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Posted December 1st 2005
Task Force S1 – Standard Scenario
Richard Buttery, Joerg Hobirk and Thierry Loarer
For the experimental campaign C15 to C17, the S1 programme follows three lines:
- High current operations will exploit the new ‘MKII-HD’ divertor to document a more ITER like shape at up to 3.5 MA with high input power. Here we will focus on measuring the Edge Localised Modes (ELMs) and pedestal, testing the confinement, plus exploring and maintaining the stability. A strongly integrated programme with the physics task forces has been planned to jointly take forward and solve all the issues for this scenario.
- ELM amelioration is essential for developing the ITER Q=10 operation. Here passive techniques include development of regimes with benign ELMs (Grassy and type II) and good confinement. Active techniques such as impurity injection and vertical position oscillations will be assessed. But we must also work to extend the type III regime as a better fall back for ITER. The priority is to demonstrate the use of these techniques in main scenarios and extend application to others.
- A major increase in work on the Hybrid scenario is planned to qualify this as a high fusion gain long pulse regime for ITER. Work will focus first on documenting the limits (beta, density), confinement and transport, comparing with baseline scenarios and pushing to ITER relevant parameters (high shape, low edge safety factor q95). Further studies will integrate with Task Force S2 to explore the role of current profile and long pulse operation. Complementary to this, collaborations on physics aspects such as transport, pedestal, stability, and burn control with the other task forces are planned particularly to validate the scenarios for the future W/Be JET wall project (see JET Bulletin, June edition).
Task Force S2 – Advanced Scenario
Xavier Litaudon, Clive Challis and Flavio Crisanti
Improvement of the tokamak concept in terms of high confinement and stability without the need for large plasma current is a crucial challenge that could finally lead to the operation of future devices in continuous mode. The long term scientific objective of the Task Force S2 activity is to develop ITER relevant regimes capable of purely non-inductive current drive operation with a large fraction of the plasma current self-generated by the neo-classical (pressure driven) ‘bootstrap’ effect. With this in mind, Task Force S2 has prepared a coherent scientific programme for 2006 in line with the priorities agreed by the Scientific and Technical Advisory Committee (STAC). To form and sustain reliable non-inductive scenarios as close as possible to ITER relevant non-dimensional parameters, Task Force S2 aims to develop and exploit advanced regimes on JET: (i) at ITER-relevant safety factor q and plasma shaping (high triangularity) taking advantage of the new divertor capabilities; (ii) at high values of pressure normalized to plasma current and magnetic field (beta) to investigate MHD stability limits and optimize the bootstrap current; (iii) where temperature and q-profiles are simultaneously controlled in real-time with advanced algorithms that include the two different time scales for heat and current diffusion; (iv) with edge conditions appropriate for future modifications to the plasma facing components (ITER-like wall project) and for ITER advanced regimes.
This is a very challenging task that requires the full heating, fuelling and current drive capability of the JET systems. But even this would be insufficient to simultaneously achieve fully non-inductive operation at high beta and high fusion performance. The approach, therefore, will be to address the various critical issues separately during 2006, with a view to full scenario integration when the future JET upgrades are complete.
Task Force M – MHD
Simon Pinches and Rudi Koslowski
Magnetohydrodynamic (MHD) stability issues continue to be at the forefront of performance development at JET: The principal limitations of the S1 and S2 target plasmas are due to various types of MHD instability. To address the most important issues facing S1 a focused programme has been formulated looking at Neoclassical Tearing Modes (NTMs, both 3/2 and 2/1) and their triggering by (fast particle stabilised) sawteeth. In addition to advancing the physics understanding, this work also includes the development of active avoidance and removal techniques. For the S2 scenario, the Resistive Wall Mode will be further studied to advance the physics understanding of this beta limiting phenomena and to provide diagnostic information on the proximity to instability. For both scenarios the MHD understanding of ELMs will be studied. Plasma disruptions and the generation and mitigation of runaway electrons will also form part of the focus of the programme. The newly installed enhanced halo sensors will provide important data on the toroidal and poloidal asymmetries of the halo currents that flow in the vessel walls as a result of a disruption. In parallel, the development of real-time neural networks promises to provide sufficient advanced warning of a coming disruption to allow time for mitigation steps to be taken, e.g. with the new disruption mitigation valve. The confinement of fast ions and the stability of fast ion driven modes is an area of importance for future ignited devices and one that will be addressed in the coming programme. Experiments will look at diagnosing the instabilities using new diagnostics techniques (e.g. reflectometry measurements) and measuring their stability with the new Toroidal Alfvén Eigenmode (TAE) antenna system in both standard and advanced tokamak scenarios.
