Alcator C-Mod Quarterly Progress Report – FY08 Q3

 
The main activities at Alcator C-Mod during the third quarter of FY08 were operation of the device and dismantling of the machine for the scheduled inspection. 

The C-Mod experimental campaign for fiscal year 2008 is now completed. A total of 15.7 weeks (62.9 days) of research operations were accomplished, producing a total of 1888 plasma discharges

Science Results

Mode Conversion Flow Drive

ICRF mode conversion plasma flow drive has been observed on Alcator C-Mod. We report the first observation of both toroidal (V_f) and poloidal (V_q) flows driven via an ICRF mode conversion (MC) process in D(³He) plasmas. At modest ³He levels (n_3He/n_e~ 8%), in relatively low density plasmas, <n_e>≤ 1.3´10²⁰m^-3, heated with 50 MHz rf power (B_t0~ 5.1 T), strong V_f in the co-current direction is observed by high-resolution x-ray spectroscopy, as shown in Fig.1. The central V_f scales with the applied rf power (≤ 30 km/s per MW), and is at least a factor of 2 more than the empirically determined intrinsic plasma rotation. The rotation near the plasma center (r/a < 0.3) responds more quickly to the applied rf power than the outer region, indicative of a local flow drive source. Localized poloidal rotation (0.3 ≤ r/a ≤ 0.5) in the ion diamagnetic drift direction is observed when P_rf ³ 1.5 MW and increases with power (~ 2 km/s at 3 MW). Turbulence spectrum broadening seen by a phase contrast imaging (PCI) system indicates strong flow also exists in the main ions. (see Fig.2) The mode converted ion cyclotron wave (MC ICW) is observed by PCI and confirmed by 2-D full wave TORIC code simulation. The simulation result shows that due to the up-shifted k_||, the MC ICW is strongly damped on ³He ions in the vicinity of the MC layer, approximately on the same flux surfaces where poloidal flow is observed, as shown in Fig.3. The involvement of ion heating and short-wavelength slow wave is consistent with theoretical considerations for efficient rf flow drive. Our experimental results are comparable to the predictions assuming similar ion interaction mechanism for the MC ICW and direct launch ion Bernstein wave. The feasibility of ICRF flow drive on ITER is promising.

Fig.1. Time histories of ICRF minority heating (green) and mode conversion (red) discharges.

Fig.2. Turbulence spectrum (top) and poloidal rotation (middle) for mode conversion flow drive.

Fig.3. 2-D TORIC power deposition simulations.

Reverse Shear Alfven Eigenmodes
 
Understanding the sawtooth phenomena is important, especially for burning plasma experiments such as ITER, where an accurate model of the physics of the crash event and the subsequent relaxation process is crucial for developing predictive capability for sawtooth control.  New observations of frequency chirping modes excited during the sawtooth cycle in the Alcator C-Mod tokamak, have been identified as q=1 reversed shear Alfven eigenmodes (RSAEs, also Alfven cascades) located near the q=1 surface.  Importantly, the observation of RSAEs implies the existence of a reversed shear q profile (i.e. negative magnetic shear), where the magnetic shear is defined as s=(r/q)(dq/dr), a feature thought to affect adversely the confinement of fast ions.  Furthermore, the fast-ion driven Alfvenic modes, including RSAEs, are a concern for burning plasmas where resonant transport of alphas is an unresolved issue.



Application of early ICRF (turned on at 0.08 seconds after plasma creation) was used to heat the plasma to retard the current penetration and create a fast ion population to drive the RSAEs at the q=2 and higher surfaces.  Multiple groups of frequency chirping modes similar to the RSAEs excited during the current ramp were observed following the second sawtooth crash and continued, at varying amplitude, for many cycles thereafter. RSAEs exist near the minimum of the q profile in a reversed shear equilibrium, i.e. hollow current density profile.  They are driven unstable by the spatial gradient of the fast ion distribution from neutral beams, fusion born alpha particles, or in the case of Alcator C-Mod, an ICRH driven minority ion species.  RSAEs and other Alfvenic modes have been used for "MHD spectroscopy", that is, the inference of equilibrium parameters such as qmin or q on axis (q0) from frequency spectra.  The phase contrast imaging (PCI) diagnostic, a type of internal reference beam interferometer, is an outstanding tool for the study of Alfvenic activity.  The output signal of the PCI system is a 1-d image, decomposed into 32 elements in the direction of the major radius, which is linearly proportional to the line integral of the electron density perturbations along the beam path.  In Alcator C-Mod the beam passes vertically through the plasma core, allowing for detection of core localized modes.  Mode frequency and spatial structure measurements from PCI are complemented by mode number identification with the Mirnov coils.  Toroidal mode numbers of 2-5 in the ion-diamagnetic direction have been identified for the modes shown in Fig. 4.  The remainder of the mode numbers is inferred through their frequency spectra and rate of frequency chirp, which is expected to scale approximately proportional to n. The ideal MHD code NOVA has been used to model the RSAEs during the sawtoothing phase.  Using experimental electron temperature profiles from the electron cyclotron emission (ECE) diagnostic, density profiles from the Thomson scattering system and a modified q profile from the equilibrium reconstruction program EFIT to include a central region (r/a < 0.25) with reversed shear, the NOVA code has produced q=1 RSAE solutions for toroidal mode numbers from 2 through 7, whose spectra are in good agreement with observations when qmin is decreased profile is relaxed while sustaining reversed shear (dashed lines in Fig.4).  A numerical scan of the magnitude of the central shear finds solutions when the q profile has flat or reversed shear and do not exist in positive shear equilibria.  As applied to the particular case presented in Fig.4, the modeling shows that qmin decreases to 0.92 prior to the third sawtooth crash.  A linear extrapolation estimates qmin to be 1.02 at the time of the second sawtooth crash. 
 

