# Presentation: 468356

## Electron clouds 

**in ion accelerators – experiments, theory and simulation**
**Art Molvik and Jean-Luc Vay**
**LLNL and LBNL**
**and the**
**Heavy-Ion Fusion Virtual National Laboratory**
**Presented at FermiLab**
- June 7, 2005
**	**

## OUTLINE

**Who we are and why we care about e-cloud****	****	****	****    Art**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency****	****	****	****       Jean-Luc**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusions**

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

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

**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/cm****2****  ********  *****1- 10 GeV*****                            **

***Near term goal: ***
***High Energy Density Physics (HEDP)***

**For A ~ 200  ******** ****	*****~ 10 ******16****** ions***
***	******	******~ 100 beams***
***	******	******1-4  kA / beam****** ***

**DT**

## The Heavy Ion Fusion Virtual National Laboratory:
			LBNL
			LLNL
			Princeton Plasma Physics Laboratory
Working on e-cloud (in addition to R. Davidson and H. Qin):
		Miguel Furman  (LBNL)
			 LBNL is lead lab for LARP on e-cloud
		Ron Cohen  (LLNL)
		Jean-Luc Vay  (LBNL) 
		Alex Friedman  (LLNL)
		Dave Grote (LLNL)
		Art Molvik  (LLNL)
		Peter Seidl   (LBNL)
		Frank Bieniosek  (LBNL)
		Michel Kireef-Covo  (GS, LLNL)
     Also:	Peter Stoltz, through coordinated SBIR at Tech-X
		Prof. John Verboncoeur, UCB NE 				

**	****	****	****LBNL**
**	****	****	****LLNL**
**	****	****	****Princeton Plasma Physics Laboratory**
**Working on e-cloud (in addition to R. Davidson and H. Qin):**
**	****	****Miguel Furman  (LBNL)**
**	****	****	**** ****LBNL is lead lab for LARP on e-cloud**
**	****	****Ron Cohen  (LLNL)**
**	****	****Jean-Luc Vay  (LBNL) **
**	****	****Alex Friedman  (LLNL)**
**	****	****Dave Grote (LLNL)**
**	****	****Art Molvik  (LLNL)**
**	****	****Peter Seidl   (LBNL)**
**	****	****Frank Bieniosek  (LBNL)**
**	****	****Michel Kireef-Covo  (GS, LLNL)**
**     ****Also:****	****Peter Stoltz, through coordinated SBIR at Tech-X**
**	****	****Prof. John Verboncoeur, UCB NE ****	****	****	****	**

**Simulation & theory**

**Experiment**

**HIF-VNL / e-cloud team (LDRD’s at LLNL & LBNL)**

## Why do we care about electrons?We have a strong economic incentive to fill the pipe.

- (from a WARP movie; see http://hif.lbl.gov/theory/simulation_movies.html)

- Time-dependent 3D simulations of HCX injector reveal beam ions hitting structure

**Notes:**
Show movie.

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## The High Current Experiment (HCX)        is exploring beam transport limits

- ESQ injector

- Marx

- matching

- 10 ES quads

- diagnostics

- diagnostics

- ESQ injector

- 10 Electrostatic quads

- diagnostics

- 4 Magnetic quads

***Parameters***
-   K+ Beam
-   0.2 - 0.5 Amp
-   1 - 1.7 MeV
-   4.5 s

## Current HCX Configuration(High Brightness Beam Transport Campaign, 2005)

**INJECTOR**

**MATCHING**
**SECTION**

**ELECTROSTATIC**
**QUADRUPOLES**

**MAGNETIC**
**QUADRUPOLES**

**Focus of Current**
**Gas/Electron Experiments**

_**Additional Experiments:**_** Fill-Factor Measurements, Head-Tail Correction, Wave Experiments**

***We are the user community!***

## We are looking at fundamental questions – experiment closely coupled with theory/simulation

**   ****Primary (ion-induced) electron emission**
**	****	****yield & velocity distribution**
**•   ****Desorbed gas**
**	****	****mechanism, yield & velocity distribution**
**   ****Accumulation & retention in Quads**
**	****loss mechanisms, sources**
**   ****Effect of electrons on beam**
**             ****	****harder - must exaggerate sources because of short experiment**
**   ****Efficacy of mitigation methods**
**roughened surface, clearing electrodes, suppressor electrode**

***Results are expected to scale to other accelerator regimes***

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission ****(of less interest to p+ rings)**
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## The Gas-Electron Source Diagnostic (GESD) measures the effect of the beam on a surface

