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Turbulence Cascade and Ion Heating in MST. A. Almagri G, Fiksel , Y. Ren , J. Sarff and MST Team. CSMO general meeting • University of New Hampshire, Durham, New Hampshire• June 4-6 2007. Motivation.
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Turbulence Cascade and Ion Heating in MST A. Almagri G, Fiksel , Y. Ren , J. Sarff and MST Team CSMO general meeting • University of New Hampshire, Durham, New Hampshire• June 4-6 2007
Motivation. • Ion temperature is observed to more than double during reconnection in MST. What is the source of this heating? It is our conjecture that this is a two steps process. First energy cascade to waves with large k then transfers to ions, both steps are not well understood Energy flow m = 0 modes are 5x larger than m =1 and critical for the energy spread in space and wave number
Motivation. • Are the magnetic fluctuation responsible for heating ions? Yes. • How does the energy cascade from longer wavelengths, meters, to short, cm, wavelengths? Nonlinear process. • What is the role of the m=0 modes? • What mechanism transfers energy from waves to the ions? Damping, dissipations, or resonance. • What part of the magnetic spectrum, k and f, do the ions get their energy from? May depend the mechanism. Experiments have started to answer some of the following questions
Outline. • Motivation • Nonlinear coupling and the role of m = 0 • Reconnection cycle • power spectra during reconnection cycle • k and k// changes • /k// evaluation • Impact of m = 0 • Effect on frequency spectrum • Effect on wave number spectrum • Effect on intermittency in magnetic turbulence
m = 1, n ≥ 6 resonances ~ 0.2 1,6 q(r) 1,7 1,8 m=0 all n minor radius , r 0 Multiple resonant surfaces mediated by m = 0 modes (1,6) (1,7) (0,1) (2,13) (0,1) (1,7) m = 0 mediates spreading in n spectrum (1,8)
Magnetic fluctuation spectrum broadens during reconnection event • Power spectrum of B at the plasma at r/a = 0.8. At reconnection Before reconnection Intermediate range Ion cyclotron range Tearing range Similar to power spectrum measured at the plasma edge
Wave number Kis predominately in the paralleldirection during reconnection. • K// and k evolve during the reconnection cycle • Before the crash k// ~ k • At and after the crash k ~ 0 K k -2 -1 0 1 -2 Time (ms)
Parallel phase velocity nearly equal to ion thermal speed • Parallel velocity increase at the crash to 100 km/s • Ion thermal speed of 200 eV ions is about 100 km/s Vion, ther
Role of m = 0 modes in the dynamics of magnetic fluctuations • Mediates the nonlinear coupling. • Strongly influences the cascade
m = 0 modes changes turbulence cascade. • Without the m = 0 modes the cascade follows a power law (?). • With the m = 0 modes the cascade is exponential. Intermitting MHD computation toroidal mode number (n)
PDF of magnetic turbulence is effected by m = 0 activity Presence of m = 0 modes increases Intermittency.
Summary. • m = 0 modes play an important role • Required for ion heating to occur. • Energy cascade from the tearing modes to short wave length is enhanced. • Energy is spread in space, higher n modes are resonant at larger radii. • Energy cascade changes from power law to exponential in wave number. • Increases the intermittency in the magnetic fluctuations.
Extra slide k B
