# Presentation: 587751

## Main Injector Electron Cloud Measurements and Simulations

- Bob Zwaska
- DOE Tevatron Operations Review
- March 27, 2007

## The Cloud at the Main Injector

- Simulations suggested that MI might be near a threshold
- 4-5 orders or magnitude increase of cloud density with a doubling of bunch intensity
- Leads to a program of studies:
- Further high intensity cloud formation simulations (for/through HINS)
- Measurements in the Main Injector
- Start of beam dynamics simulations
- Future program fir NOvA – ANU (nee SNuMI)

- M. Furman (LBL) *FERMILAB-PUB-05-258-AD** *

- Bill Ng (FNAL)

## Electron Cloud at MI

- Currently run with 53 MHz bunches of 6-10 x 1010 protons / bunch
- Question for upgrades: Can the bunch population be increased?
- Proton Plan & ANU: more bunches to 10 x 1010 / bunch
- SNuMI: up to 16 x 1010 / bunch
- HINS: up to 30 x 1010 / bunch
- Note: the Main Injector does not suffer from the e-p instability
- Transverse dampers have substantial operating margin
- Dominant mode is resistive wall
- Have seen evidence of electron cloud formation
- Vacuum activity in isolated areas
- Direct electron measurements in 1 area

## Measurements of Dynamic Pressure Rise

- See fast rise over the course of a cycle (1s)
- No rise without beam
- Cloud exacerbated by ceramic (high electron yield)

**Ion Pump Current**

- Ceramic beam pipes

## Dynamic Rises Around the Ring

**Locations of **
**vacuum rises**

- Dynamic vacuum effects seen at ~ 5% of MI ion pumps

## Electron Probe

- “Retarding Field Analyzer”
- Borrowed from Argonne
- Two electrodes connected externally
- Shielded multiple times from beam
- Being used as a simple electron counter
- Directly measure electron current on the beam pipe
- Still has some pick up from beam

- Collector

- Retarder

## Cycle Measurement

- DC signal seento spike at middle of cycle
- Required lots of filtering and noise reduction
- Rapid increase of signal occurs into acceleration
- Dip occurs at transition
- Maximum occurs shortly after transition
- Electron count decreases toward the end of the cycle

## Collected results

- Large number of cycles sampled at maximum current
- Clear turn-on at  higher intensities
- Noise is bad due to amplifier/MADC system
- 0.2 uA ~ 1% neutralization

## Closer look at Transition

- Better filtering/amplifying allow a closer look
- Introduces time delay
- Some cloud before transition
- Biggest effect after
- The dip definitely occurs
- Bunch length dependence looks complicated
- Naïve expectation was that shortest bunch length gave highest electron current

## LBL Simulations

- As part of HINS R&D LBL has provided us with MI simulations
- Furman, Vay, Sonnad
- POSINST code of electron cloud formation
- Used for simulations with APS, PSR, LHC, ILC, PEP, etc.
- 2 (+0.5) -D PIC particle tracker
- Extensive libraries for primary and secondary electron generation
- WARP code for beam interaction
- 3-D beam tracker (originally meant for IF)
- Application to MI still preliminary

## Effects of bunch length

- LBNL now thinks that very short bunches can suppress ecloud in simulation, two causes:
- Electrons have too high of an energy
- Too much time between bunches
- However, parameter space is different for MI, so we still don’t have a clear correspondence

## Vessel Geometry Affecting Cloud Formation

- Long bunches attract electrons into the center
- Short bunches may not be too quick
- Elliptical geometry constrains cloud extent
- Image charge increases force towards the wall

## Simulation Issues

- Secondary emission is a complicated process
- Measurements suggest ours is high
- However, simulation saturates well before that level
- Issue with SEY models
- Multidimensional phase space
- Electric field at surface

## Variation with SEY

- Simulated hreshold moves significantly with SEY max
- SEYs > 1.4 should already show electron cloud saturation  in MI
- Present intensites – not observed

## Secondary Emission Measurement

- Measured at SLAC’s facility with actual MI beam pipe
- Bob Kirby
- On average, 1.9-2 electrons produced per incident 400 eV electron on MI pipe
- Difference is conditioning
- SEY maximum is far beyond that used in simulation
- Kirby believes his numbers, Furman believes his measurement
- This comparison is not typically performed

## Study Plans

- Improve MI electron cloud installation
- Better shielded cables
- Better electronics/DAQ
- Possible new measurements in MI
- Bunch-by-bunch tunes
- Tests of material coatings/electrodes/solenoids
- Considering installations in Booster and Recycler Ring
- RR for ANU – will need new detectors
- Constrain simulations (and hopefully improve them)
- Continue comparison and simulation with POSINST
- Consider whether WARP is a good tool for MI
- Try to get going with Synergia framework

## Summary

- The electron cloud does not limit the MI presently, and is not expected to for ANU
- Still a possibility for any scheme that further pushes bunch intensities
- The cloud has been observed in MI
- Vacuum increases and direct electron detection
- Behavior of electron current is complicated
- Simulations suggest MI might be near a significant threshold
- But, for measured SEY values, MI should already be past threshold
- Simulations have to be improved
- Have a plan to expand measurements and simulation efforts
- Goals are to guide further upgrades and to improve our predictive abilities