# Presentation: 064591

## LBNL Interests in the PS2

- Miguel A. Furman
- LBNL
- _[mafurman@lbl.gov](mailto:mafurman@lbl.gov)_
*LARP CM11*
*FNAL, 27-28 October 2008*

***bnl - fnal- lbnl - slac***

_**US LHC Accelerator Research Program**_

## Items

- Laser stripping of H–
- Laser systems
- Integration with beam optics
- Space-charge effects
- Halo formation, particle losses
- Space-charge compensation
- Conventional collective effects
- Impedances and instabilities
- Multibunch broadband feedback
- Electron-cloud effects
- EC build-up and mitigation
- Effects of EC on beam

## H– Stripping for the PS2

- Benefits from the ongoing SNS developments
- Encouraging results in R&D tests
- Based upon a three-step approach:
- Magnetic stripping of outer electron
- Laser excitation of inner electron
- Magnetic stripping of excited inner electron
- Key goal is ~100% stripping
- Needs powerful laser
- LBNL is well equipped for the task (strong experience w/ lasers)
- CERN requested a study (deliver a “White Paper”)
- Potential synergies with Project X

**Notes:**
Russell Wilcox built the laser distribution farm for NIF
Femtoslicing experience at ALS (R. Schoenlein, A. Zholents)
LOASIS lab (W. Leemans, one of leading laser acceleration labs in the world)
Time and synchronization for ALS and LCLS (R. Wilcox, J. Byrd, J. Staples)

Laser stripping for H- injection is not a new idea, but is nowhere in use at present
Being developed at SNS by Slava Danilov

## Optical cavity and H– stripping setup

- reflected
- power

- transmitted
- power

- transport
- optics

- waist, 40 m

- evacuated
- tube

- piezoelectric
- positioner,
- controller
- (cavity tune)

- from
- laser

- Four-mirror cavity
- Round-trip time relatively independent of focal spot size
- Cavity length is actively tuned to stay on optical resonance
- Pulse length is long enough to not require stabilization of laser offset frequency
- Transport system needs evacuated tube, active alignment for stable cavity injection

- H– beam

- active
- alignment

- ~20cm

**Beam travels**
**perpendicular to page**

## Stripping laser source concept and timing

- 352 MHz
- machine
- timing

- synch-
- ronizer

- modelocked
- Nd:YAG
- laser

- optical
- switch

- high voltage
- pulser

- Nd:YVO
- preamp

- double-
- passed
- Nd:YVO
- amp

- pulsed
- power
- supply

- isolator

- 2 Hz trigger

- 2 Hz trigger

- 1.2 ms
- pulse
- generator

- to transport
- optics and
- buildup
- cavity

- Peak output laser power is 10 kW in 50 ps
- Average power during 1.2 ms pulse is 200 W
- Assumes 100x buildup cavity
- All components are commercially available
- Lasers are diode-pumped for stability, reliability

- 50 ps

- 1.2 ms burst
- 352 MHz, 50 ps
- 200 W during

******=1.064 ********m**

**Notes:**
352 MHz is bunch frequency of the injection linac
2 Hz is cycling frequency of PS2 (check this). 
Garoby says 0.3 Hz (=3.3 sec). Maybe 2 Hz is just the ramping freq (=0.5 sec ramp), and the difference (=2.8 sec) is for injection and extraction flat bottom and flat top?)
1.2 ms is linac pulse

## Laser stripping status and plans

- Assessed early feasibility
- Optical parameters for the buildup cavity have been calculated to provide required focal spot size and roundtrip time
- SNS personnel (S. Danilov) has confidence of achieving 100% efficiency with this design
- We have identified vendors for all the laser components and cavity optics
- Plans:
- Complete feasibility study
- Specify major components
- commercial parts selection and pricing
- Integrate with design of injection optics and hardware
- laser system consistent with complete system conceptual design
- include specifications of magnets and transfer lines
- beam pipe and windows
- Include possible beam diagnostics

