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1 Phase Space Instability with Frequency Sweeping H. L. Berk and D. Yu. Eremin Institute for Fusion Studies Presented at IAEA Workshop Oct. 6-8 2003
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2 “Signature” for Formation of Phase Space Structure (single resonance) Explosive response leads to formation of phase space structure Berk, Breizman, Pekker
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3 Simulation: N. Petviashvili
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4 “BGK” relation Basic scaling obtained even by neglecting effect of direct field amplitude Examine dispersion with a structure in distribution function (e.g. hole) v
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5 Power Transfer by Interchange in Phase Space Ideal Collisionless Result
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6 TAE modes in MAST (Culham Laboratory, U. K. courtesy of Mikhail Gryaznevich) IFS numerical simulation Petviashvili [Phys. Lett. (1998)] L linear growth without dissipation; for spontaneous hole formation; L d. =(ekE/m) 1/2 0.5 L With geometry and energetic particle distribution known internal perturbing fields can be inferred Predicted Nonlinear Frequency Sweeping Observed in Experiment
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7 Study of Adiabatic Equations Study begins by creating a fully formed phase space structure (hole) at an initial time, and propagate solution using equations below.
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8 Results of Fokker-Planck Code sweeping terminates why? sweeping goes to completion
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9 Normalized Adiabatic Equation, eff =0 Dimensionless variables: “BGK” Equation Take derivative with with respect to b
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10 Propagation Equation;Difficulties Problems with propagation a.H T ( ) = 0, termination of frequency sweeping b.1- = H T ( ) = 0; singularity in equation, unique solution cannot be obtained
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11 Instability Analysis Basic equation for evolving potential in frame of nonlinear wave (extrinsic wave damping neglected), 1 = P(t) cos x + Q(t) sinx; f satisfies Vlasov equation for: Spatial solutions are nearly even or odd
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12 Analysis (continued) F(J)-F 0 ( ) G T J Find equilibrium in wave frame: Linearization: Perturbed distribution function
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13 Dispersion Relation Identity Consequence: Adiabatic SweepingTheory “knows” about linear instability criterion for both types of Breakdown: (a)sweeping termination (b) singular point Onset of instability necessitates non-adiabatic response
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15 Comparison of Adiabatic Code and Simulation
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16 Evolution of Instability Trapping frequency, b bi Spectral Evolution, L slope in passing particle distribution Indication that Instability Leads to Sideband Formation
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17 Side Band Formation During Sweeping
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18 Summary 1. Ideal model of evolution of phase structure has been treated more realistically based on either particle adiabatic invariance or Fokker-Planck equation 2. Under many conditions the adiabatic evolution of frequency sweeping reaches a point where the theory cannot make a prediction (termination of frequency sweeping or singularity in evolution equation) 3. Linear analysis predicts that these “troublesome” points are just where non-adiabatic instability arises 4. Hole structure recovers after instability; frequency sweeping continues at somewhat reduced sweeping rate 5. Indication the instability causes generation of side-band structures
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19 Finis
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20 Linear Dispersion Relation Linear Instability if H T < 0 Hence H T ( b ) =0 is marginal stability condition of linear theory. Adiabatic theory breakdown due to frequency sweeping termination, or reaching singular point is indicative of instability. Then there is an intrinsic non-adiabatic response of this particle-wave system
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