Similar ity experiments on RFX-mod and MST standard discharges: magnetics

Similarity experiments on RFX-mod and MST standard discharges: magnetics T. Bolzonella, P. Franz, D. Terranova, L. Zanotto and the RFXteam A. Almagri, D.J. Den Hartog, G. Fiksel , J. Reusch, J. Sarff and the MST team 13th RFP Workshop, Stockholm, 9-11 October 2008

Motivations & aims • Definition of a general framework and methodology to compare different RFPs (in non-standard discharges also) • Establish standard discharges on both MST and RFX-mod under similar operative conditions and in the same parameters space • Compare results with common“processing” methods • Beginning of an exercise which can be extended to old RFX machine and other RFPs (e.g. T2R). Preliminary results, tools refinement ongoing

Machine assembly(1) MST R=1.5m, a=0.52m (R/a~3) Thick (50mm) alluminium shell b/a=1.04 Vessel = shell RFX-mod R=2.0m, a=0.46m (R/a~4) Thin(3mm) copper shell b/a=1.17 Vessel ≠ shell ≠ support structure

Machine assembly(2) MST RFX-mod Shell = single-turn toroidal wind. 48 separate coils (12 sectors) Support struct. Internal toroidal gap Thin shell

Machine assembly(3) MST RFX-mod magnetizing wind. Field shaping wind. 1 poloidal gap 2 poloidal gaps (shell, support structure)

Mode and error field control MST • Passive control (thick shell + careful design of passive structure and power supplies) • Active control at poloidal flange RFX-mod • Active control (4x48 saddle coils independently controlled) • Various real-time control algorithms available. Virtual Shell can mimic a passive MST-like control

Magnetic measurements MST • Internal Rogowski coil for plasma current • Toroidal (ex-vessel) and poloidal (in-vessel) loop voltages, fluxes • Toroidal array of 64 internal pick-up coils @241°P measuring Bt, Bp, Br • 2 Polodal arrays of 16 internal pick-up coils @0°T and 180°T measuring Bt, Bp, Br RFX-mod • Pick-up coil arrays (4x48 + 8x4 ) for Bt, Bp • 8 Rogowski probes (plasma current) • Poloidal and toroidal loop voltages and fluxes • Saddle loops for Br (4x48) • Halo probes • In-vessel sensors (ISIS, magnetic+electrostatic arrays) All probes but ISIS are placed between the vacuum vessel and the shell (vessel filter)

Criteria for comparison and analysis • Perform discharges as close as possible on plasma parameters and machine operation: • Choice of some fixed normalized parameters: F, Ip (Bp(a)), density (norm. I/N) • Mimic operation: ideally, self-reversal, “parallel” operation of toroidal windings, virtual shell operation to mimic thick shell, non-sustained plasmas • Process data with common algorithms: a similarity pulsefile is being built and filled up to this purpose

Similarity shots: general MST RFX-mod • Shallow (-0.05) & deep (-0.2) F similarity shots tried, in H2.

First results: F-Theta plan • For a given F, the flat-top theta value is clearly different in the two machines (Time interval is [0:40]ms for MST and [0:80]ms for RFX-mod)

First results: a-Theta0 MST RFX-mod • Average profiles flatter in RFX-mod

First results: dynamo behavior MST RFX-mod • Discrete dynamo events are very clear in MST for both F values, while in RFX-mod at shallow F large bursts seem to disappear

First results: modes(shallow F) MST RFX-mod • QSH can develop more frequently in MST, while in RFX at 400 kA standard plasma is always in MH state.

First results: modes(deep F) MST RFX-mod • QSH can develop in MST also at deep F (not in this case). DRE are more evident in the MST case

First results: modes(deep F) • Only slow rotations can be seen (induced) in RFX-mod

First results: modes (deep F) • Fast mode rotations can be seen in MST, at least in the startup phase, with strong variations due to dynamo cycles.

First results: modes (shallow F) • The phase locking seems weaker than the deep F case. Less dynamo required?

First results: modes(shallow F) • Fast mode rotations can be seen in MST, at least in the startup phase

Comments • Standard discharges setup for similarity studies are feasible, exercise just started • It is confirmed that, for a given F, the two machines have different magnetic equilibrium profiles • Dynamo in MST exhibits more discrete behavior • Tearing mode spectrum is more often in MH state (especially in RFX-mod). • TM spontaneous fast rotations are observed in MST while only slow rotations can be induced in RFX-mod.

Future work • Add a point at same Theta and different F in the comparison? Maybe more similar dynamo activity • Assess the relevance of startup? More can be done in both machines (e.g. Virtual shell, self-reversal in RFX-mod, new Bt power supply in MST). Comparison also in terms of efficiency (volt-second consumption) • Pulse file “Similarity” created, to be filled up after definition of common calculation procedures • Extend RFX-mod similarity database (few dedicated shots up to now), better mimic of MST is possible • In MST gathering consistent amount of data and accessing measurements seem easier

Similarity shots: startup MST RFX-mod • In RFX-mod deep F discharges normally need higher toroidal voltage

Similar ity experiments on RFX-mod and MST standard discharges: magnetics