# Presentation: 587756

## Electron Cloud InstabilitiesRobert ZwaskaFermilabDec. 5, 2006

## Outline

- Electron Cloud Intro
- Formation Process
- Interaction with beam
- Observations at Fermilab

## Electron Cloud Basics

- Positively-charged beam
- Produces an electric field
- Supports a persistent plasma of nonrelativistic electrons within the vacuum of the beampipe
- Other notes:
- Beam and electrons interact
- Electrons must be produced somehow
- Seconday/photo - emission
- Primary Production

## Why Fermilab Needs to Understand the Cloud

- Fermilab has high-intensity, positive beams
- However: the cloud does not limit operation of our accelerators ... yet
- When considering upgrades (intensity increases), we might produce an intense cloud and have to deal with it
- Proton Plan/Driver - SNuMI
- Fermilab is part of other projects that could likely be limited by the cloud
- LHC will produce a cloud
- ILC positron damping ring will produce a cloud
- Almost any higher intensity, positive beam will have to be designed with the electron cloud in mind
- Performance limitations may be by crippling, or mitigation expensive after the fact

## Electron Cloud Research

- Study of the cloud split among 3 major topics:
- How does the cloud form?
- How does the cloud interact with the beam?
- How can the cloud be prevented or mitigated?
- Whole variety of physical processes and parameters involved
- First question: what has been seen so far?

## e-cloud beam instabilities at various machines

**INP Novosibirsk, 1965**

**Argonne ZGS,1965 **

**BNL AGS, 1965**

**Bevatron, 1971**

**ISR, ~1972 **

**PSR, 1988**

**AGS Booster, 1998/99**

**KEKB, 2000**

**CERN SPS, 2000**

- [F.Z. PRST-AB **7**, 124801 (2004)]

## “critical mass phenomenon”?

| hadron machine | ZNb/(xyz) [108/mm3] |
| --- | --- |
| ISR | 0.14 |
| CPS | 0.28 |
| SPS (LHC) | 0.21 |
| SPS (FT) | 0.13 |
| PSR | 0.15 |
| RHIC | 0.10 |
| ISIS | 0.006    no e- cloud |
| SNS | 0.30 |
| J-PARC (3 GeV) | 0.04 |
| FNAL 8-GeV PD | 0.03 |
| J-PARC (50 GeV) | 0.17 |
| FAIR SIS-18/100 | 0.23/0.31 |
| LHC | 159 |

**Weiren Chou**,
- Oliver Bruning,
- Massimo
- Giovannozzi,
- Elias Metral,
- ECLOUD’02

**e- cloud effects**

**planned,**
**or under**
**construc-**
**tion or **
**commis-**
**sioning**

**safe?**

**safe?**

## Different Models of Cloud Formation

- Resonant Production
- Similar to multipactoring in RF cavities
- Multiple bunches accelerate electrons at a specific resonance, producing more through secondary emission
- Assumes an unrealistic number of symmetries
- Photoproduction
- Huge number of synchrotron photons produce electrons through photo-emission
- Doesn’t require much secondary emission or a cascade
- Not relevant to proton machines and can be dealt with vacuum antechambers
- Quasi-adiabatic heating
- Long bunches slowly attract and heat electrons into the center of the beam vessel
- Expelled at the end of the bunch and produce electrons through secondary emission
- Fast heating – (most relevant to Fermilab)
- Short bunches shock electrons into the center – heating them
- Collisions with beam pipe produce a sea of secondary electrons that mill around the walls
- Subsequent bunches heat the electrons multiple times, producing a cascade
- Repulsion within a strong cloud can further contribute to heating

## electron cloud in the LHC

**electron cloud in the LHC**

- schematic of e- cloud build up in the arc beam pipe,
- due to **photoemission**** **and **secondary emission**

- [Courtesy F. Ruggiero]

- empirical e-cloud threshold scaling:

*N**b**~L**sep*

*N**b**~L**sep*

- [F.Z., EPAC’02]

## Model at Fermilab

- Considering the MI beam
- 1-8 ns long bunches every 19 ns
- 1-5 mm transverse sigma
- Bunch intensities of 1011 protons
- Produce a few initial/primary electrons
- Residual gas ionization
*O*( e- / m / torr / proton)
- Lost protons
- Can produce 100’s in beam pipe
- Beam produces strong potential
- Nonadiabatic appearance
- Electrons Accelerate
- Beam disappears
- Electrons collide with wall

- r

- V

- few kV

## Secondary Emission

- More electrons produced upon collision with wall
- Conversion of energy to multiplicity
- On average, 2 electrons produced per incident 400 eV electron on MI pipe
- Secondary electron yield (SEY) depends on incident electron’s energy
- Different materials and geometries can have different SEYs
- Produced electrons have much lower energies, typically 1-10 eV

## Secondary Electrons Reheated

- Secondaries are reheated in the same way as the primaries
- Bunches must reappear before secondaries are reabsorbed
- Potential for exponential growth
- Collective electron charge can increase heating effect
- Eventually, electrons will screen the proton’s charge leading to a saturation density
- Peak electron linear density comparable to peak proton density

- r

- V

- few kV

## Processes Involved

- Proton Beam

- Primary Electrons

- Secondary Electrons
- (cloud)

- Many Processes

- Beam Pipe

- Heating

- Secondary Emission

- Ions (plasma)

- Gas

- Secondary Emission

- Desorption

- Scattering, tune shift, plasma instabilities ...

