experiments and simulations with electron clouds in magnets – application to cesr

The Heavy Ion Fusion Science Virtual National Laboratory

Experiments and simulations with electron clouds in

magnets– application to CESR

Art MolvikLawrence Livermore National Laboratory

Heavy-Ion Fusion Science Virtual National Laboratory

atCornell UniversityJan. 31-Feb. 2, 2007UCRL-PRES-227649

2Molvik – CesrTA – 2/2/07


Outline

1. Introduction and e-cloud

diagnostics on HCX

2. Possibilities for CESR

3. Clearing electrode experiments

4. Application to CESR

5. Summary

Experiments and simulations with electron clouds in magnets

– application to CESR



HIFS-VNL has unique tools to study ECE

•WARP/POSINST code goes beyond previous state-of-the-art

– Parallel 3-D PlC-adaptive-mesh-refinement code with accelerator lattice follows beam self-consistently with gas/electron generation and evolution

•HCX experiment addresses ECE fundamentals relevant to HEP (as well as WDM and HIF)

– trapping potential ~2kV (~20% of ILC bunch potential?) with highly instrumented section dedicated to e-cloud studies

•Combination of models and experiment unique in the world

– unmatched benchmarking capability provides credibility ‘Benchmarking’ can include: a. Code debug

b. validation against analytic theory

c. Comparison against codesd. Verification against

experiments– enabled us to attract work on LHC, FNAL-Booster, and ILC (2007)

Who we are – The Heavy Ion (Inertial) Fusion Science Virtual National Laboratory (HIFS-VNL) has participants from LLNL, LBNL, and PPPL.



(a) (b) (c)Capacitive

arrayClearing electrodes

Suppressor

e-

End plate

Retarding Field

Analyser (RFA) GESD

Q1 Q2 Q3 Q4

K+

Short experiment => need to deliberately amplify electron effects:

let beam hit end-plate to generate copious electrons which propagate upstream.

INJECTOR

MATCHINGSECTION

ELECTROSTATICQUADRUPOLES

2 m long MAGNETICQUADRUPOLES

1 MeV, 0.18 A, t ≈ 5 s,

6x1012 K+/pulse, 2 kV beam potential, tune depression to 0.10

HCX used for gas/electron effects studies (at LBNL)

Location of CurrentGas/Electron Experiments

2 m



Diagnostics in two magnetic quadrupole bores, & what they measure.

MA4MA3

8 “paired” Long flush collectors (FLL): measures capacitive signal + collected or emitted electrons from halo scraping in each quadrant.

3 capacitive probes (BPM); beam capacitive pickup ((nb- ne)/ nb).

2 Short flush collector (FLS); similar to FLL, electrons from wall.

2 Gridded e- collector (GEC); expelled e- after passage of beam

2 Gridded ion collector (GIC): ionized gas expelled from beam

BPM (3)

BPM

FLS(2)

FLS

GIC (2)

GIC

Not in service

FLS

GECGEC



Expelled ions

0

2

4

6

8

10

12

0 5 10 15 20

Argon gas density (1E10/cc)

GIC4.7-5 current (micro A)

Expelled ions Pbackground

Expelled ion current

[mA]

n(Ar) [1010 cm-3]

We measure electron sources – ionization

1.Ionization of gas by beam (ne/nb ≤

3%)

Beam

Expelled ions

Beam Potential+2 kV +1 kV

Beam current known; from expelled ion current infer-Ionization rate -Also, gas density in beam



B

-50 V +50 V

We measure electron sources – walls

2. Electron emission

– beam tube (ne/nb

≤ 7%)

3. Electron emission

– end wall (ne/nb,

0, 100%)

Quadrupole magnets(a)

(b)

( c)Internal diag

Clearing electrodes a, b, c

K+ beam

Suppressor

e- from end

For low ion scrape-off, expect current near-zero 4 t(µs) 10

-10 mA

40 mA

140 mA

Vs=0, Vc_a,b=+9 kV

-30

-20

-10

0

10

0 2 4 6 8 10

Bias on clearing electrode-c (kV)

Clearing electrodes (mA)

I_aI_bI_c

040415

Clearing electrode-c bias

(a)

(b)

(c)

0 2 6 kV 10-

30

10

Clearing

current

(mA)

0



Point source of electrons to simulate synchrotron radiation photoelectrons

Electron current vs. cathode-grid potential at various cathode temperatures

1

10

100

1000

10000

0 500 1000 1500 2000Potential between cathode & grid (V)

Current (mA)

950 C980 C1005 C1035 C1060 C1085 CPower (950 C)Power (980 C)Power (1005 C)Power (1035 C)Power (1060 C)Power (1085 C)

Electron gun operates over range

~10 eV to 2000 eV (cathode & grid indep.)

