Dear Burning Plasma Aficionados:

This newsletter provides a short update on U.S. Burning Plasma Organization activities. Comments on articles in the newsletter may be sent to the editor (Tom Rognlien trognlien at llnl.gov) or assistant editor (Rita Wilkinson ritaw at mail.utexas.edu).

Thank you for your interest in Burning Plasma research in the U.S.!

Director's Corner by J. W. Van Dam

IITER International Summer School on MHD and Plasma Control
Here’s another reminder to encourage graduate students, postdoctoral researchers, and young scientists to attend this Summer School (http://w3fusion.ph.utexas.edu/ifs/iiss2010/). Senior scientists are also very welcome to attend. This is the first time the ITER International Summer School is to be held in the US, and it probably will not return to this country for some time—so take advantage of the opportunity now. On-campus housing for the student participants is a bargain. Registrations and abstracts are due by April 30.

Reminder about the USBPO Council Election
The twelve members of the USBPO Council serve staggered three-year terms. Every year at this time, four members rotate off. An election is held to replace two of the members, with the other two being appointed. The Council’s Nomination Committee, chaired by John Sarff, has finished collecting nominations from the USBPO membership and will soon propose a slate of qualified candidates to the Council. You will then receive a message about the voting procedure.

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USBPO Topical Group Highlights

The BPO Confinement and Transport Topical Group seeks to facilitate U.S. efforts to understand and predict particle, momentum, and energy confinement in the plasma core region through a combination of experimental and theoretical studies (the leaders are Edward Doyle and John Rice). This month's research highlight from John Rice and colleagues describes their recent work to understand the causes and consequences of toroidal rotation in tokamaks.

Observations of Toroidal Rotation Drive without External Momentum Input
John Rice, Yijun Lin, Matthew Reinke, Y. Podpaly, Steve Wukitch, G. Wallace, and R. Parker (MIT)

Toroidal and poloidal flows and gradients are beneficial in confined plasmas USBPO eNews, May 19, 2010, Issue 44 Page 3 of 11 because they can suppress instabilities across a range of scales, reducing plasma turbulence and transport, and allowing higher plasma pressures to be obtained. In most tokamak experiments, toroidal plasma flow is driven by externally injected neutral beams, though in some cases, flow can also appear spontaneously without direct momentum input. However, in reactor scale devices like ITER, the neutral beams wil likely not be adequate to drive sufficient plasma flow to suppress turbulence. Experiments at Alcator C-Mod are focusing on intrinsic rotation and radio frequency wave driven flow.

A new imaging x-ray spectrometer has allowed unprecedented measurement of toroidal rotation. The spontaneous/intrinsic rotation in enhanced confinement plasmas is mainly aligned with the current, and has a relatively simple parameter dependence, with the magnitude of the core velocity proportional to the stored energy normalized to the plasma current. The co-current rotation is observed to propagate in towards the center from the plasma edge following the transition to the high confinement mode (H-mode) of operation, on a time scale similar to the energy confinement time. The profile shapes range from relatively flat to centrally peaked, which in the latter case is indicative of the presence of an inward momentum convection or pinch. An example of a velocity profile exhibiting peaking due to an inward co-current pinch is shown in Fig. 1. The velocity gradient region is typically in the outer 2/3 of the plasma radial profile.

Fig. 1. Recent x-ray spectrometer data of the radial profile of toroidal velocity in C-Mod during high and low confinement mode operation.

