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. McMichael Argonne National Laboratory, Argonne, IL 60439, USA presented to the Midwest Accelerator Physics Collaboration Meeting June 11, 2003 Argonne National Laboratory

2. Introduction—Electron Sources 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

3. Ionization and Neutralization Ave. pressure in IPNS RCS: 1-2  T N 2 RCS  n ~0.5 ms at injection; acceleration period: 14 ms At PSR,  n ~20 ms; store period: <1 ms At 4E12 injected, n g is 2 orders greater than n b

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

5. Beam electric field and potential Assuming a uniform beam, radius r b =1.5 cm and wall radius, r w =3.8 cm 3x10 12 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 k  76 dB into 50 

10. RFA data InjectionScrambler

11. RFA issues 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

12. Pie electrodes 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)

13. Pie data—H position near PM. Noise increases significantly 9 ms10 ms

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

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

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

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

18. Higher Frequency Spectra 500 MS/s, 20  s window, 10 kS No scramblerWith scrambler

19. Conclusions 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

20. Acknowledgement 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.