Fig.4. PCI spectrogram showing the modes appearing prior to the third sawtooth crash.  NOVA results of the frequency spectra are overlaid in dashed line and have been used to infer the evolution of qmin from 0.99 to 0.92 over the period of RSAE activity.
 
A one dimensional cylindrical model is used to describe the relaxation of the current following redistribution from the sawtooth crash.  Experimental temperature profiles as a function of time are given.  The current density profile is solved on an array of 201 radial grid points using a fourth order Runga-Kutta method for the time evolution.  Sawtooth inversion in the ECE and soft x-ray data place the q=1 surface near r/a=0.35.  A series of q profiles from this model are shown in Fig.5, where it is seen that a reversed shear q profile develops within a few milliseconds and persists until the end of the sawtooth cycle (19 ms).  A reversed shear profile is created only when the initial temperature profile is sufficiently flat.  For a temperature profile of the form T(r)=T0(1 - rg)h, where r=r/a, reversed shear is initially generated when g > 2.  The core temperature profile immediately following the crash is well modeled with g=2.5 and h=2.6.  The large second derivative of the current density at the mixing radius results in an inward flow of current which, due to the higher conductivity near the core, builds up just interior to this point, an effect similar to the generation of reversed shear during the current ramp phase.  
 

Fig.5. Numerical results of current diffusion in cylindrical geometry.  The q profile is initially flat with q=1 out to the mixing radius at r/a = 0.4 and develops reversed shear with q0 - qmin ~ 0.01.
 
 
 
 
 
 
 
 
 
 
 

Meetings

 
 
Paul Bonoli hosted the 13th Steady State Operation ITPA Topical Group Meeting at the PSFC-MIT April 15-17, 2008. The meeting was attended by approximately 25 physicists from the US, EU, Russia, Japan, and Korea. Presentations and discussions were made on issues related to integrated scenario modeling and actuator benchmarking for different operating scenarios in ITER including the ELMY H-mode Scenario 2, the Hybrid Scenario, and the Steady State Scenario 4. Results and plans related to joint experiments were also made. Presentations from MIT were made by Dr. Amanda Hubbard on "Recent Lower Hybrid Current Drive Results from Alcator C-Mod" and by Paul Bonoli on "Physics Research in The SciDAC Center for Simulation of Wave-Plasma Interactions".
 
 
 
Reviews of the MIT proposal for operation of Alcator C-Mod in 2009-2013 were held at the PSFC the week of May 5. The technical review of the proposed C-Mod five-year program was conducted on Thursday-Friday (May 7-8, 2008). Reviewers included Ed Synakowski (LLNL, Chair), Dick Majeski (PPPL), Mike Kotschenreuther (U. Texas, IFS), Thomas Pedersen (Columbia U.), Rich Groebner (GA), Steve Knowlton (Auburn U.), and John Sarff (U. Wisc). A separate review of the Facilities and Operations was conducted on Friday, May 9, by Peter Petersen (GA, Chair), Janos Kirz (LBNL), and Al von Halle (PPPL). Rostom Dagazian (OFES) participated in both reviews and provided the charge for the reviewers. Links to vugraphs of the five year program presentations are available on the web at 
 
http://www.psfc.mit.edu/research/alcator/program
 
 
 
Alcator C-Mod was well represented at the the 18th International Conference on Plasma Surface Interactions in Controlled Fusion Devices, held in Toledo, Spain, May 26-30.
 
An invited talk was given by Bruce Lipschultz on "Hydrogenic retention in a high-Z plasma facing component tokamak Alcator C-Mod". Co-authors included D.G. Whyte, J. Irby, B. LaBombard, and G.W. Wright.
 