Due to major vacuum breach that followed a vacuum turbopump failure on 10 November 2005, the JET experimental campaign C15 has been postponed. Following this event, an intervention for changing three faulty neutral beam injectors and repairing the shutter on the infrared endoscope was advanced. The JET Restart is expected to continue in January 2006. The High Level Commissioning is planned to start on the 6 February followed by the Experimental Campaigns C15-C17 from 20 February to 23 June 2006.
Task Force T – Transport
Paola Mantica, Volker Naulin and Tuomas Tala
Task Force T activity during C15-C17 will continue to focus on the understanding of transport processes, with close comparison between experimental results and theoretical modelling. Electron transport studies will be extended to hybrid regimes and to a larger variety of Internal Transport Barrier (ITB) plasmas. Thanks to improved diagnostic resolution, the new topics of ion heat transport and momentum transport will be addressed with focussed experiments. Experimental assessment of momentum transport and its modelling are of special importance, as this is a major player in the process of ITB formation. The issue of density peaking due to turbulence generated pinches will be further investigated at ITER relevant collisionality, within the frame of an ITPA (International Tokamak Physics Activity) coordinated experiment. Also an extensive experimental characterization of impurity transport in various regimes for a wide range of impurity species is planned. More detailed experimental data on pedestal and ELMs will allow for comparison with available models of edge transport.
Task Force D – Diagnostics
Andrea Murari, Elena De La Luna and Jerzy Brzozowski
One of the main aspects of the recent JET enhancements was a significant upgrade of JET diagnostic capability, to better support JET’s broad scientific programme in preparation for ITER. The main lines of development were the diagnostics for a burning plasma, a better characterisation of the profiles, particularly at the edge, and some advanced diagnostics, in particular in the field of fast instability and turbulence measurements. Since all the foreseen systems are due to be commissioned, in the next campaign we expect a very significant increase in the amount of data per shot. During the 2004 experimental campaigns, the maximum amount of data acquired per shot was about 2 Gbyte, while now 10 Gbyte or more are expected. Since the quality of the data should also improve, JET diagnostics should be able to provide a wealth of interesting information to support the scenario development and allow progress on the most advanced physical studies in Tokamak physics on the route to ITER.
Task Force E – Exhaust
Richard Pitts, Wojtek Fundamenski and Volker Philipps
Task Force Exhaust has identified four main areas of investigation in the coming campaigns: (i) characterising ELM power exhaust in high triangularity ITER-like shaped plasmas, (ii) designing integrated scenarios with tolerable (Type-III) ELMs, obtained by increasing the radiative fraction with the help of nitrogen seeding, (iii) investigating plasma-wall interaction and the associated migration of first wall material, and (iv) mitigating them effect of disruptions by massive gas injection. The first two goals are made possible by the installation of the MkII-HD divertor, including a new array of divertor Langmuir probes, as well as the main chamber infra-red camera, which can be used to study the heat loads due to ELMs on the main vessel wall. The third goal will benefit greatly from the newly installed Quartz Microbalance monitors, which permit material deposition rates in the divertor to be measured in real time.
Task Force H – Heating
Jef Ongena and Joëlle Mailloux
We all look forward to an exciting and challenging set of experiments for Task Force H in the coming campaigns. The technical emphasis this year is to deliver a maximum amount of Ion Cyclotron (IC) and Lower Hybrid (LH) wave power to ITER like plasmas in use by the other Task Forces. Several developments will help to reach this goal, but a new one is the availability of 3dB couplers for one of the IC antennae this campaign. Dedicated experiments to study IC and LH wave coupling issues relevant to ITER, including non-linear effects in the scrape-off layer and edge plasma during gas puffing near the antennas, are also planned. Feedback controlled phasing of the waveguides of the LH system is another study topic and – if successful – promises to be very useful for all scenarios relying on the presence of non-inductive plasma currents. In addition to optimizing power delivery or current drive to the plasma, Task Force H plans a variety of experiments aimed at a better understanding of the physical mechanisms underlying the various heating methods. Highlights are (i) the study of the origin of plasma rotation in the presence of IC resonance heating, a puzzle waiting for a solution for a long time, (ii) on and off-axis heating by Neutral Beam Injection (NBI) in order to try to elucidate the discrepancy in NBI power deposition and current drive, first observed in ASDEX-Upgrade, and (iii) varying the position of the q=1 surface in real time with IC Current Drive using the newly developed extreme shape controller, to understand the role of fast particle populations in stabilizing/destabilizing sawteeth and Neoclassical Tearing Modes.
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