EP Feedback Mini-Workshop IUCF March 13-15, 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

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

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

EP Feedback Mini-Workshop IUCF March 13-15, Ionization cross section for N 2 and DC neutralization folding time A background gas pressure of 1.5  Torr is a reasonable approximation

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

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

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

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

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

EP Feedback Mini-Workshop IUCF March 13-15, Radial beam matching conditions An equilibrium (matched) radius exists during the beam. a eq = (8k B T e  o /Z i e 2 n b ) 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:  n = 2a(  k B T ┴ /m o c 2 ) 1/2

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

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

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

EP Feedback Mini-Workshop IUCF March 13-15, Consider densities n b = 1.05x10 14 p/m 3 n i = 1.39x10 11 N/m 3 n g = 5.31x10 16 molecules/m 3 (mainly H 2 0 and N 2 ?) 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).

EP Feedback Mini-Workshop IUCF March 13-15, Electron and proton beam ionization cross sections in H 2 gas—as function of  and T(eV)* *M. Reiser, Theory and Design of Charged Particle Beams, Wiley, New York, 1994, p. 276 A 50 MeV proton has ionizing power similar to a 27 keV electron. Once generated, electrons typically have much lower energies than 27 keV.

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

EP Feedback Mini-Workshop IUCF March 13-15, RFA data—Horizontal 3-stage trans- impedance amplifier: 300 k   A→300 mV) Integrated di/dt on RFA signal shows negative going beam signal

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

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

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

EP Feedback Mini-Workshop IUCF March 13-15, 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, RFA data with PIE signals The integrated s5 PIE data appears to be slightly ahead of the s6 RFA data l 56 /  c 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, 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, 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, Other evidence for electrons and ions

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

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

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

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

EP Feedback Mini-Workshop IUCF March 13-15, 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, Conclusion and Further Work 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