Simulation of Coulomb Collisions in Plasma Accelerators for Space Applications

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1 Simulation of Coulomb Collisions in Plasma Accelerators for Space Applications D. D Andrea 1, W.Maschek 1 and R. Schneider 2 Vienna, May 6 th Institut for Institute for Nuclear and Energy Technologies - Research Center Karlsruhe 2 Institut for Pulsed Power and Microwave Technology - Research Center Karlsruhe

2 Contents Introduction to space propulsion SIMPLEX An example of applied accelerator Physical and numerical model A time scaling analysis for : Intra-species Inter-species Coupled case Future works

3 Introduction: Space Propulsion Spacecraft acceleration generated by propellant discharge: T = dm / dt v e Rate of expulsion of propellant W = dm / dt g 0 Specific impulse: I sp = T / W = ve / g 0 For constant exhaust velocity: m 0 v = ve ln m f

4 The Electric Propulsion Acceleration of propulsion gases by electrical heating and/or by electric and magnetic body forces. Electrothermal : electrical heat addition and expansion through nozzle. Resistojets and Arcjets. Electrostatic : application of electric fields. Ion Thrusters and Colloid Thrusters Electromagnetic : application of electromagnetic fields. Hall Thrusters, Pulsed Plasma Thrusters(PPT) and Magnetoplasmadynamic Thrusters (MPDT). Robert G. Jahn - Physics of Electric Propulsion.(1968)

5 To the Moon with SIMPLEX Stuttgart Instationary MagnetoPlasmadynamic thruster for Lunar Exploration Pictures from Uni-Stuttgart website Pulse time: Peak current: Capacitor voltage: 8 µs 40 ka 2000 V Exhaust velocity: Mean thrust: Mass ablated/bit 12 km/s 1,4 mn 160 µg

6 Physical Mathematical Modelling Electrical quasi neutrality conditions Non-equilibrium conditions in several degrees of freedom. Failure of continuous models,e.g. Fluid models Presence of external and self-induced E-B fields Charged-Neutral collisions and Chemical reactions Elastic charged particle interactions

7 Intra-Species Collision: Governing Equations δf δt α Col = v 2 ( α ) 1 ( α ) [ F f ] + : D α 2 v v t [ f ] α Fokker-Planck Equation (FP) KEY QUANTITIES: THE ROSENBLTUH POTENTIALS F t D ( α ) ( α ) H v β 2 G v v β Dynamical Friction Diffusion Tensor H G β β + ( x, w, t ) m f α β 3 = d w µ v w αβ + ( x, w, t ) 3 = v w f d w β r r V = V ( t ) THE STOCHASTIC DIFFERENTIAL EQUATION (SDE) Stochastic variable; satisfying the SDE: r r with transition probability P P 2 Euler scheme r V r = V r + F t t + B r dv ( n+1 ) ( n) ( n) ( n) ( n+1) p ( V, t / V t ) 2 0, p 0 t r η r ( t) = F dt + B dw ( t) fullfills a FP equation p t t D = r t t B B T

8 Numerical Framework: PIC Scheme Evaluate the key-quantities on the grid Interpolate the coefficients onto the particles Reconstruct the distribution function at t n+1 = t n + t Push the particles in velocity space

9 Time Scaling Analysis Delta vs. Maxwell Possible approaches: FULLY SELF-CONSISTENT Mutual Influence of beam and background particles TEST-PARTICLE ANSATZ Separation of friction and diffusion effects unrealstic NOT SELF-CONSISTENT Stochastical modelling of beam particles evolution with fixed background Maxwell

10 Moments Analysis Mean value investigation Variance investigation Mean value time evolution for selfconsistent and reference simulation Transversal variance time evolution for self-consistent and reference simulation

11 Inter-Species Collision: Governing Equations Preliminaries: c >> m << 1 c = c r const. e w X e m X Friction and Diffusion known from the simple form of: H ˆ 2 SDE becomes:, where: α r ( c t) c and G ( c t) X = 1 r, X, = dc t First and second moment time development : ˆ e e = ( ) = α C( t) dt +α H dw ( t) ( ex ) 3 2 = ΓP n X c and H t c t r = I t cc ˆˆ T M i = e α 2 ( t t ) 0 M 0, P ij 1 1 = δij + Pij ( t0) ij exp 3 3 δ 3 α { 2 ( t t )} 0

12 (e,x) Collision lectron beam impinging a background Ion distribution: V ( e) ( e) ( e) x, 0 = V, V y,0 = 0, V z, 0 = 0 0 Final electron distribution function Moment temporal evolution

13 Coupled Calculations:(e,X) + (e,e) Collision Non-equilibrium electrons impinging a background Ion distribution: Initial electron distribution function Moment temporal evolution

14 Coupled Calculations:(e,X) + (e,e) Collision Non-equilibrium electrons impinging a background Ion distribution: Initial electron distribution function Moments temporal evolution

15 Comparison Comparison between the mean value decay for the x-direction of the velocity in the (e-x) case and the coupled case (e-e)+ (e-x) Delay in the coupled case due to 2 non constant α

16 Summary and Conclusions Development of a three dimensional, self-consistent code for Coulomb collisions simulations Qualitative time scaling analysis performed for intra- and inter-species case Comparison with coupled calculation indicate the fundamental role of the momentum transfer collision frequency Coupling with a Maxwell-Vlasov solver will give a better insight of the influence of the electromagntic fields, external and self-generated