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Introduction—Electron Sources

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Introduction—Electron Sources

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  1. Observations of electrons in the Intense Pulse Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS)J. C. Dooling, F. R. Brumwell, W. S. Czyz, K.C. Harkay, M. K. Lien, and G. E. McMichaelArgonne National Laboratory, Argonne, IL 60439, USApresented to theMidwest Accelerator Physics Collaboration MeetingJune 11, 2003Argonne National Laboratory

  2. Production of electrons in high intensity machines can have significant impact on beam stability and lifetime. Electrons come from several sources: Stripping of H- Photo-ionization Background gas ionization (high pressure) SE, Multipacting, may lead to E-P and other instabilities; also, plasma formation Introduction—Electron Sources

  3. Ionization and Neutralization • Ave. pressure in IPNS RCS: 1-2 mT N2 • RCS tn~0.5 ms at injection; acceleration period: 14 ms • At PSR, tn~20 ms; store period: <1 ms • At 4E12 injected, ng is 2 orders greater than nb

  4. Profile and Position System (PAPS) IPM Retarding Field Analyzer (RFA) Pie Electrodes Resistive Wall Monitor (RWM) IPNS RCS Diagnostics

  5. Beam electric field and potential • Assuming a uniform beam, radius rb=1.5 cm and wall radius, rw=3.8 cm • 3x1012 protons • Bunching factor and frequency folded into peak current

  6. Profile and Position System IPM

  7. PAPS IPM Data • IPM profiles respond to pressure and bias • Gaussian fit to data tracks horizontal position (Pie) electrode centroid • Ionization bursts observed at injection and after phase modulation (PM) • BW~5 kHz

  8. IPM responds to ionization bursts • RWM Q and PM

  9. Retarding Field Analyzer—installed in the L5 straight section Trans-impedance amplifier, 300 kW 76 dB into 50 W

  10. Injection Scrambler RFA data

  11. Electron signals intermittent Power supply noise (switching—filter helps) Beam noise (di/dt) Magnetic shielding (not done with present amplifier) Radiation effects from probe at beam elevation RFA issues

  12. Split can (short strip line yielding di/dt) Provides horizontal position information for tuning by operators Beam phase for fast phase feedback control of the rf amplifiers Vertical position Raw, single channel signal provides broadband beam information (motion) Pie electrodes

  13. 9 ms 10 ms Pie data—H position near PM. Noise increases significantly

  14. Without scrambler With scrambler Pie Electrode spectra from time data Lower hybrid waves? E perpendicular to B

  15. Pie Spectra, con’t • Spectra without scrambler, full cycle (80 ms frames)

  16. Pie Spectra, con’t • With scrambler, full cycle (80 ms frames)

  17. Lower Hybrid Drift (LHD) Frequencies • LHD frequencies for N+ and OH+

  18. No scrambler With scrambler Higher Frequency Spectra500 MS/s, 20 ms window, 10 kS

  19. Electrons present, so are ions (plasma) PM modifies plasma channel, adds stability Large electron signals usually not observed Rarely at injection, more often after scrambler Add magnetic shielding to RFA Include background ions in simulations, especially when intense electron signals suggest significant neutralization. Diagnostics to look directly at or near the beam region e.g., interferometry, spectroscopy, Langmuir probes, Rogowski coils Conclusions

  20. This work would not be possible without the dedication and hard work of the IPNS Accelerator Operations Group, the IPNS and MSD Divisions, and DOE support. Acknowledgement

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