recently, the ICRF effort has required fewer people, and work on the DNB has resumed. Finite Element Analysis calculations are now being performed on the DNB superstructure from the test cell wall to the tokamak. Results will determine the size and cross section of the beams to be used to carry the load.
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A Cost and Schedule Review for the proposed C-Mod Lower Hybrid Current Drive System was held at MIT on June 30-July 1. This project is planned as a collaboration between MIT and PPPL. The panel (T. Bigelow (ORNL), F. Soldner (JET), R. Pinsker (GA), T. Intrator (LANL), and W. Ferguson (LLNL)) heard technical presentations by Miklos Porkolab, Ron Parker, Ian Hutchinson, Paul Bonoli, and Joel Hosea (PPPL).
Collaborations and Participation in the Fusion Science Community
Abstracts for the American Physical Society - Division of Plasma Physics meeting, to be held in November, have been submitted. From C-Mod there will be 4 Invited Oral presentations, 11 Contributed Orals, and 32 Contributed Posters. A list of the first authors and titles of the oral presentations follows. The abstracts are now available on the Web at
http://www.psfc.mit.edu/cmod/aps99/abstracts.html
Invited:
Bonoli, P., (MIT), ``Mode Conversion Electron Heating in Alcator C-Mod: Theory and Experiment''
Boivin, R., (MIT), ``Effects of Neutral Particles on Edge Dynamics in Alcator C-Mod Plasmas''
Rice, J., (MIT), ``Toroidal Rotation in Alcator C-Mod Plasmas with No Direct Momentum Input''
Pitcher, C.S., (MIT), ``The Effect of Divertor Baffling on Alcator C-Mod Discharges''
Contributed Orals:
Terry, J., (MIT), ``Recent Results from Alcator C-Mod''
Mossessian, D., (MIT), ``H-Mode Pedestal Measurements on Alcator C-Mod''
Granetz, R., (MIT), ``Impurity Transport and H-mode Edge Pedestals in Alcator C-Mod''
Pedersen, T., (MIT), ``Poloidal Asymmetries in Soft X-ray Pedestals from Alcator C-Mod''
Mikkelsen, D., (PPPL) ``Ion Temperature Gradient Scale Length in Alcator C-Mod: Theory and Experiment''
Taylor, G., (PPPL) ``Electron Power Deposition During ICRF Heating on C-Mod''
In, Y. (MIT) ``Observation of n=1 Resistive Interchange Mode in low-beta Plasmas''
Mazurenko, A., (MIT) ``New Results from the Phase Contrast Imaging on Alcator C-Mod''
LaBombard, B. (MIT) ``Cross-Field Particle Transport through the Separatrix and Scrape-off Layer in Alcator C-Mod''
Goetz, J., (MIT) ``Impurity Screening Studies on Alcator C-Mod''
Boswell, C., (MIT) ``2-D Profiles of Volume Recombination in the Alcator C-Mod Divertor''
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After the lower hybrid cost and scheduling review in early July, Randy Wilson stayed on site to help with J-port antenna-conditioning.
Gary Hallock (UT at Austin) visited the PSFC on 7/7/99-7/9/99 to help with the PCI upgrade and data analysis.
Dr. Christopher Watts of Auburn Univ. visited for the week of July 12th. He was working with John Heard on hardware modifications and documentation for the ECE correlation radiometer.
Dr. Ron Bravenec was on-site beginning July 16. He remained through July 22, measuring D-alpha profiles and working on BES CAMAC data acquisition.
Gary Taylor was on-site working on the GPC2 ECE instrument. He made good progress in reducing the pickup which has been experienced during operation of the new J-port RF antenna.
Matt Sampsell (U.Texas) visited C-Mod the first week in September to make some adjustments to the BES viewing geometry. David Winslow (U. Texas) was on-site for two weeks to start driven divertor probe experiments and explore divertor fluctuation measurements. R. Chatterjee (U. Texas) visited for three weeks to continue work on temperature fluctuations with the ECE radiometer.
Dr. Ricky Maqueda from LANL was on site for a week in August to investigate edge fluctuations and "filaments" as seen by gated ( ~ 10 msec), fast framing (1000 fps) camera. He will also be running the IR camera system. This is part of the C-Mod collaboration with LANL.
Ron Bravenec (U. Texas) was at C-Mod the third week in August looking at Da fluctuations with the BES system optics.
Perry Phillips was on-site in August working on the UT ECE system. Software was developed to look for changes in local Te gradients, the slow ECE channels were reconfigured to improve signals, and a new temperature fluctuation IF system was added.
Joel Hosea and Randy Wilson visited C-Mod during the last week in August to participate in the new J-port antenna startup experiments.
Selected Domestic Travel
Considerable work went into preparing for and attending the ``Snowmass'' meeting on ``Opportunities and Directions in Fusion Energy Science for the Next Decade'', which took place July 11-23 in Snowmass, Colorado. Seven of the C-Mod staff and a number of collaborators participated.
Jesus Ramos attended the US-Japan workshop on high-beta systems at GA in September. He gave a talk on ``MHD-stability and current-drive studies for advanced tokamaks''.
In late July Bruce Lipschultz went to the APS DPP `sorters' meeting where the program for the APS-DPP November meeting was organized.
Miklos Porkolab attended the FESAC Panel meeting in Knoxville, Tenn, from Aug. 18 through Aug. 21.
Miklos Porkolab attended the FESAC meeting Sept. 1-2 in Gaithersburg, Md.
Martin Greenwald attended a meeting of the ESnet Steering Committee (ESSC) on which he represents the interests of the fusion energy sciences community.
International Travel
In early July Joe Snipes presented a talk at the CRPP in Lausanne on ``Fast Particle Driven Modes in Alcator C-Mod''.
In mid-July Josh Stillerman attended the 2nd IAEA Technical Committee Meeting on Control, Data Acquisition and Remote Participation on Fusion Research which was held in Lisbon, Portugal. Josh gave an oral presentation titled ``WWW interfaces for runtime relational database applications''. After this meeting, he visited EPFL in Lausanne, installing logbook/database software and discussing general MDSplus issues.
Near Term Plans
Our near term plans are focussed on the current run campaign, which is scheduled to run into October. The C-Mod Run Schedule on the Web has been updated. Overall plans for the 1999 Campaign can be found at
http://www.psfc.mit.edu/cmod/run_schedule.html.
Those with access to the online OPS bulletin board are advised to check there for the most up-to-date schedule information.
A new WEB feature that should improve off-site collaboration possibilities and increase outside interest has been added. Live streaming video and audio from the control room are now available during C-Mod operations. The video shows the control room activity between shots and switches to video of the discharge itself during each pulse. The URL is
http://www.psfc.mit.edu/cmod/control_room_rv1.html.
The Alcator C-Mod 1999 Ideas Forum has been preliminarily scheduled for the week of December 6, 1999.
| New Publications |
J.E. Rice, et al., ``Central Impurity Toroidal Rotation in ICRF Heated Alcator C-Mod Plasmas'', Nuclear Fusion 39 (1999), p 1175.
I.H. Hutchinson, et al., ``Plasma Rotation during Ohmic H-modes in the Alcator C-Mod Tokamak'', submitted to PRL
J.E. Rice, et al., ``The Rydberg Series of He-like Cl, Ar, and S and Their High-n Satellites in Tokamak Plasmas'', submitted to the New Journal of Physics.
X. Bonnin and W. L. Rowan, ``Investigation of scrape-off layer up-down asymmetries in diverted plasmas in TEXT-Upgrade'', Nucl. Fusion 39 (8), (1999) p. 1009-1023.
