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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 Innovative contributions to confinement understanding M. Zuin

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 Large operative space explorable in terms of I p and B(a) How may non standard RFP plasmas help in RFP understanding? What kind of experiments can be suggested by non standard RFP plasmas in order to better understand RFP dynamics and confinement?

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 How may non standard RFP plasmas help in RFP understanding? What kind of experiments can be suggested by non standard RFP plasmas in order to better understand RFP dynamics and confinement? RFP “Other” Large operative space explorable in terms of Ip and B(a)

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M. Zuin RFX ws 2011 Padova, 7-9/2/ planned RFX activity How may non standard RFP plasmas help in RFP understanding? What kind of experiments can be suggested by non standard RFP plasmas in order to better understand RFP dynamics and confinement?

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 b / B(a) vs q(a) m=0 m=1 m=2 Total fluctuation n≠0 The fluctuation level and the m/n combination are functions of q(a) The dependence is not regular, many minima (and maxima) are found. RFP is one of these minima

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 b / B(a) vs q(a) m=0 m=1 m=2 Total fluctuation n≠0 The relation between confinement time and fluctuation level is not trivial E∼E∼ 2-3 ms E≲E≲ 1 ms E>E> 20 ms 3 ≲ 1 20

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 ULQ towards RFP m=0 m=1 m=2 Total fluctuation n≠0 What information can we get from ULq plasmas, characterised by a relatively low level of magnetic fluctuations but by poor confinement properties (high n e /n G )? ULq

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 m=0 m=1 m=2 Total fluctuation n≠0 The recent space of parameters explored by RFX- mod is extremely narrow in terms of q(a) [=> SHAx] Slight decrease of magnetic fluctuation with decreasing |F| Not reversed plasmas require extremely large Ohmic sustainment Even deep F equilibria require high V loop (see P. Innocente) F=-0.07 F=-0.15

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 F > 0 : Not (steadily) sustainable plasmas? m=0 m=1 m=2 Total fluctuation n≠0 The transition to F ≥ 0 (loss of reversal) is associated to the disappearance of QSH and to a rapid quench of the plasma current Is the increased magnetic fluctuation (high n) responsible for this? Are these modes controllable by means of the FB system of RFX-mod F=-0.07

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 F > 0 : Not (steadily) sustainable plasmas? q profile almost flat in the plasma => very low stabilizing shear effect MHD simulations do not show any abrupt transition at F>0 Unstable plasma current profile, ideal kink instability? Is the Suydam criterion violated (i.e. pressure driven modes, interchange)? What about the role of resistivity ?

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 High frequency fluctuations (deep F) kHz kHz m=0 m=1 Resistive g-modes (resonant at the edge) m=1

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 High frequency fluctuations (F>0) kHz kHz m=1 m=0 m=1 Pressure driven ? The fluctuation level is found to “explode” at F>0 even at high frequency (mainly the m=1, 10

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 m=0 islands and confinement The presence of m=0 islands (along with the m=1 modulation) strongly affect plasma wall interaction At F>0, it is possible that the expulsion of the O–points of the m=0 chain of islands would deteriorate PWI (increase of resistivity) To experimentally determine the role of the m=0 islands in confinement properties of RFP, we could try to drive them outside of the plasma F= V SOL = m3 F= V SOL = m3

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 TPE: transient F>0 => QSH TPE (Hirano 2008): When a little positive Bt w pulse is applied, transition to QSH state can be triggered (m/n=1/6 ideally MHD unstable) Confinement is not good in the QSH state produced by positive Bt. Not only primary mode (n=6) but also secondary modes (n=7,8,9) grow.

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 RELAX: Very deep F, no sawtooth RELAX has recently shown the existence of a new state of reduced magnetic fluctuation and enhanced SXR emission at extremely deep F (see Masamune tomorrow) What is the role of the aspect ratio? F < - 1 (!)

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M. Zuin RFX ws 2011 Padova, 7-9/2/2011 The flexibility of RFX-mod has allowed the exploration of a wide range of experimental conditions in terms of q(a) The confinement time of RFP is better than that of ULq plasmas, despite the larger fluctuation level. Why? Would a detailed study of ULq plasmas (high n e /n G ) help in understanding RFP confinement and limits? Loss of reversal is associated to a plasma current quench: violation of stability criterion? Low I p operations would allow to explore a wider range of experimental conditions requiring large V loop (density and beta limits, F 0, ULq) What is the role of the transient character of F>0 operations in TPE in producing QSH (OPCD also)? We observed many minima of the fluctuation level vs q(a). RELAX probably found a new one at extremely deep F values. What about RFX?

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