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Large operative space explorable in terms of Ip 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?
Large operative space explorable in terms of Ip 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? RFP “Other”
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? 2011 planned RFX activity
dbf / B(a) vs q(a) • 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 m=0 Total fluctuation n≠0 m=1 m=2
dbf / B(a) vs q(a) • The relation between confinement time and fluctuation level is not trivial m=0 Total fluctuation n≠0 m=1 3 m=2 ≲ 1 20 tE∼ 2-3 ms tE> 20 ms tE≲ 1 ms
ULQ towards RFP m=0 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 ne/nG)? m=1 ULq m=2
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 Vloop (see P. Innocente) m=0 Total fluctuation n≠0 m=1 m=2 F=-0.15 F=-0.07
F > 0 : Not (steadily) sustainable plasmas? • 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 m=0 Total fluctuation n≠0 m=1 m=2 F=-0.07
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 ?
High frequency fluctuations (deep F) m=1 m=0 m=0 m=1 1 -10 kHz m=1 m=0 10 -100 kHz Resistive g-modes (resonant at the edge)
High frequency fluctuations (F>0) The fluctuation level is found to “explode” at F>0 even at high frequency (mainly the m=1, 10<freq<100 kHz) m=1 m=0 m=0 m=1 1 -10 kHz m=1 m=0 10 -100 kHz Pressure driven ?
F=-0.013 VSOL=0.7843 m3 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) F=-0.057 VSOL=1.8552 m3 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
TPE: transient F>0 => QSH • TPE (Hirano 2008): • When a little positive Btw 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.
RELAX: Very deep F, no sawtooth • RELAX has recentlyshown 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 (!)
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 ne/nG) help in understanding RFP confinement and limits? • Loss of reversal is associated to a plasma current quench: violation of stability criterion? • Low Ip operations would allow to explore a wider range of experimental conditions requiring large Vloop (density and beta limits, F << 0, 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?