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RFA Measurements and Ideas related to Background Gas Ionization J. C. Dooling, S. Wang, K.C. Harkay, R.L. Kustom, G.E. McMichael, M.E. Middendorf, and A. Nassiri. presented at the Midwest Accelerator Physics (MAP) Meeting, Indiana University Cyclotron Facility March 14, 2007.

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  1. RFA Measurements and Ideas related to Background Gas IonizationJ. C. Dooling, S. Wang, K.C. Harkay, R.L. Kustom, G.E. McMichael, M.E. Middendorf, and A. Nassiri presented at the Midwest Accelerator Physics (MAP) Meeting, Indiana University Cyclotron FacilityMarch 14, 2007 EP Feedback Mini-Workshop IUCF March 13-15, 2007

  2. Coasting beam injection (70 ms at 2.2 MHz~154 turns) Pseudo SR mode—bunching is initially weak BF~1 At 1 mTorr, background gas density is 300 times the beam density. Electron and ion generation in the IPNS RCS EP Feedback Mini-Workshop IUCF March 13-15, 2007

  3. Early in the RCS acceleration cycle Fast scope, 800 ps sample window Spectrum Analyzer, S5T, 0.3 ms after inj., 50 ms gated window, 30 kHz VBW Bins 18 and 1600 from the FFT (Df=125 kHz) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  4. Ionization cross section for N2 and DC neutralization folding time A background gas pressure of 1.5 mTorr is a reasonable approximation EP Feedback Mini-Workshop IUCF March 13-15, 2007

  5. RFA—have installed two on the IPNS RCS One is mounted above the beam; one is placed in a horizontal, outboard port EP Feedback Mini-Workshop IUCF March 13-15, 2007

  6. Comparison of installation configurations—PSR and IPNS RCS IPNS RCS Initial PSR Photo courtesy of R. Macek Only evanescent, near-field energy can reach the rfa (and it does) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  7. Uniform beam electric field and space-charge potential as well as other beam characteristics EP Feedback Mini-Workshop IUCF March 13-15, 2007

  8. Low Energy Temperature-dependent density enhancement High ionization cross section Oscillates many times through the beam center Radial beam (electrons) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  9. KV: Equations of motion in the beam field (1-D): Equations Beam and initial electron and ion distributions electrons charge density: protons background ions Though the same form, wb, the bounce frequency is not to be confused with the plasma frequency. EP Feedback Mini-Workshop IUCF March 13-15, 2007

  10. Starting with a KV distribution for beam and electrons Te Radial electric beam: ave. density is temperature dependent. For temperatures of few eV, density is peaked near the center. Density oscillates at 2fb. EP Feedback Mini-Workshop IUCF March 13-15, 2007

  11. An equilibrium (matched) radius exists during the beam. aeq = (8kBTeeo/Zie2nb)1/2 The radial electron beam radius depends on the average temperature (velocity) as well as the ion charge and the non-neutralized beam density. Normalized emittance: en = 2a(gkBT┴/moc2)1/2 Radial beam matching conditions EP Feedback Mini-Workshop IUCF March 13-15, 2007

  12. Radial beams • Electron bounce frequency: • Ion bounce frequency: = 65 MHz at injection = 1.52 MHz at injection (mass 1) = 0.41 MHz “ “ (mass 14) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  13. Mainly repelled Move slowly Species variable Charge-state +1 but could be higher Radial beam (ions) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  14. Ion distribution(mass 14) 50,000 macro charges EP Feedback Mini-Workshop IUCF March 13-15, 2007

  15. Why the peak at higher radius? Consider the following two skiers on a frictionless hill: The skier with the scarf starts on the maximum slope, the other starts higher up the hill. Initially, the scarved skier goes faster. However, near the bottom of the hill, the skier with the greater kinetic energy catches up. EP Feedback Mini-Workshop IUCF March 13-15, 2007

  16. Evolved electric field and potential from the background ions (one revolution) Strictly 1-D, radial EP Feedback Mini-Workshop IUCF March 13-15, 2007

