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Experimental study of the transport limits of intense heavy ion beams in the High Current Transport Experiment. Lionel Prost, F. M. Bieniosek, C. M. Celata, A. Faltens, P. A. Seidl, W.L. Waldron, R. Cohen, A. Friedman, M. Kireeff Covo, S. M. Lund, A.W. Molvik, I. Haber LBNL, LLNL , UMD

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  1. Experimental study of the transport limits of intense heavy ion beams in the High Current Transport Experiment Lionel Prost, F. M. Bieniosek, C. M. Celata, A. Faltens,P. A. Seidl, W.L. Waldron, R. Cohen, A. Friedman, M. Kireeff Covo, S. M. Lund,A.W. Molvik,I. Haber LBNL, LLNL, UMD HIF 2004 Symposium Princeton, NJ June 9, 2004

  2. Talk outline • Experiment objectives, layout • Matching, transport through electrostatic quadrupoles • Transverse phase space • Longitudinal phase space • Transport through magnetic quadrupoles

  3. Fill factor = amax/Rpipe amax ~$1B aavg Beam Rpipe Clearance Beam pipe Heavy ion fusion system studies show that driver cost is very sensitive to fill factor IBEAM results: Robust Point Design (2.8 B$) range being explored W. Meier, LLNL (fixed number of beams, initial pulse length, and quadrupole field strength)

  4. HCX explores two driver options for low energy transport • Electrostatic quadrupoles provide clearing fields which sweep out unwanted electrons (clarifies study of other beam dynamics issues without electrons). • Vary fill factor (emittance growth, halo, beam loss: sensitivity to mismatch, alignment, focusing & image nonlinearities). • Simulations predict beam fill factors of 0.8 of the aperture radius (R=23 mm) may be possible, with negligible beam degradation. • Magnetic quadrupoleexperiments at high beam current • first with four pulsed magnets; later with superconducting magnets. • Explore secondary e-, Ao, A- production from beam scraping and collisions with background gas. This gives information on needed clear aperture and on surface conditioning.

  5. HCX is exploring driver-scale dynamics • Transverse phase space evolution, fill factor, halo,… • Longitudinal bunch control, space charge waves,… ≈11 m Current monitor TOF pulser Bunch control module D2 4 RT pulsed magnetic quadrupoles 1-1.8 MeV ESQ injector Matching section (6 quadrupoles) 10 electrostatic transport quadrupoles QD1 D-end, Energy Analyzer, GESD K+ source & triode I = 0.2-0.6 A 15 diagnostic systems, + halo pickups on each electrostatic quadrupole

  6. Electrostatic transport section 1 Azimuthally rotatable ESQ Allows variable skew coupling ( Dq = ± 4o Max) 1 QD (Diagnostic)ESQ Removable from beam axis to allow insertion of intercepting slit-scanner diagnostics. 4 QI ESQ’s Independent voltages to allow beam envelope manipulations Six-strut Mounting System and Kinematic Support Structure For rail alignment decoupled from vacuum tank 2 QS ESQ’s Translatable in x and y to allow centroid steering

  7. Magnetic transport section (4 room temperature pulsed magnets with elliptical bore) Capacitive probe Support rail Electron suppressor

  8. Talk outline • Experiment objectives, layout • Matching, transport through electrostatic quadrupoles • Transverse phase space • Longitudinal phase space • Transport through magnetic quadrupoles

  9. Relectrode Kapton exposed to beam @ injector exit. Envelope based on QD1 data 50 y(mm) 0 vertical QD1 horizontal -50 -40 0 40 x(mm) en = 0.48 p mm mrad en = 0.44 p mm mrad Verticalphase space Horizontal The matching section compresses the beam transversely and prepares it for periodic transport. F(x,y), crossed slit time slice 41(28 mm  30 mm) Verticalprofile Horizontal

  10. BEAM LOSSES THROUGH THE 10 QUAD TRANSPORT SECTION FOR 60% & 80% FILLING FACTOR ≤1% Measured two ways:  Total beam current measurements (Faraday cups) at entrance and exit.  e- current collected on quad electrodes throughout the HCX channel (Electrode capacitive monitors). N ≈ 6x1012K+/pulse (1 MeV) Beam loss: 0.5% (SV(capacitive mon.)) to 1% (Faraday cups), Expect ≈ 0.05% loss from beam-background gas collisions assuming a stripping cross-section (on N2 and/or O2) of 3.5 x 10-16 cm2 and a pressure of2 x 10-7 Torr.

