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Space Charge in Isochronous Regime (IR)

Space Charge in Isochronous Regime (IR). E. Pozdeyev, BNL* *work partly done at Michigan State University in 2001-2003 (experiments and simulations) together with J.A. Rodriguez. Isochronous regime. Several types of machines operate / run into IR:

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Space Charge in Isochronous Regime (IR)

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  1. Space Charge in Isochronous Regime (IR) E. Pozdeyev, BNL* *work partly done at Michigan State University in 2001-2003 (experiments and simulations) together with J.A. Rodriguez

  2. Isochronous regime • Several types of machines operate / run into IR: • rings for precise nuclear mass spectrometry • some isochronous-optics light sources. • hadron synchrotrons during transition crossing • cyclotrons (FFAG?) • Studies of beam dynamics of intense beams around transition have been conducted and documented (including text books, K. Ng) • Effect of space charge (SC) on transverse motion and coupling of radial and longitudinal motion must be included in consideration in IR (usually omitted) E. Pozdeyev, HB08

  3. Longitudinal impedance at short λ • Long wavelength approximation • (includes image charges) • Short wavelength approximation • (no image charges) SC impedance peaks at short wavelength: λm~2.5  E. Pozdeyev, HB08

  4. Transverse SC field Linear charge density modulation => Energy modulation => Radius modulation => Radial electric field Er x z Er The radial field comes from the snaky shape and can come from image charges. (We neglect images assuming flat vacuum chamber (like in a cyclotron).) Er Field from a slice Ez The radial field due to snaky shape E. Pozdeyev, HB08

  5. Dispersion function and slip factor Steady state solution Exactly at the transition If there is dispersion function error, the slip factor is Negative Mass instability below γtr ?! Sure… E. Pozdeyev, HB08

  6. Growth rate with SC in Isoch. Reg. The growth rate for the microwave instability: E. Pozdeyev, HB08

  7. Small Isochronous Ring (SIR), Circa end of 2003 FFC E. Pozdeyev, HB08

  8. Beam dynamics simulations in SIR CYCO simulations (Np=3105) Bunch breaks up within a few turns throughout the bunch Contour plot of bunch charge density in median plane for turns 0 to 75 X Ipeak=10 A Turn J.A. Rodriguez Ph.D. dissertation, MSU Z E. Pozdeyev, HB08

  9. Experimental results:Longitudinal beam dynamics Measured longitudinal bunch profile Turn# 10 (fixed), Current increases Vertical axis: peak current measured by Faraday Cup Horizontal axis: arrival time to the Faraday Cup (equivalent to Z) E. Pozdeyev, HB08

  10. Comparison Experiments to Simulations # of clusters as a function of # of turns for 5, 10 and 20 A Simulations Experiments Simulations included only SC and image charges on the vacuum chamber J.A. Rodriguez, Ph.D. dissertation, MSU E. Pozdeyev, HB08

  11. Experimental Results:Scaling with Beam Current I = 19.9 μA I = 9.9 μA I = 5 μA Growth rate depends linearly on the beam current!!! Not as sqrt(I)!!! J.A. Rodriguez Ph.D. dissertation, MSU E. Pozdeyev, HB08

  12. Simulation results:Dependence on Emittance Simulations: number of clusters vs. turn # for different beam emittance J.A. Rodriguez Ph.D. dissertation, MSU E. Pozdeyev, HB08

  13. Experimental Results:Scaling with Bunch Length TBUNCH = 600 ns TBUNCH = 300 ns TBUNCH = 150 ns Breakup happens throughout the bunch (no roll-up from the ends as thought by some). Size of clusters and number per unit length does not depend on current. J.A. Rodriguez, Ph.D. dissertation, MSU E. Pozdeyev, HB08

  14. Experimental results:Transverse beam dynamics E1 y0 - E2 E2 > E1 + 7.1 Energy spread grows from 0 to 5% in 10-20 turns I=20 A, E=17 keV, T=1 sec E. Pozdeyev, HB08

  15. Conclusions • Effect of space charge (SC) on transverse motion and coupling of radial and longitudinal motion can play a crucial role at IR (usually omitted) • This can drive Negative Mass Instability at and below γtr • Simulation results (CYCO) and experimental data (SIR) agree remarkably well • They show that • the instability causes very fast beam fragmentation and energy spread growth • the growth rate is proportional to the beam current and inversely proportional to the beam emittance • Landau damping most likely exist through modification of the dispersion function non-coherently E. Pozdeyev, HB08

  16. Acknowledgements Special thanks:J.A. Rodriguez, F. Marti, R.C. York J. Bierwagen, D. Cole, D. Devereaux, R. Fontus, S. Hitchcock, D. Lawton, D. Pedtke, D. Sanderson, J. Wagner, A. Zeller, R. Zink Personnel of the NSCL machine, welding, electronic shops, assembly group, computer department, designers and detailers. E. Pozdeyev, HB08

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