**Gas-electron source diagnostic  (GESD)**

-   **electron emission yield**
**  ****gas emission yield**
**  ****mitigation methods**

## Gas desorption / electron emission measurements have led to understanding and mitigation

**•****	****Gas desorption coefficients very large, less variation with ******** **
**•****	****Roughened surface reduces desorption**

**•****	****Ion-induced electron emission scales with 1/cos******** ****	****	****       (******** = angle incidence)**
**•****	****Target surface roughened by glass-bead blasting  – reduces emission, but warps sheet metal.**
**•****	****Alumina dust-blasted: nearly as effective mitigation, with little warping of 0.4 mm thick SS**

- Earlier gas emis: E. Mahner, et al., PRST-AB 6, 013201 (2003).

- A. W. Molvik, et al., PRST-AB 7, 093202 (2004).
- Earlier e- emis: P. Thieberger, et al., PR A 61, 042901 (2000).

## We discovered that copious gas desorption in accelerators is due to electronic sputtering*

- * Suggested by Thomas Schenkel, LBNL EBIT

**Gas desorption scales with electronic component of dE/dx**

- •	**Conventional Sputtering** – driven by nuclear scattering of ions in matter
- •	**Electronic sputtering** – driven by electronic slowing of ions in matter

**HIF-VNL facilities ideal**: energy range from nuclear dominating to electronic slowing  dominating. **M. Kireeff Covo: e- emission scaling, to be published**

**Desorption not due to nuclear or total slowing.  GSI results confirm.**

## Intense beam excitation of gas – enables measuring velocity distribution of desorbed gas

_**Observation: desorbed gas in beam emits light**_

**View expanding gas cloud from side – f(v****0****) normal to hole plate [with gated camera or streak camera]**

**Follow-on Options:**
**•****	****Tilt target – f(****v****0****) near grazing incidence**
**•****	****Narrow ribbon target – f(****v****0****,********) with gated camera**
**•****	****Absolute calibration to obtain desorption coef.**
**•****	****Other targets: NEG, ...**

**Dual benefits – ****needed to model time for gas to reach beam**** and provides clues to physical mechanisms of gas desorption**

**Notes:**
1st bullet: RPA will also measure beam potential, from peak of ion energy distribution. It can also be biased to measure electrons that detrap during the current fall time.

4th bullet: data from HCX and STS-500 is consistent with the electronic component of dE/dx responsible for gas desorption with no significant contribution from nuclear scattering, so we've identified the energy source driving desorption. Experiments up to 4 MeV, with a variety of ions and targets will recheck this conclusion, but more importantly will begin to study the various particle sources. [The HCX/STS work will be submitted to a major journal, probably Nature - so maybe leaving 4th bullet on S18 as you have it is best.

## Images of the gas cloud show growth and decay as a function of time.  (F. Bieniosek) 

***Note:  Measurement requires***
***high line charge density beam***

## Line integral of images indicates an expansion velocity 

**of up to a few mm/μs**

**Estimated velocity:**
**Slope ~1 mm/μs**

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## WARP simulations show that the 	nature of the electron source matters

- Electrons ejected from end wall

- Electrons from ionization of gas

- Electrons desorbed from beam pipe in quad upon ion impact

*Ecloud in a Quadrupole*

*beam pipe*

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

**MA4**

- 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

- GEC

- GEC

**Notes:**
In the third magnet we have: 
-8 paired FLush collector Long (FLL)

In the fourth magnet we have:

4 capacitive probes (BPM); @ 0, 90, 180, 270 DEGREES, NOT GRIDDED, RECESSED FROM WALL TO REDUCE ION SCRAPING.

2 FLush collector Short (FLS); @ 45, 135, 225. NOT GRIDDED, 

2 Gridded Electron Collector (GEC); GRIDDED, RECESSED FROM THE WALL, @ 315 DEGREES, CORRESPONDING LOCATION TO FLS POSITIONS  - MEASURES EXPELLED SECONDARY ELECTRONS AFTER PASSAGE OF BEAM PULSE/TAIL.  ***NOT WORKING YET***