  11. en = 0.48 p mm mrad en = 0.40 p mm mrad en = 0.40 p mm mrad en = 0.48 p mm mrad In both fill-factor cases measured so far, no evidence of emittance growth, within diagnostic sensitivity. 60% fill factor(so = 64o, s=12ospace charge tune depression s /so = 0.2) 80% fill factor(so = 44o, s=7o, s /so = 0.2)  QD1  10 quads downstream Expect ~±10% fluctuations in e(z) due to non-equilibrated distribution.

  12. Envelope calculation, initialized with QD1 data for 80% fill factor Improvements to envelope modeling include: 1. Realistic fringe field model based on 3D field calculations. 2. Quadrupole Ez and corresponding radial focusing force. 3. Corrections for the grounded slit plates of the intercepting diagnostics that short out the self-field of the beam near the diagnostic. 4. More thorough crosschecks on the beam current and energy. Mismatch ≈ 1 mm so = 44o, s=7otune depression s/so = 0.2

  13. Experimental envelope parameters compared to envelope model predictions at the exit of the electrostatic section Experimental uncertainties: standard deviation from 5 repeated measurements Envelope model uncertainties: standard deviation of a Monte Carlo distribution of envelope predictions, representing measurements uncertainties and equipment accuracies (e.g.: stability of the quadrupole voltages)

  14. Talk outline • Experiment objectives, layout • Matching, transport through electrostatic quadrupoles • Transverse phase space • Longitudinal phase space • Transport through magnetic quadrupoles

  15. Beam energy determination from fitting cold fluid model to data The wave (beam current perturbation) observed 5.5 m downstream, is compared to a cold fluid model of the beam. The beam energy is the ‘free’ parameter that is adjusted. 1 ms DIb(t) Standard g-factor model: g = ln(b/a) / 2pε0assumes beam is incompressible Beam velocity: vb = 220 cm/ms Space charge wave velocity: cs = 6 cm/ms

  16. calibrated absolute E via detection of K++ & biasing stripping grid. Electrostatic energy analyzer for independent measurement of the beam energy 1 MeV

  17. Beam energy calibration valuable for envelope control & input to PIC simulations • Electrostatic energy analyzer (EA) measures longitudinal phase space • s(E)/E ≈ 0.5% • <E> relative accuracy ±0.2% • E is constant to within 0.5 % for 3.1 s. • TOF and EA diagnostics determine absolute Ebeam to ±1.5%, with both measurements agreeing within these uncertainties. (preliminary)

  18. Talk outline • Experiment objectives, layout • Matching, transport through electrostatic quadrupoles • Transverse phase space • Longitudinal phase space • Transport through magnetic quadrupoles

  19. Ion Beam Electron Fraction 0 % 2  2% 52 cm 10%  10% 5 cm Next: measurement of the evolution of desorbed neutrals and secondary e-’s in magnetic quadrupoles (Molvik et al. Th.I.01) WARP simulations with mocked-up electron distributions (200 quads): R. Cohen, LLNL

  20. Optical data from early transport experiments showed very distorted beam distributions due to the diagnostics intercepting the beam in a field free region Installation of an electron suppressor between the last magnet and the diagnosing slits Upstream beam images for reference Sum of images Single slit image Sum of images Single slit image Sum of images Single slit image

  21. Addition of clearing electrodes between magnets influences the beam distribution 2 different magnet gradient solutions Single pulse images Single pulse images   Clearing electrodes grounded Clearing electrodes @ +9 kV

  22. en = 0.13 p mm mrad en = 0.09 p mm mrad Good transport of the apertured beam (I = 32 mA) – Negligible emittance growth Horizontal direction (upstream of magnets) Horizontal direction (downstream of magnets)

  23. Apertured beam 2-D PIC simulation initialized with measured a, a′, b, b′, <x>, <x′>, <y>, <y′> Magnet aperture 53% fill factor Expt data I = 32 mA PIC statistical edge Magnet aperture Z (m) Clearing electrodes Good agreement between measurements and simulations