Intrinsic plasma rotation, however, is not well understood and does not allow simple external control. Therefore, tokamak researchers around the world have been pursuing other means to drive plasma flow, particularly via externally launched rf waves. At the C-Mod tokamak, it has been demonstrated for the first time that significant toroidal and poloidal plasma flow can be driven by radio frequency (rf) waves. In the experiments, waves with a frequency of 50 MHz are launched into plasmas consisting of deuterium and helium-3 (³He), and a magnetic field of ~5.1 Tesla. The rf wave undergoes a process called “mode conversion” inside the plasma. During the mode conversion process, the launched rf wave slows down, the wavelength becomes shorter, and the wave is converted from the launched fast wave into a pair of slow waves, the so called ion cyclotron wave and ion Bernstein wave. When the amount of the helium-3 ions is at an optimal level, these shorter wavelength wave modes can interact with the plasma ions and generate plasma flow, dubbed mode conversion flow drive (MCFD). Using this method, rotation velocities up to 100 km/s have been generated, also in the same direction as the plasma current. Shown in Fig. 2 is a comparison of the MCFD results withthe intrinsic rotation scaling. Unlike the intrinsic rotation that depends on underlying transport mechanisms and plasma performance, MCFD has several external control knobs for manipulating the rotation level. The rotation can be regulated by plasma parameters such as the magnetic field, ³He concentration, electron density and plasma current, in addition to rf properties such as the power level, frequency and wave phase.

Fig. 2. Increase in peak toroidal velocity versus stored-energy/plasma-current for Mode Conversion Flow Drive (purple) and intrinsic rotation (red) without MCFD.		Fig. 3. Radial profile of toroidal velocity in C-Mod during LHCD (red) compared to the profile in the low confinement mode without LHCD.

Another method for rf flow drive is to utilize higher frequency waves (4.6 GHz, lower hybrid current drive, LHCD) which couple to high energy electrons. These waves have been found to generate substantial rotation in the opposite direction, counter to the plasma current. Shown in Fig. 3 is a rotation velocity profile which is strongly peaked in the counter-current direction, produced using these microwaves. It turns out that these higher frequency waves, which generate counter-current rotation, can be used in conjunction with the lower frequency waves, which drive co-current rotation, in order to produce complicated rotation velocity profiles which are hollow in the core. This may turn out to be a useful tool for regulating transport.

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Reports

Summary of the April ITER TBM Workshop
Erol Oktay (OFES)

The Workshop on ‘TBM Impact on ITER Plasma Physics and Potential Countermeasures' was held in Cadarache, France on April 13-15, 2010. The meeting was organized by the ITER organization (IO) to assess the impact of Test Blanket Modules (TBMs) on ITER physics operations and to consider countermeasures. The primary impact is due to the magnetic ripple introduced by TBM massive ferromagnetic structures, and the potential available countermeasures include a reduction of ferromagnetic mass, increasing the recession of TBM from the plasma, and using correction coils. About 45 representatives participated from the IO, TBM teams, international plasma physics community, India Domestic Agency, TBM Program Committee (PC), and the ITER Science and Technology Advisory Committee (STAC). This workshop provided a very productive JOINT discussion between the fusion scientists and TBM experts for the first time on this complicated and important issue.

Three sets of experimental results were discussed during the physics session. These results were from the past ripple experiments on JET and JT-60U, and the recently completed DIII-D ‘ITER TBM Mock-up’ experiment conducted by an international team led by General Atomics. The DIII-D experiment, which was requested by the IO, was designed specifically to address the impact of ripple on plasma performance so that the IO can provide guidance to the TBM teams on their design of TBMs. All of these past and present ripple experiments were discussed extensively at the workshop with the following basic conclusion: The impact of ripple is negligible in low performance and low pressure (beta) plasmas; however the plasma performance is degraded in high beta plasmas as the ripple increases, potentially jeopardizing the first priority ITER mission, which is to achieve fusion gain Q~10 plasmas. These experiments also show that ripple impacts plasma rotation, density pump out, and fast particles. The ripple theory and modeling are not mature, and the predictive capability is lacking to quantify the impact of these issues on ITER performance.

In order to mitigate the ripple issue, the TBM teams have been evaluating the impact of ferromagnetic mass reduction and TBM recession on their technical objectives. Each team presented their findings on the second day, as well as a conceptual design of correction coils. A general conclusion from most of the TBM teams is that a reduction in the TBM ferromagnetic mass would jeopardize their technical objectives. Increasing the recess of TBMs from the plasma surface would help some, but its impact on neutron flux on TBMs and shielding of toroidal field coils need to be assessed.