  17. Consider densities Cross section for nitrogen In the RCS, the background gas density is roughly 500 times the beam density and the beam density 800 times the generated ion density (on the first pass). nb = 1.05x1014 p/m3 ni = 1.39x1011 N/m3 ng = 5.31x1016 molecules/m3 (mainly H20 and N2?) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  18. Electron and proton beam ionization cross sections in H2 gas—as function of b and T(eV)* A 50 MeV proton has ionizing power similar to a 27 keV electron. Once generated, electrons typically have much lower energies than 27 keV. *M. Reiser, Theory and Design of Charged Particle Beams, Wiley, New York, 1994, p. 276 EP Feedback Mini-Workshop IUCF March 13-15, 2007

  19. RFA data—Horizontal Using a 3-stage amplifier—electron signals positive Peaks appear with a period of (2fs)-1 EP Feedback Mini-Workshop IUCF March 13-15, 2007

  20. RFA data—Horizontal 3-stage trans-impedance amplifier: 300 kW (1 mA→300 mV) Integrated di/dt on RFA signal shows negative going beam signal EP Feedback Mini-Workshop IUCF March 13-15, 2007

  21. RFA data-Vertical Using LANL 2-stage amplifier—electron signals negative Oscillations are much faster EP Feedback Mini-Workshop IUCF March 13-15, 2007

  22. RFA data-Vertical 2-stage trans-impedance amplifier: Integrated beam di/dt on RFA signal should be positive going signal EP Feedback Mini-Workshop IUCF March 13-15, 2007

  23. Comparison with unshielded RFA data from PSR (courtesy of R. Macek) EP Feedback Mini-Workshop IUCF March 13-15, 2007

  24. RFA data-Vertical EP Feedback Mini-Workshop IUCF March 13-15, 2007

  25. RFA data-Vertical FFT of the time data shown on the previous slide—SB are not indicative of the tune EP Feedback Mini-Workshop IUCF March 13-15, 2007

  26. RFA data with PIE signals The integrated s5 PIE data appears to be slightly ahead of the s6 RFA data l56/bc not accounted for. RFA scope in a different location than PIE scope; cable length and triggering delays also contribute EP Feedback Mini-Workshop IUCF March 13-15, 2007

  27. RFA data with PIE signals—at extraction Again at extraction, the integrated PIE signal appears to slightly lead the integrated RFA signal. Maximum deflection voltage applied (600 V). EP Feedback Mini-Workshop IUCF March 13-15, 2007

  28. RFA data-Vertical Sometimes large positive signals are seen. Integration producing a curve with a different characteristic from primarily negative going signals EP Feedback Mini-Workshop IUCF March 13-15, 2007

  29. Other evidence for electrons and ions EP Feedback Mini-Workshop IUCF March 13-15, 2007

  30. IPM data EP Feedback Mini-Workshop IUCF March 13-15, 2007

  31. Near injection a rising vertical tune is seen EP Feedback Mini-Workshop IUCF March 13-15, 2007

  32. Tune and Chromaticity Pinger measurements from pie data Single-ended to ID fundamental harmonic Differenced to increase S/N on betatron SB EP Feedback Mini-Workshop IUCF March 13-15, 2007

  33. Tune and Chromaticity—early and late in the RCS cycle 2 ms H V EP Feedback Mini-Workshop IUCF March 13-15, 2007

  34. Tune and Chromaticity—early and late in the RCS cycle Chromaticity scan with sextupole A current, 11 ms EP Feedback Mini-Workshop IUCF March 13-15, 2007

  35. Tune and Chromaticity—early and late in the RCS cycle 11 ms, chromaticity scan with sextupole A 82 A is nominal, octupole component evident in the horizontal EP Feedback Mini-Workshop IUCF March 13-15, 2007

  36. Coherent tune shift (vertical—little seen in horizontal) Data fit with third-order polynomial EP Feedback Mini-Workshop IUCF March 13-15, 2007

  37. To be certain we are not seeing di/dt from the beam, shielding is being added to the RFA EP Feedback Mini-Workshop IUCF March 13-15, 2007

  38. Simple 1-D model with KV to understand physics Electrons are present in the RCS—Source: background ionization, SE from wall Central density is temperature dependent If electrons are present, then so are ions I’s repelled by beam, but are slow During beam space charge, electrons form radial beam Low-energy electrons have higher ionization cross section than protons Conclusion and Further Work EP Feedback Mini-Workshop IUCF March 13-15, 2007

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