2 Gridded Ion Collector (GIC), DOUBLE GRIDDED TO SUPRESS TO HIGH DEGREE THE CAPICITIVE SIGNAL.

## Sources of electrons and expelled or secondary ions can be measured

**Electron emission from wall**** **
**Axial current of electrons from end**

**Bias on clearing electrode-c (kV)**

**Electrode **
**current **
**(mA)**

**(a)**

**(b)**

**(c)**

**Beam head**

**Beamtail**

**Sources of electrons and expelled or secondary ****ions can be measured**

## Retarding potential analyzer (RPA) measures energy distribution of expelled ions

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

**Ref: Michel Kireeff Covo, to be published.**

## Anomalous clearing electrode currents:

**Anomalous clearing electrode currents:**
**	****	****not due to ionization of gas**

## •	Final spike of GEC current may be trapped electrons.

**•****	****Earlier portion has beam loss produced component**

**Gridded Electron Collectors (GEC) current correlates with beam loss, especially at head & tail**

## Integral of GEC final spike bounds accumulated electron charge, lack of electron suppression limits accuracy

**Integral of GEC final spike bounds accumulated electron charge, lack of electron suppression limits accuracy**

**Positive signals to -GEC indicate energetic ions hit collector, emitting e-**
**Lower limit on trapped e-**

**Negative signals to + GEC**
**(b) Coincident with (a) e- off opposite wall (E****e****≥40 eV @ head) **_**or off grid**_
**(c ) Not coinc: upper limit on trapped e-**

- e/ b = 0.17  Upper limit

- e/ b = 0.037  Lower limit

***Particle balance studies with e- source diag., and comparison with RPA and other diag. will reduce these uncertainties.***

## ECE observed in only 4 quads: sim. compared with exper.

*  ***beam ions**

*  ***secondary electrons**

*  ***electrons from ions hitting surface**

**Exp / Sim Comparison suggest semi-quantitative agreement.**

**e-supp**

## •	Continue to apply diagnostics within and between magnetic quads and compare with simulations

***•******	******Apply imaging techniques to measure first pass of gas desorbed from a NEG (Non-Evaporable Getter)***
***•******	******Simulate ECE multipactor by driving electrodes with rf***

***Possible future activities***

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## “Roadmap” toward self-consistent modeling of beam with e-cloud & gas effects.

**Key: ****operational; ****partially implemented **** (5/6/05)**

- WARP ion PIC, I/O, field solve
- 
- fbeam, , geom.

- electron dynamics
- (full orbit; interpolated drift)

- wall electron source

- volumetric (ionization) electron source

- _gas module_

- emission from walls

- ambient

- charge exch.

- ioniz.

- nb, vb

- fb,wall

- fb,wall

- 

- sinks

- ne

- ions

- Reflected
- ions

- fb,wall

- gas transport

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## WARP has many well-tested features ...

**Geometry: ****3D, (x,y), or (r,z)**
**Field solvers:**** FFT, capacity matrix, multigrid**
**Boundaries:**** “cut-cell” --- no restriction to “Legos”**
**Bends:**** “warped” coordinates; no “reference orbit”**
**Lattice description:**** general; takes MAD input**
**solenoids, dipoles, quads, sextupoles, ...**
**arbitrary fields, acceleration**
**Beam injection: ****Child-Langmuir, and other models**
**Diagnostics:**** Extensive snapshots and histories**
**Parallel:**** MPI**
**Python and Fortran: ****“steerable,” input decks are programs **
**a GUI is also available**

## ... and new features advancing the                                         state of the art ...

**	****- LHC ****3-D ****simulation: Cell count reduced ****20,000x**
** **
**New electron mover that allows large time steps**
**E-cloud and gas models**
**Prototype Vlasov solver (for halo)**

**... ****and *****new features***** advancing the****                                         state of the art ...**

*Beam coming off source ****x 11 speedup***

## Problem: Electron gyro timescale 
     << other timescales of interest
	 brute-force integration very slow due to small t
Solution*: Interpolation between full-particle dynamics (“Boris mover”) and drift kinetics (motion along B plus drifts)	

-      << other timescales of interest
- 	 brute-force integration _very_ slow due to small t
- Solution*: Interpolation between full-particle dynamics (“Boris mover”) and drift kinetics (motion along B plus drifts)

**We have invented a new “mover” that relaxes the problem of short electron timescales in magnetic field***

**Magnetic quadrupole**

**Sample electron motion in a quad**

*******R. Cohen et. al., Phys. Plasmas, May 2005**

- small t=0.25/c
- Standard Boris mover
- (**reference case**)

- large t=5./c
- New interpolated mover

- large t=5./c
- Standard Boris mover
- (**fails in this regime**)

- Test: Magnetized two-stream instability

## POSINST has been used extensively for e-cloud calculations

**Follows slice of electrons at one location along beam line  **
**	****	****2-D PIC  for e****–**** self force**
**	****	****analytical kick for force of beam on electrons**
**Effect of electrons on beam -- minimally modeled**
**	****	****dipole wake**
**Good models for electron production by:**
**	****	****synchrotron radiation**
**	****	****residual gas ionization**
**	****	****stray beam particles hitting vacuum wall**
**	****	****secondary electron production (detailed model)**