In the final session, the results from these two sessions were further discussed to develop a set of workshop consensus, recommendations, and action items. An initial consideration is to deploy TBMs on ITER with the first plasma until the beginning of Q=10 campaign with high beta plasmas. If experiments during the early period indicate that TBM ripple would jeopardize the Q=10 mission, the TBMs would be replaced by stainless steel ‘dummy’ TBMs. The implication of this scenario needs to be evaluated in the context of the ITER Research Plan. The workshop recommended a continuation of the DIII-D experiments in order to study further plasma rotation and lock-mode issues in higher beta plasmas, and a repeat of these experiments on JET in order to investigate the impact of periodic toroidal coil ripple with the local TBM ripple on the same device.

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Status of Fusion Energy Sciences' Joint Facilities Research Target 2010 on Thermal Transport in the Scrape-off Layer
Rajesh Maingi (ORNL/NSTX/Coordinator), Brian LaBombard and Jim Terry (MIT/C-Mod), Charles Lasnier (LLNL/DIII-D), and the 2010 JRT Team

Beginning in 2008, the DOE Office of Fusion Energy Sciences asked the three major domestic fusion facilities Alcator C-Mod, DIII-D, and NSTX to coordinate a yearly set of common experiments aimed at critical scientific questions that impact the extrapolability of results from present day devices, especially toward ITER. These specially focused experiments have become known as Joint Research Targets (JRT). For FY 2010, the JRT focuses on scrape-off layer transport, with the following wording:

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Summary Report of the Meeting of the ITPA Topical Group on Transport and Confinement at Culham, UK
Stan Kaye (PPPL)

The fourth meeting of the ITPA Transport and Confinement Topical Group was held in Culham Laboratory, UK, on March 22-25, 2010. In attendance were approximately 40 participants from the US, EU, Japan, Korea, Russia and the IO. There were a number of remote participants and presentations as well. Topics covered during the meeting were: rotation and momentum transport, impurity transport, Internal Transport Barrier (ITB) physics, physics model validation, and the Joint Expt TC-2 (hysteresis and access to high-confinement regimes with confinement factor of H~1). Results and discussion for these topics are summarized in the paragraphs below.

Rotation and momentum transport: This session focused on two topics: intrinsic rotation and momentum transport. Intrinsic rotation studies on ASDEX-U were carried out using electron cyclotron heating (ECH), finding counter-current rotation peaking with central deposition. With ion cyclotron radio frequency (ICRF) heating on JET, counter-current rotation was also observed, but did not exhibit scaling with plasma pressures observed in other devices. Tore-Supra adjusted the edge magnetic ripple and found that the resulting ion loss led also to counter-current rotation, and that subsequent ICRF or lower hybrid (LH) heating changed the rotation only in the core. There were questions raised as to whether it was valid to identify the rotation as “intrinsic,” in light of torque due to particle loss with RF heating. There were additional presentations on observations of intrinsic rotation in CSDX, potential fluctuations in TJ-II, drag due to ion-neutral collisions, and reports on Joint Experiments. The Momentum Transport presentations discussed the determination and scaling of momentum diffusivity and inward pinch derived from results on a number of devices, specifically in joint experiments on NSTX, DIII-D and JET, each of which performed collisionality scans to determine the relation between particle and momentum radial transport coefficients. The data indicate weak relation between c/v, the ratio of momentum diffusivity to pinch, with collisionality on DIII-D and NSTX, and the JET results showed that the particle and momentum pinches have an opposite dependence on collisionality. There was a discussion of the momentum database as well as on the methodology to determine the momentum transport coefficients.

