Direct Electron Cloud Measurements in the Main Injector

1
Direct Electron Cloud Measurements in the Main
Injector

• Reminder of Situation
• Direct Measurements

• Bob Zwaska
• August 23, 2006
• AP Department Meeting

2
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

Bill Ng (FNAL)
M. Furman (LBL) FERMILAB-PUB-05-258-AD
3
Electron Cloud Effect (my view)
Proton Beam
Many Processes
Heating
Primary Electrons
Scattering, tune shift, plasma instabilities
Secondary Electrons (cloud)
Secondary Emission
Ions (plasma)
Secondary Emission
Beam Pipe
Desorption
Gas
4
First Measurements of Dynamic Pressure Rise
Ion Pump Current

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

Ceramic beam pipes
5
Nonlinear Response
Different beam structure
first pulses
Turn on
6
50 Hz Pump
Type of pump installed at two locations

• Higher resolution reveals cycle details
• Injections
• RF capture
• Ramp
• Transition
• Flattop
• Extraction

1.0 1011 / bunch
0.5 1011 / bunch
7
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

Collector
Retarder
8
Electron Detector

• See some activity on all production cycles
• Have some noise issues
• Device - ICLOUD
• V correspond to uA current (amplifier MADC)
• Very noisy
• Sampled to Datalogger at time of maximum

9
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)

10
Transition Effect

• Definite decrease in cloud signal at transition
• Not expected
• Simulations have suggested that cloud current
  only increases with smaller bunch length
• Will follow this in the future

11
Collected results

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

12
SSNuMI Histogrammed
13
Conclusions

• Electron Cloud Detector is installed and
  operating in the Main Injector
• New measurements are consistent with pressure
  measurements, but differ in details
• Expected due to different conditions at devices
• Directly measured turn-on of electron cloud
  current with intensity
• Unexpected dip of current at transition
• With reduction of noise and more careful
  electronics, a wide range of measurements should
  be possible
• Will also consider new detectors for further
  investigation of the MI electron cloud