Contributed orals included
 
"On the consequences of neutron and plasma induced damage for fuel retention in the bulk of plasma facing materials", D.G. Whyte
 
"Observations and Interpretation of the Spatial Structure of Edge Turbulence at the Outboard Midplane and High-Shear Region of Alcator C-Mod", J.L. Terry, S.J. Zweben, B. LaBombard, I. Cziegler, O. Grulke, R.H. Cohen, D.D. Ryutov, M. Umansky, T. Stoltzfus-Dueck, J. Krommes, D. Stotler.
 
Posters presented were
 
"H-mode edge profile modification and density control with pumping and magnetic balance regulation in Alcator C-Mod", J.W. Hughes, A.E. Hubbard, B. LaBombard, B. Lipschultz, R. McDermott, M.L. Reinke, J.L. Terry, D.G. Whyte
 
"Relationship between Edge Gradients and Plasma Flows in Alcator C-Mod", B. LaBombard, N. Smick, A. Graf, K. Marr, R. McDermott, M. Reinke, M. Greenwald, J.W. Hughes, B. Lipschultz, J.L. Terry, D.G. Whyte
 
"ICRF Specific Impurity Sources and Plasma Sheaths in Alcator C-Mod", S.J. Wukitch, B. Labombard, Y. Lin, B. Lipschultz, E. Marmar, M. Reinke, D. Whyte, and the Alcator C-Mod Team (presented by Bruce Lipschultz).
 
 
 
 

Collaborations

 
 
Run time was devoted to MP#535a "Rotation inversion vs density and current, in limited and diverted L-mode plasmas" as a collaboration with TCV, addressing ITPA Joint Experiment TP6.1 on Spontaneous Rotation. Basil Duval (EPFL-Lausanne, TCV) served as Session Leader using remote communications tools. The purpose of this experiment was to map out the rotation inversion (where the intrinsic rotation changes direction) as a function of density and plasma current, and compare results with similar observations in the TCV tokamak. A density scan was first carried out in LSN discharges with current of 0.65 MA and a magnetic field of 3.0 T (q95 ~ 3). For a range of average densities between 0.35 and 0.7 x 1020/m3 no rotation inversion was found. Subsequently inner-wall limited discharges were run, and rotation inversions were observed with average density nebar~ 0.7 x 1020/m3. For higher and lower densities, no inversion was seen. The observed rotation inversion occurred inside of r/a=0.6 only, and seems qualitatively similar to the TCV results.
 
Dennis Whyte traveled to DIII-D to participate in experiments on fuel retention. The DIII-D experiments repeated the fuel retention measurement technique started on C-Mod, where all active pumping is closed off during the plasma pulse and for 5-10 minutes afterwards; this results in very high accuracy measurements of the retained fuel in the vessel walls. Surprisingly, the DIII-D Ohmic discharges exhibited nearly identical retention to that seen on C-Mod: ~1% of incident D ion fluence to the wall was retained repetitively on each shot, without any sign of the wall saturating. This result is contradictory to simple expectations of the difference between the C-Mod metal molybdenum wall and the DIII-D graphite wall.
 

Domestic Travel

Miklos Porkolab visited Lawrence Livermore National Lab on March 31st where he had discussions on collaboration among C-Mod experimentalists and the BOUT code authors (Maxim Umansky and Bill Nevins).
 
Paul Bonoli attended the Sherwood Fusion Theory Conference in Boulder, CO that was held from March 30 - April 2, 2008. He presented a poster on behalf of John Wright and other co-authors titled "Full-wave electromagnetic field simulations of lower hybrid wave propagation in ITER relevant regimes".
 
On April 15 Brian LaBombard traveled to Dartmouth College and gave a Plasma Seminar at the Department of Physics and Astronomy entitled "Plasma Physics at the Edge of a Tokamak Fusion Reactor". He met with Barrett Rogers to discuss edge transport physics and to make plans for simulating C-Mod's boundary region with an electromagnetic fluid turbulence code.
 
Joe Snipes participated in the Confinement Database and Modeling ITPA meeting at ORNL the week of April 22. He presented the latest C-Mod results on the dependence of the H-mode threshold low density limit on plasma current and toroidal field.
 
Ron Parker served on the DIII-D 5-year Program Review Committee at GA, April 28-30. Rui Vieira served on the DIII-D Facilities Review at GA on April30 -May 2.
 
Catherine Fiore attended the semi-annual meeting of the Committee on the Status of Women in Physics in College Park on Tuesday April 29th. Catherine is the 2008 Chairperson.
 
Amanda Hubbard participated in the 14th meeting of the Pedestal ITPA Topical Group in San Diego, April 30-May 2nd. She gave a presentation on "LHCD Effects on the pedestal in Alcator C-Mod". Jerry Hughes and Dennis Whyte participated remotely in parts of the meeting. Jerry made a presentation on H-mode pedestal modification by topology variation on C-Mod, and Dennis on "Exporing ELM control on C-Mod with n=1 perturbations from external coils".
 