Core Impurity Transport: This session reviewed and compared recent and past observations in the different devices with particular emphasis on He transport. The aim was to identify common generic features that have to be understood and predicted by theoretical modeling. Modeling of turbulent impurity transport indicate that turbulent convection can reverse direction depending on the type of turbulence, and that a total convection directed outwards is usually difficult to obtain in simulations of plasma conditions at which it is observed, particularly for impurities like B or C. He transport was determined in DIII-D, and the diffusivity was found to be gyroBohm-like in the core, but Bohm-like farther out in radius, similar to the results for the thermal diffusivity. Particle diffusivities of impurities in JET are always significantly higher than the neoclassical level, while convection velocities are near neoclassical levels in the core in H-mode plasmas. This level of the diffusivity is consistent with results from C-Mod, which also indicate even greater impurity diffusivities during the low (L) and intermediate (I) confinement modes. Results from Tore-Supra also indicate larger than neoclassical impurity diffusion and that the impurity transport is dominated by electron drift wave turbulence in the core of the plasma. In the outer half of the plasma, the transport is dominated by ion turbulence. In contrast, in the low aspect ratio NSTX device, the impurity transport appears to be consistent with neoclassical theory, including the impact of rotation on neoclassical transport.

Internal Transport Barriers (ITB): The session was divided into electron and ion ITB presentations. Electron ITBs in NSTX require strong negative magnetic shear to form; the negative shear leads to a reduction in turbulence identified as electron temperature-gradient (ETG) modes and its associated transport. The development of electron ITBs in MAST and JET are also associated with strong negative shear. Electron density ITBs in TCV, JET and JET60-U are associated with electron temperature ITBs, and are also seen when the magnetic shear is strongly negative. Work was presented on mechanisms giving rise to “canonical” pressure and rotation profiles. Ion ITB formation in C-Mod is very sensitive to both magnetic field and ICRF resonance location, with significant ExB shear measured at the ITB foot location. JET also reported the importance of strong ExB shear for an ion ITB Advance Tokamak (AT) scenario. Also reported from JET was the reduction of ion profile stiffness with high rotation gradient and low magnetic shear, and it was suggested to re-examine the common ExB quenching rule with an alternative version of this rule based on Resistive Ballooning, which is found to be more consistent with JET results. LHD plasmas showed both a reduction of ion-scale turbulence and thermal conductivity inside the ITB, consistent with gyrokinetic predictions of ion temperature-gradient (ITG) mode stabilization in this region.

Transport Model Validation: This was the first meeting of those involved in this designated ITPA Joint Activity. Presentations showed quite clearly that standard, published models fail to reproduce the evolution of the full T_e profile, and, thus, time evolution of the internal inductance, l_i, (critical for ITER) during the ramp-up phases of both ITER-Demo-like discharges as well as standard discharges. The reason for this is primarily the lack of agreement in the outer region of the plasma, which is where the current profile has the biggest impact on l_i. Only with ad-hoc modifications of these models can agreement be achieved, but these modifications leave little confidence for extrapolation. The group will move forward in a two-pronged effort. It was found that T_e profiles in the outer region of the plasma are essentially linearly decreasing as a function of poloidal flux for ITER-Demo discharges from a range of devices, and these can be used as a basis to probe heating and confinement requirements needed to achieve ITER target profiles. Longer-range plans include the detailed testing of both published models and more first-principles, gyrokinetic calculations. High-Confinement Regimes with H~1: These regimes arise shortly after the L-H transition. Various experiments reported on these results, and it was clear that no specific recipe for obtaining this confinement regime, without Type I edge-localized modes (ELMs), exists. Several presentations were given on the I-mode, which has H-mode like energy confinement (0.8-1xH), but L-mode like particle confinement, and, therefore, no ELMs. The I-mode can be obtained only at high power in, so far, a counter-injection plasma; these powers are higher than those required for transition into the H-mode with co-injection, making the I-mode presently not relevant for reactor scenarios. It was felt that there is much physics to be learned from the I-mode, specifically in its subsequent transition to the H-mode and possibly as the first step in the L-H transition process. The next meeting of the ITPA Transport and Confinement group will be held following the IAEA FEC in October in South Korea.

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Announcements

Submit BPO-related announcements for next month’s eNews to Tom Rognlien at trognlien at llnl.gov.

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