_***POSINST calculates the evolution of the electron cloud***_

**POSINST has been used extensively for e-cloud calculations**

## We have merged WARP and POSINST (with support of coordinated LBNL & LLNL LDRD’s)

***POSINST***

- Python

**field calculator**

**ion mover**

**image forces**

**electron source**
**modules**

**kicks from beam**

**diagnostics**

**lattice description**

***x******i******, v******i***

***framework***
***& ***
***user interface***

**electron mover**

**We have merged WARP and POSINST (with support of coordinated LBNL & LLNL LDRD’s)**

## Recent additions

**Gas module**
**emit neutrals (as macro-particles) from beam ion impact according to incident particle energy and angle of incidence**
**Ionization module**
**create macro-ions and macro-electrons resulting from impact ionization of gas molecules**

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## Beam hits end-plate in HCX to generate copious electrons leading to observable effects on the beam

- Capacitive
- Probe (qf4)

**Clearing electrodes**

**Suppressor**

- Time-dependent beam loading in WARP from moments history from HCX data:
-  current
-  energy
-  X-Y (assume semi-gaussian)
-  RMS envelopes, emittances
-  slopes
-  beam centroids
-  slopes average
- (4-D phase-space reconstruction in develop.)

***200mA K******+***

**e****-**

## Comparison sim/exp: clearing electrodes and e-supp. on/off

**Comparison sim/exp: clearing electrodes and e-supp. on/off**

*  ***beam ions**

*  ***secondary electrons**

*  ***electrons from ions hitting surface**

***200mA K******+***

## Comparison sim/exp: 3 clearing electrodes biased (no sec.)

**Comparison sim/exp: 3 clearing electrodes biased ****(no sec.)**

**(a)**

**(b)**

**(c)**

**(qf4)**

**(a)**

**(b)**

**(c)**

**(qf4)**

** **

## Comparison sim/exp: 3 clearing electrodes biased

**Comparison sim/exp: 3 clearing electrodes biased**

**(a)**

**(b)**

**(c)**

**(qf4)**

**(a)**

**(b)**

**(c)**

***200mA K******+***

**e****-**

**+9kV                 +9kV                     0V                            0V**

**(qf4)**

## Comparison sim/exp: first clearing electrode biased

**Comparison sim/exp: first clearing electrode biased**

**(a)**

**(b)**

**(c)**

**(qf4)**

**(a)**

**(b)**

**(c)**

***200mA K******+***

**e****-**

**+9kV                   0V                       0V                           0V**

**(qf4)**

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusion**

## Study of e-cloud in LHC FODO cell

**SPS sees surprisingly long-lasting e-cloud  (~ second)**
**The problem:****  **
**	****Simulate “multibunch, multiturn” passage of beam through FODO cell (100 m):**
**	****	****	****   ****	****dipoles**
**	****	****	****	****quadrupoles**
**	****	****	****	****drifts**
**	****Electrons  ********  synchrotron radiation, secondary emission**
**Study:**
**	****Electron accumulation and trapping in quads**
**	****Power deposition from electrons**
**   **

## We are beginning to use WARP to study e-cloud in LHC

**(particles colored according to radius)**

**beam (scaled 10x)**

**electrons**

**We are beginning to use WARP to study e-cloud in LHC**

**1 LHC FODO cell**

**F B  B   B D B   B   B**

**T=2********s**

## OUTLINE

**Who we are and why we care about e-cloud**
**Experimental facilities and issues**
**Gas desorption and electron emission **
**Electron sources, accumulation, and effects**
**The “roadmap” to self-consistency**
**Our simulation tools**
**Example of comparisons of WARP-POSINST with HCX**
**Application to High Energy Physics: 1 LHC FODO cell**
**Conclusions**

## Conclusions

**Developed new diagnostic capabilities to quantify gas, electrons, &  effects**
**Near completion of a new capability that follows “roadmap” toward self-consistent modeling of beam with e-cloud and gas**
**PIC; Lattice; AMR; new electron mover; secondary e****-****; photo-e****-****; neutrals generation and tracking; ionization.**
**Comparison with HCX experiment are very encouraging**
**demonstrated importance of secondary electrons,**
**inclusion of desorbed neutrals and ionization from halo, using newly developed modules, should help to close remaining gap.**
**Started to apply to High-Energy physics**
**first 3-D time-dependent simulation of LHC FODO cell,**
**study application to FNAL, RHIC, and maybe others?**