  24. en = 0.42 p mm mrad en = 0.38 p mm mrad en = 1.67 p mm mrad en = 0.43 p mm mrad Full beam (I = 175 mA) transported with minimal loss but emittance increases ~4x in the horizontal direction Horizontal direction Vertical direction  Upstream of the magnets  Downstream of the magnets

  25. 2-D PIC simulations helped getting enough clearance for diagnostics insertion Expt data 67% fill factor + diagnostics I = 175 mA PIC statistical edge + Magnet aperture Extreme particle edge Inconsistencies remain between data and simulations, which cannot explain the observed emittance growth

  26. Summary – upcoming plan • Transport results through ten electrostatic quadrupoles show good beam control. 80% beam filling factors at the front end of a heavy-ion induction linac might be possible with acceptable emittance growth and beam loss. • Details of the measured phase space distribution are being used to initialize particle-in-cell simulations for comparison of data with theoretical models. • New optical diagnostics will provide previously unmeasured correlations and accelerate data acquisition time. • Transport through four magnetic quadrupoles for studying secondary electron and gas effects: Beam dynamics for highest beam current not yet fully understood. New diagnostics providing interesting results. (A. Molvik et al., Th.I-01)

  27. Summary – upcoming plan (con’t) • Induction cores will be installed between magnets to study the effect of accelerating fields on the secondary electrons distribution. • Bunch control induction module will be installed this fall to study longitudinal space-charge field chromatic effects. • Other fill factors will be measured. • Results will have a direct impact on future heavy ion induction accelerators.

  28. EXTRAS

  29. What must we understand? Intense Beam Physics Stability, waves, phase space changes, nonlinear dynamics Beam Manipulations Beam production, acceleration, longitudinal compression, bending, focusing, chamber transport Multiple-Beam Interactions Pulse-length Limits Emittance growth Beam loss

  30. Matching section quadrupole electrode monitors show little scraping during the flat-top of the beam RMonitor IBeam (t) Good agreement between the capacitive signal derived from the current transformer and the electrode monitor signal collected in the matching section minimal beam loss (except at the beam tail)

  31. Phase space scan x,x crossed-slit • Spatial hollowing is a common feature simulated x,x simulated x,y Simulation distributions, initialized with upstream data, are in coarse agreement with data at end of electrostatic lattice Other correlations, e.g. (y,px), influence dynamics; to be resolved with new optical diagnostics (Bieniosek et al., this conference,Th.I-10) Scintillator imaged after beam passes thru slit line is mean x´(x=0) vs. y; note the correlation

  32. V(t) TOF pulser diagnostic calibrates beam energy. Space charge waves puts a precise time mark on beam. A short-pulse perturbation in the beam energy within the matching section launches forward and backward traveling space charge waves in the beam frame, providing an accurate time-of-flight measurement to determine the beam energy. This also has allowed the study of longitudinal dynamics. V(t) 0.3 ms Generated by a thyratron pulsed capacitive discharge Initial DV

  33. 1 Ebeam (MeV) 1 MeV head/tail spread -> longitudinal space charge calibrated absolute E via detection of K++ & biasing stripping grid. 0.925 4 10.5 Time (s) measurement 3/20/2003 WARP3d simulation (scaled voltage) Electrostatic energy analyzer shows longitudinal distribution • 3D WARP simulations (preliminary) : • uses experimental injector extraction voltage waveform (shortened) • simulation ran to the end of the matching section; measurements were made at the HCX exit

  34. “200kV ears” Bunch control induction module will be tested on HCX, and will be needed in next-step transport & acceleration experiments • Apply agile control of the acceleration waveforms to correct for space charge field effects on the head/tail of the beam. • Deliverables: Complete system of induction cores and modulators for installation in the HCX lattice between matching section and 1st HCX ESQ tank: • Regulate 20kV variations during the flattop (±0.1%VMarx) to study consequences of pulse energy variations. • ± 200kV “ear” waveforms, actively regulated (±3%) to allow analysis and control of the bunch ends. Electrostatic transport K+ beam Matching section induction module

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