<1 mA to 1000 mA

Electron gun

enables

quantitatively

controlled

injection of

electrons



Retarding field analyzer (RFA) measures energy distribution of expelled ions

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

• RFA an extension of ANL design (Rosenberg and Harkay)• Can measure either ion (shown) or electron distributions• Potential of HCX beam edge ~1000 V, beam axis ~ 2000 V

Ref: *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, 054801 (2006); Instrument details to be pub. NIM-A



/RFA• Retarding field analyzer (RFA) measures potential on axis = ion-repeller potential

• + beam/e- distr. => f=Ne/Nbeam

– A first –time-dependent measurement of absolute electron cloud

density*

Clearing electrode measures e- current

+ e- velocity drift => f=Ne/Nbeam

Absolute electron fraction can be inferred from RFA and clearing

electrodes

Beam neutralization

B, C, S on

B, C off S on

B, C, S off

Clear. Electrode A

~ 7% ~ 25% ~ 89%

RFA (~ 7%) ~ 27% ~ 79%*Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, 054801 (2006).



Outline

1. Introduction & e -cloud

diagnostics on HCX




5. Summary



CESR – RFA measures energy distribution of expelled ions?





• Bunch 1.6x1010 e+, 14 ns separation• For b ~ 50 V, expulsion time of H2 ion ~ 0.6 µs• Need separate ion and electron repeller grids. Measure potential at which ion current zero.

• Difficult measurement: low energy, low current with ultrahigh vacuum, need to shield against rf pickup, poor time dependence, detector in wiggler B-field?

Ref: *Michel Kireeff Covo, et al, Phys. Rev. Lett. 97, 054801 (2006).



CESR – RFA to measure e-cloud density (in drift or wiggler)?

• Bunch 1.6x1010 e+, 14, 28, or 42 ns separation now (≥4 ns later) – what is the energy of expelled ions?

• Beam line charge: at 4 ns spacing ~2 nC/m, at 14 ns ~0.6 nC/m

• Beam potential: ~180 V, ~50 V [Assuming rb/rw ~0.1, 0.01

better]

• My conclusion is that the peak ion energy is determined by the beam line charge, averaged over a bunch spacing.

• Time to expel H2+ ion: 0.3 µs, 0.6 µs (0.12-0.24 of

revolution time)

• Expelled ion current extremely small:

- HCX: 180 mA, ~1E-15 cm2, P~3E-7 torr, 100 nA/cm2

- CESR: ~1 mA, Cross section much smaller, P ~1E-9, fA/cm2?

- CESR-100%ionized gas: 3.3E9cm-3(0.0012cm3)q/[(1µs)(12cm)~0.5nA/cm2

• Conclusion: difficult measurement – what other possibilities?



CESR – other diagnostics to measure e-cloud density?

• Capacitively coupled electrodes

- Simple diagnostic, well developed for BPMs and other applications

- But, also measures emission and collection of electrons, which can

have a magnitude similar to capacitive.

• Perhaps adjacent biased electrodes, e.g., ±bias. + measures

capac. - e- collection, and - measures capac. + e- emission

• Perhaps adjacent bare and gridded diagnostics: capacitive

electrode and grid-shielded electrode (or RFA). The latter

measures only e-, the former both.

These ideas need more analysis – Is there a pony in this pile?



Outline


diagnostics on HCX




5. Summary



Trapping depth of electrons depends upon their source, in a quadrupole magnet (without multipactor)

Electrons ejected from end wall

Electrons from ionization of gas

Electrons desorbed from beam pipe in quad upon ion impact

E-cloud in a quadrupole magnet[Electron mover also speeds simulation in wiggler fields]

beam pipe

Deeply trapped electrons

Weakly trapped electrons



Gridded Electron Collectors (GEC) current measures electron depth of trappingGEC collects e- along B-field lines, detrapped at end of pulse

-800

-600

-400

-200

0

200

4 6 8 10 12 14

Time (µs)

FaradayCup(mA) - GEC4.5-2 + GEC4.5-2

Weakly-trappedelectrons

Deeply-trappedelectrons

Beam-tailscrapes wall



Beam potential contours

In drift region

HCX current flattops for ~4 µs (like super-bunch)



Weakly trapped electrons cleared with ~300 V bias, whereas deeply trapped require >1000 V

GEC4.5-2: integrated around 5.25us (positively biased), Data 6/23/05

-70,000

-60,000

-50,000

-40,000

-30,000

-20,000

-10,000

0

0 500 1000 1500 2000 2500

clearing electrode biases (V)

lambda (pC/m)Weakly trapped e-Deeply trapped e-

• Weakly trapped electrons originate on or near a wall (beam tube) – turning points near wall.