## Space-charge effects

- Space charge is a serious intensity limitation in the PS and PS Booster
- Effects of space-charge on dynamic aperture and beam quality (eg., haloes)
- Combine impedance with space charge on single-bunch effects
- Investigate possible space-charge compensation schemes
- eg., octupoles, electron lens or neutralization
- Support the effort of collimator designers
- build tools to enable work flow between space-charge simulations and collimator design
- Use existing IMPACT and MaryLie/IMPACT codes
- We will add time-dependent B-fields
- Determine required simulation parameters for acceptable numerical accuracy
- eg., order of transfer maps, number of space-charge kicks, etc.
- Perform long-term simulations with realistic lattice
- Include fully 3D space-charge effects
- This will be a collaborative effort with other LARP labs
- FNAL: benchmark with SYNERGIA code (P. Spentzouris)

## Example of long-term simulation:space-charge-driven dynamic emittance exchange

- Sample result
- MARYLIE/IMPACT code (3D parallel)
- 1 GeV protons, sp.-ch. tune shift ~0.1–0.3
- Ramp longitudinal tune from below to above resonance
- Propagate beam through a linac
- 1 linac period: 35o phase adv.
- constant focusing lattice approx.
- Goal: check scaling law (I. Hofmann and G. Franchetti, PAC07)
- this is the first 3D numerical check
- Part of a longstanding collaboration with GSI on space-charge simulations
- 106 macroparticles, 643 grid
- ~1.3x106 space-charge kicks (5 kicks/cell)
- 32 hrs on 64 processors (IBM/SP5)

- _emittances vs. time_

**Notes:**
The period here is actually refers to a lattice period in linac. This study wasoriginally targeted for linac 
application. 
2) The equivalent machine turn is about several thousand to 10 thousand turnsdepending on the tune of machine.
3) If a turn has 30 periods, there will be 150 sc kicks per turn.
4) These are protons. The equivalent tune shift is in the range of 0.1 to 0.3.
5) The transverse emittance is a sum of horizontal and vertical emittance.The phase advance for 1 period 
is 35 degree. If machine consists 30 periods, the tune will be 35*30/360 = 2.917
6) It is a 3d constant focusing lattice based on the smooth approximation to the AG focusing in real 
machine. The beta function is set by the matching to the given initial normalized emittance 
0.12 mm-mrad, 0.24 mm-mrad and 0.48 mm-mrad.
7) I. Hofmann and G. Franchetti, "Scaling laws for crossing nonlinear space-charge resonances,"PAC07 proceedings.

## Conventional collective effects

- PS2 will be high-current machine (I~2.5 A, Ipk~12-25 A)
- Need impedance reduction relative to PS
- Estimate single-bunch instabilities in PS2
- Based on best guesses for impedances and vacuum chamber geometry
- Develop impedance model and revisit single-bunch instabilities
- Contribution of components like bellows etc.
- Include RF system impedances
- based on FNAL booster cavity or a better model if available
- Analyze LLRF feedback
- Include 2nd-harmonic RF impedance model
- Revisit instability growth rates for single- and multi-bunch cases
- Design a multi-bunch broadband feedback system
- Estimate feedback parameters
- Develop concepts for kickers
- Work will be a SLAC-LBNL-FNAL collaboration

## Electron-cloud effects

- EC is a serious limitation in the SPS and possibly in the upgraded injectors
- PS2 EC build-up studies at LBNL started in 2006
- At the request of CERN personnel
- Results presented at LUMI2006, ECL2 and other venues
- Effect can be strong
- Sensitivity to the bunch spacing scenarios
- Sensitivity to the vacuum chamber radius
- may influence the RT vs. SC choice
- Simultaneously, we are assessing the EC for the FNAL MI upgrade
- At the request of FNAL personnel
- ~15 papers since early 2006
- Experimental calibration of simulations at MI increases confidence in simulations

## PS2 and MI upgrade

- In same ballpark vis-à-vis ecloud
- PS2 EC studies will benefit from simulations, measurements and code validation at MI

|  | PS2 | MI upgrade |
| --- | --- | --- |
| Circumference [m] | 1256 | 3319 |
| RF freq. [MHz] | 40 | 53 |
| Inj. kin. energy [GeV] | 4 | 8 |
| Extr. kin. energy [GeV] | 50 (75) | 120 |
| Cycling time [sec] | ~3 | ~2 |
| Bunch spacing [ns] | 25, 50, 75 | 19 |
| Bunch popul. [1011] | 2, 5.4, 6.6 | 3 |
| Chamber material | St. St. (?) | St. St. |
| Transition gamma | (30-40)i | 20 |

## Examples: simulated ecloud density in PS2(*)dipole bend, Eb=50 GeV, peak SEY=1.2, 1.3, 1.4

- Estimated ecloud densities ~(a few) x 1011 m–3
- ==> mandatory to examine effect on the beam
- “Gröbner (y)” is a simple estimate of the vertical beam-induced multipacting condition.
- (*) PS2 parameters used as input to the simulation taken from “psplusetcparameters.xls” (F. Zimmermann, 2006). Bunch intensities from R. Garoby, BEAM07.