## Possible Effects of the Cloud

- Vacuum bursts caused by gas desorption
- Can activate machine protection
- Hurt lifetime of storage ring
- New impedance – electrons act as a wake field
- Tune Shifts
- Normal space charge tune shift can be considered to be the sum of electric and magnetic parts in the lab frame
- Magnetic partially cancels the electric
- Electron cloud can neutralize electric, but leave the magnetic portion
- Tune shift can be potentially large
- Like a beam-beam effect around the entire ring
- Potential is also very nonlinear -> emittance growth
- Also time-varying in bunch and in bunch train

## Electron cloud evolution

- Position vs. time of e- at different initial amplitude (0.5,...,3b). e depends on initial amplitude.

- z/z

- x

- 	e- motion during the passage of a Gaussian bunch:
- x < ~ x	harmonic oscillations (~ 4)
- 
- x >> x	non-linear oscillations
- 	(x >12x, e perform less then 1⁄4 oscillat.)

- y/b

- Log[e(x,z)]

- EC density at (x,y=0,0)

**Electron cloud evolution**

- z/z

*HEAD*

*TAIL*

- t=0

- t=tend

- → EC density function of longitudinal position

- b

- 2b

- 3b

- Pinch Model from CERN

## simulation

- horizontal linear invariant of a
- proton vs. turn number

- Elena Benedetto &
- Giuliano Franchetti

**simulation**
**HEADTAIL**
**code**

- two mechanisms of incoherent e- effect & shrinkage:
- periodic resonance crossing → halo growth
- periodic linear-instability crossing → core growth

**T****s**

- ingredients: (1) synchrotron motion, (2) e- induced tune shift along the bunch (E. Benedetto, G. Franchetti, F. Zimmermann, submitted to PRL)

## 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 brought to 30 x 1010?
- At a review this question was asked for the electron cloud
- Weiren Chou convinced Miguel Furman (LBNL) to simulate electron cloud build-up with POSINST
- Results prompt further investigation at Fermilab
- Note: the Main Injector does not suffer from the e-p instability
- However, we can see some evidence of cloud formation
- Intend to study with observations and LBNL simulations

## Considering the Cloud

- Simulations suggest that MI might be near a threshold
- 4-5 orders or magnitude increase of cloud density with a doubling of bunch intensity
- Not yet established:
- How well code pertains to Main Injector (question of SEY)
- What the effects of electron neutralization will be on the beam

## Measurements of Dynamic Pressure Rise

- See fast rise over the course of a cycle (1s)
- The control system barely keeps up

**Ion Pump Current**

- Ceramic beam pipes

## Dynamic Rises Around the Ring

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

## Electron Probe

- “Retarding Field Analyzer”
- Borrowed from Argonne
- Two electrodes connected externally
- Retarder can be biased to allow energy measurements
- Currently being used as a simple electron counter
- Directly measure electron current on the beam pipe

- Collector

- Retarder

## Collected results

- Clear turn-on at  higher intensities
- Noise is worse due to amplifier/MADC system
- 0.1 uA ~ 1% neutralization at 20e12

## SS+NuMI Histogrammed

## Time Measurements

- Unbiased signal
- Required lots of noise reduction
- Could not get a good zero for subtraction
- Dip at 1.1 s
- Rapid increase of signal occurs into acceleration
- Dip at transition (next slide)

## Transition Effect

- Definite decrease in cloud signal at transition
- Not expected
- Simulations have suggested that cloud current only increases with smaller bunch length
- Looking into with simulation

## More Transition

- Better filtering/amplifying allow a closer look
- Introduces time delay
- Some cloud before transition
- Biggest effect after
- Bunch length dependence looks complicated

## New Simulations

- 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

## Simulation Issues

- Secondary emission is a complicated process
- Measurements suggest ours maxes at 1.9-2.0
- However, simulation saturates well before that level
- Issue with SEY models
- Multidimensional phase space
- Electric field at surface

## Summary

- The electron cloud is a potentially limiting collective effect in positive particle accelerators
- Fermilab accelerators are not limited by the cloud, but
- We do observe some cloud activity
- Simulations suggest that we may be near a threshold
- Upgrades may double/triple bunch intensities
- Electron cloud under study with observations and simulation
- Progress has been ongoing, still looking for a clear picture
- Cloud has been observed in isolated locations
- Decrease of cloud intensity has been observed at very short bunch lengths
- May or may not be consistent with simulation
- Consideration of the cloud will be important for any path the Fermilab starts on