• Deeply trapped electrons originate from beam impact ionization of gas, or scattering of weakly trapped electrons – turning points within beam.



Clearing electrode removes all electrons from a drift region

• Clearing electrode ring (C) – blocks electrons from (B) when biased more negatively than -3 kV

• Clearing electrode ring (B)

[with Vc= 0] blocks

electrons from (A) when biased more negatively than -3 kV

Vs=0, Vc_a,b=+9 kV

-30

-20

-10

0

10

0 2 4 6 8 10

Bias on clearing electrode-c (kV)


I_aI_bI_c

040415

Vs=0, Vc_a=+9 kV, Vc_c = 0.0

-40

-30

-20

-10

0

10

0 2 4 6 8 10

Bias on clearing electrode-b (kV)


I_aI_bI_c

040415

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/RFA

Suppressor bias = 0 V, electrons can leak back into quads along beam.



Clearing electrode fields, above 2 kV bias, dominate over beam space-charge field







Vc = 0 kV

b = +2 kV

Vc = +2 kV

b = +2 kVVc = +10 kV

b = +2 kV

Beam space-charge potential b = +2 kV

For CESR-TA, clearing field should dominate over beam space charge averaged over a few bunches (Vc > 50 V), or remove electrons in period between bunches (Vc ≥ 100 V).



1

WARP-POSINST code suite is unique in four ways merge of WARP & POSINST

Key: operational; partially implemented (4/28/06)

+ new e-/gas modules

2

+ Adaptive Mesh Refinement

Z

R

concentrates resolution only where it is needed3Speed-up x10-104

beam

quad

e- motion in a quad

+ New e- moverAllows large time step greater than cyclotron period with smooth transition from magnetized to non-magnetized regions

4 Speed-up x10-100



And also

Electron oscillations – simulation & experiment agree

WARP-3DT = 4.65s

0. 2. time (s) 6.

Simulation Experiment0.

-20.

-40.

I (m

A)

Current to clearing electrode (c) agrees in frequency ~ 6 MHz

0

100

200

300

400

500

600

-20-15-10-505Axial position from center of magnet (cm)

HCXWARP

FFT 1.9 to 2.9 µs, averaged 1 to 31 MHz, Data 26 January, 2006, Shot 6

Currents to capacitive electrode array agree in wavelength ~5 cm, and amplitude (below)

e-

(a)

(b)

(c)

200mA K+

OscillationsElectrons bunching

Beam ions hit end plate

0V 0V 0V/+9kV 0V

Q4Q3Q2Q1

200mA K+

ElectronsSimulation

ExperimentSimulation



Outline


diagnostics on HCX




5. Summary



Clearing electrodes for CESR

• Drift regions easiest: clearing electrode can clear

entire region

• Quadrupole magnets next: Clearing electrodes clear

upstream in each drift region

• Dipole and wigglers hardest: Clearing electrodes

clear only along intercepted field lines



Outline


diagnostics on HCX




5. Summary



• Extensive diagnostics on HCX, some may be useful

on CESR

• Simulations benchmarked against experiment –

accurately reproduce many details of experiment

• May measure e-cloud density on CESR, along with

measuring effects on beam: need more analysis

- RFA (more difficult than on HCX)

- Multiple capacitive electrodes with varying

bias

- Capacitive and grid-shielded electrodes

• Clearing electrodes

- Effective at clearing drift region

- In magnets (dipole, wiggler) effective only on

intercepted field lines, or (quad) nearby

drift surfaces

Summary



Backup slides



Simulations with beam reconstructed from slit scans – improved agreement

• Effects of electrons on beam– beam loading in simulation now uses reconstructed data from slit-plate measurements

leads to improved agreement between simulation and experiment

Semi-gaussian load =>

New load from reconstructed data =>

X'

X X

X'

X'

Reconstructed X-Y distribution from slit-plate measurements

QuickTime™ and aGraphics decompressor


Low e- High e-

No e-

No e-

High e-

High e-



1.6MeV; 0.63A/beam; 30s1.6MeV; 0.63A/beam; 30s

~1km4.0GeV; 94.A/beam; 0.2s4.0GeV; 94.A/beam; 0.2s

4.0GeV; 1.9kA/beam; 10ns4.0GeV; 1.9kA/beam; 10ns

Heavy Ion Inertial Fusion or “HIF” goal is to develop an accelerator that can deliver beams to ignite an inertial fusion target

Target Requirements:

3 - 7 MJ x ~ 10 ns ~ 500 Terawatts

Ion Range: 0.02 - 0.2 g/cm2 1- 10 GeV

Near term goal: High Energy Density Physics (HEDP)

For A ~ 200 ~ 10 16 ions

~ 100 beams

1-4 kA / beam

DT

experiments and simulations with electron clouds in magnets – application to cesr

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