- _ecloud density vs. bunch intensity_

## Ecloud proposed tasks

- Refine assessments of electron-cloud build-up
- Sensitivity to physical and numerical parameters
- Assess the need to combine space charge with electron-cloud effects
- Explore parameter space, especially RT vs. SC choice
- Compare and understand differences of EC build-up at the PS2 vs. FNAL MI upgrade
- Assess electron-cloud mitigation mechanisms
- low-SEY coatings, grooved chambers, clearing electrodes
- Assess impact of EC on the PS2 beam
- start with quasistatic approximation
- then extend it to 3D self-consistent simulations, including effects from space-charge, gas desorption and its ionization
- If above indicates an SPS-style instability, design and propose a broadband feedback system
- Work in collaboration with SLAC

## Summary: effort level and personnel(this is LBNL’s portion only)

- Laser stripping:
- 1/2 FTE per year in FY09 and FY10, plus $20k/yr in travel to CERN and SNS
- J. Byrd, R. Wilcox, + ???
- Space-charge studies:
- For FY09 and beyond, total 1 FTE/per year plus $30k/yr travel
- R. Ryne, J. Qiang, M. Venturini
- Conventional collective effects:
-  1/2 FTE (FY09), 1/2 FTE (FY10)
- J. Byrd, S. de Santis, M. Furman
- Electron-cloud**:**
- FY09: 0.7 FTEs plus $30k/yr travel
- FY10 and FY11: 0.6 FTEs/yr plus $30k/yr travel
- M. Furman, M. Venturini, J.-L. Vay, G. Penn

## Additional material

## Laser stripping plans

- Complete feasibility study
- Specify major components
- commercial parts selection and pricing
- Integrate with design of injection optics and hardware
- laser system consistent with complete system conceptual design
- include specifications of magnets and transfer lines
- beam pipe and windows
- Include possible beam diagnostics

## MI simulated ecloud build-up C=3319.4 m, h=588 , fRF=53 MHz

- Present operation: Nb=6x1010, 548 bunches/fill (Ntot=3.3x1013)
- But have reached Nb~1x1011 for some fill patterns
- Upgrade goal: Nb=3x1011 (Ntot=1.64x1014)

## Laser stripping: technical approach

- High intensity is achieved in optically resonant buildup cavity
- Interaction point is some distance away from narrow focus, to utilize beam spread angle
- as Doppler frequency spread
- 352 MHz, 1.2 ms burst, 2 Hz repetition rate
- High peak power (in 50 ps) mode-locked laser (at 352 MHz) is amplified in high average power amp
- Repetition rate and pulse timing is slaved to machine frequencies
- Laser beam is transported to optical buildup cavity via actively aligned transport optical system
- Cavity is actively tuned to maintain coherence with incoming pulses, obtain >100x buildup
- Laser is diode-pumped for high stability and reliability

## Ecloud proposed tasks(combined LBNL-SLAC effort)

- Refine assessments of electron-cloud build-up (4 person-months in FY09)
- Assess the need to combine space charge with electron-cloud effects in the simulation effort. Assess the feasibility of combining the corresponding simulation codes (2 PM in FY09).
- Explore parameter space, especially RT vs. SC choice (4 PM in FY09)
- Compare and understand differences of EC build-up at the PS2 vs. FNAL MI upgrade (3 PM in FY09)
- Assess electron-cloud mitigation mechanisms
- eg., low-SEY coatings, grooved chambers and clearing electrodes (4 PM).
- Assess impact of EC on the PS2 beam
- start with quasistatic approximation
- then extend it to 3D self-consistent simulations, including effects from space-charge, gas desorption and its ionization (12 PM in FY10).
- If above indicates a single-bunch instability, design and propose a broadband feedback system (4 PM in FY10 or FY11)