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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. - PowerPoint PPT Presentation
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EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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 FacilityMarch 14, 2007
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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-WorkshopIUCF March 13-15, 2007
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Early in the RCS acceleration cycle
Bins 18 and 1600 from the FFT (f=125 kHz)
Spectrum Analyzer, S5T, 0.3 msafter inj., 50 s gated window, 30 kHz VBW
Fast scope, 800 ps sample window
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Ionization cross section for N2 and DC neutralization folding time
A background gas pressure of 1.5 Torr is a reasonable approximation
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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-WorkshopIUCF March 13-15, 2007
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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-WorkshopIUCF March 13-15, 2007
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Uniform beam electric field and space-charge potential as well as other beam characteristics
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Radial beam (electrons)
• Low Energy
• Temperature-dependent density enhancement
• High ionization cross section
• Oscillates many times through the beam center
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Equations
• KV:
• Equations of motion in the beam field (1-D):
222 2x ax y by1
2 2x ya b
x xx
F qEa x
m m
e,
q e,
Ze,
2 2x r
o o o
x rE E x y
2 2 2
b b ij ij ej
e Z n Z n n
22bb
o
e nr r r
2 m
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 electronand ion distributions
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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 2fb.
Te
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Radial beam matching conditions
• An equilibrium (matched) radius exists during the beam.
aeq = (8kBTeo/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: n = 2a(kBT┴/moc2)1/2
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Radial beams
• Electron bounce frequency:
• Ion bounce frequency:
1/ 22be b
beo e
e n1f
2 2 2 m
1/ 22bi i b
bio i
Z e n1f
2 2 2 m
= 65 MHz at injection
= 1.52 MHz at injection (mass 1)= 0.41 MHz “ “ (mass 14)
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Radial beam (ions)
• Mainly repelled
• Move slowly
• Species variable
• Charge-state +1 but could be higher
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Ion distribution(mass 14)
50,000macrocharges
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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-WorkshopIUCF March 13-15, 2007
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Evolved electric field and potential from the background ions (one revolution)
mr k i1m k
k 0m o
rE r
r
mr1 r1 r1m m k
m k 0V r E E
Strictly 1-D, radial
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Consider densities
nb = 1.05x1014 p/m3
ni = 1.39x1011 N/m3
ng = 5.31x1016 molecules/m3
(mainly H20 and N2?)
Cross section for nitrogen
In the RCS, the background gas densityis roughly 500 times the beam density andthe beam density 800 times the generatedion density (on the first pass).
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Electron and proton beam ionization cross sections in H2 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-WorkshopIUCF March 13-15, 2007
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RFA data—Horizontal
Peaks appear with a period of (2fs)-1
Using a 3-stage amplifier—electron signalspositive
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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-WorkshopIUCF March 13-15, 2007
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RFA data-Vertical
Oscillations are much faster
Using LANL 2-stage amplifier—electron signals negative
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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RFA data-Vertical
2-stage trans-impedance amplifier:
Integrated beam di/dt on RFA signal should be positive going signal
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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Comparison with unshielded RFA data from PSR (courtesy of R. Macek)
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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RFA data-Vertical
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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RFA data-Vertical
FFT of the time data shown on the previous slide—SB are not indicative of the tune
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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RFA data with PIE signals
The integrated s5 PIE data appears to be slightly ahead of the s6 RFA data
l56/c not accounted for. RFA scope in a different location than PIE scope; cable length and triggering delays also contribute
EP Feedback Mini-WorkshopIUCF March 13-15, 2007
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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).
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RFA data-VerticalSometimes large positive signals are seen. Integration producing acurve with a different characteristic from primarily negative going signals
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Other evidence for electrons and ions
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IPM data
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Near injection a rising vertical tune is seen
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Tune and Chromaticity
Pinger measurementsfrom pie data
Single-endedto ID fundamentalharmonic
Differencedto increase S/N on betatron SB
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Tune and Chromaticity—early and late in the RCS cycle
2 ms
H V
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Tune and Chromaticity—early and late in the RCS cycle
Chromaticity scan with sextupole A current, 11 ms
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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-WorkshopIUCF March 13-15, 2007
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Coherent tune shift (vertical—little seen in horizontal)
Data fit withthird-order polynomial
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To be certain we are not seeing di/dt from the beam, shielding is being added to the RFA
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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