presented at the

EP Feedback Mini-WorkshopIUCF March 13-15, 2007

1

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


2

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.


3

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


4

Ionization cross section for N2 and DC neutralization folding time

A background gas pressure of 1.5 Torr is a reasonable approximation


5

RFA—have installed two on the IPNS RCS

One is mounted above the beam; one is placed in a horizontal, outboard port


6

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)


7

Uniform beam electric field and space-charge potential as well as other beam characteristics


8

Radial beam (electrons)

• Low Energy

• Temperature-dependent density enhancement

• High ionization cross section

• Oscillates many times through the beam center


9

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


10

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


11

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


12

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)


13

Radial beam (ions)

• Mainly repelled

• Move slowly

• Species variable

• Charge-state +1 but could be higher


14

Ion distribution(mass 14)

50,000macrocharges


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.


16

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


17

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).


18

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.


19

RFA data—Horizontal

Peaks appear with a period of (2fs)-1

Using a 3-stage amplifier—electron signalspositive


20

RFA data—Horizontal

3-stage trans-impedance amplifier:

300 k

A→300 mV)

Integrated di/dt on RFA signal shows negative going beam signal


21

RFA data-Vertical

Oscillations are much faster

Using LANL 2-stage amplifier—electron signals negative


22

RFA data-Vertical

2-stage trans-impedance amplifier:

Integrated beam di/dt on RFA signal should be positive going signal


23

Comparison with unshielded RFA data from PSR (courtesy of R. Macek)


24

RFA data-Vertical


25

RFA data-Vertical

FFT of the time data shown on the previous slide—SB are not indicative of the tune


26

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


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).


28

RFA data-VerticalSometimes large positive signals are seen. Integration producing acurve with a different characteristic from primarily negative going signals


29

Other evidence for electrons and ions


30

IPM data


31

Near injection a rising vertical tune is seen


32

Tune and Chromaticity

Pinger measurementsfrom pie data

Single-endedto ID fundamentalharmonic

Differencedto increase S/N on betatron SB


33

Tune and Chromaticity—early and late in the RCS cycle

2 ms

H V


34


Chromaticity scan with sextupole A current, 11 ms


35


11 ms, chromaticity scan with sextupole A

82 A is nominal, octupole component evident in the horizontal


36

Coherent tune shift (vertical—little seen in horizontal)

Data fit withthird-order polynomial


37

To be certain we are not seeing di/dt from the beam, shielding is being added to the RFA


38

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

presented at the

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