Yijun Lin attended the 4th US-PRC magnetic fusion collaboration workshop held at University of Texas, Austin from May 5 to 6. He had discussions with Chinese fusion researchers on C-Mod collaborations and also presented recent C-Mod ICRF results at the workshop.

Earl Marmar visited Princeton on Wednesday, May 14, where he presented the PPPL Colloquium. His talk was entitled "Alcator C-Mod Research Highlights and Plans". While at PPPL, he also had discussions with Dale Meade, Masa Ono, Michael Bell, Stan Kaye, Stefan Gerhardt, Mike Zarnstorff, Hutch Neilson, and Sam Cohen.
 
Alex Ince-Cushman, Matt Reinke, Bill Rowan, Igor Bespamyatnov, Eric Edlund, Jinseok Ko, and Greg Hanson (ORNL) presented papers based on C-Mod research at the 17th Topical Conference on High Temperature Plasma Diagnostics in Albuquerque, NM, May 11-15. Alex gave an invited talk on the new Johann imaging x-ray spectrometer system.

Robert Granetz gave an invited seminar on fusion energy to the IEEE Nuclear and Plasma Sciences Society and the IEEE North Shore Subsection. The talk was presented at the U. Mass campus in Lowell, MA on Thursday evening, 22 May, and was well received by the audience.

Pat MacGibbon was in California the week of May 22 working with our vendor on the evaluation of the #117 klystron.

Earl Marmar traveled to San Diego to participate at General Atomics in joint meetings of the Executive Committees of two IEA Implementing Agreements: The Large Tokamak IA and The Poloidal Divertor IA. Dr. Marmar attended as a member of the Poloidal Divertor Executive Committee and as an alternate member of the Large Tokamak Executive Committee. During the meetings he also made two presentations. The first was a summary of recent C-Mod results and plans, and the second was on the topic of experience in C-Mod with high-Z metal plasma facing components. Following these meetings, Dr. Marmar also participated in a meeting of representatives of the Fusion Facilities Coordinating Committee, the main purpose of which was to discuss potential joint experimental research JOULE milestones for FY2010.
 
Paul Bonoli attended the annual SciDAC Advisory Committee Meeting at PPPL on June 5-6, 2008. As Principal Investigator of the RF SciDAC Center he gave a talk titled "Scientific Progress in the SciDAC Center for Simulation of Wave Plasma Interactions"

Paul Bonoli attended the Joint IEA Workshop (W68) on Development of high beta-N scenarios for ITER. It was held at General Atomics on June 24-25, 2008. The title of his talk was "ITER Relevant Physics Studies in the Lower Hybrid Range of Frequencies on Alcator C-Mod".
 
Dennis Whyte traveled to DIII-D the last week in June to participate in experiments on fuel retention.

International Travel

Ian Hutchinson attended the International Conference on the Physics of Dusty Plasmas at Ponta Delgada (Azores). He presented a plenary talk entitled "Direct Calculation of Plasma-Dust Interactions".

Bruce Lipschultz, Dennis Whyte, Brian LaBombard, and Jim Terry attended the 18th International Conference on Plasma Surface Interactions in Controlled Fusion Devices, held in Toledo, Spain, May 26-30.

John Rice, Joe Snipes, Jerry Hughes, Ron Parker, and Miklos Porkolab attended the 35th EPS meeting in Crete. John gave an invited talk on "Spontaneous Rotation in C-Mod". After the EPS meeting, Miklos Porkolab attended the satellite meeting: EFTSOMP2008- Workshop on E fields, Turbulence and Self Organisation in Magnetized Plasma, June 16-17, Hersonissos, Greece, where he presented an invited talk: Turbulence studies in Ohmic plasmas in the Alcator C-Mod tokamak with PCI and GPI diagnostics.
 
Greg Wallace and Orso Meneghini were in Cadarache, France, until June 27th to work with the Tore Supra Lower Hybrid group. 
 
Joshua Stillerman and Thomas Fredian held meetings at Consorzio RFX in Padova Italy on future MDSplus development. The main topics discussed were object based APIs for data access and event based data acquisition for long pulse experiments. They also attended the EFDA remote participation workshop. Fredian presented a summary of MDSplus usage in the community. Stillerman presented an overview of US remote collaboration efforts, the ESnet RCWG (Remote Collaboration Working Group), as well as remote collaboration in use by Alcator C-Mod.

Near Term Plans

Schedule

A draft schedule for the FY08 run period can be found at
 

http://www.psfc.mit.edu/research/alcator/facility/Operations/operation_schedule.pdf