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Electron Cloud Measurements

at the Fermilab Main Injector

Bob Zwaska

Fermilab

ECloud07 Workshop

April 9, 2007

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The Main Injector Program

(Double) Batch 1 (PBar)

Batch 2

Batch 3

Batch 4

Batch 5

Batch 6

Booster

Main Injector

• Provides high power, 120 GeV proton beam 80 kW for antiproton production 180 kW for neutrino production

• Takes 6 or 7 batches from the 8 GeV Booster @ 15 Hz 4-5 × 1012 protons per Booster batch

• Total cycle time ≥ 1.4 s + batches/15

NuMI

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Main Injector Operation• 53 MHz beam

• H=588

• 84-500 bunches

6-10 x 1010 protons/bunch

• Bunch length: 0.2-1.5 m

• Transverse size : 1-5 mm

• Ramps 8 – 120 GeV

0.8 s ramp period

• Passes through transition

• 3-D dampers needed for operation

Resistive wall instability

No evidence of e-p

• Linear growth rate scaling

• Operation limited by fractional loss

Losses < 10%

Maximum charge is secondary

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Upgrade Plans at Fermilab• Medium term Proton Intensity upgrades

Intended for neutrino program (at first)

• Proton Plan (in progress, done by ~ 2008)

Use slip stacking in Main Injector to increase

proton bunch intensity

• NOvA-ANU (planned for ~2011)

Increase cycling rate of MI by using Recycler

for stacking

• SNuMI (early planning)

Increase proton bunch intensity by using

accumulator for stacking

• HINS (Proton Driver, on hold)

Increase proton bunch intensity through new

8 GeV Linac

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Evolution of Proton Intensities

• Early plans were to go straight from Proton Plan to HINS Start to get big bunch intensities, and worry about electron cloud

• More recently, upgrade path has lengthened First leg, using the Recycler, does not significantly increase bunch intensity

Other legs still involve some increase

• Miguel Furman did initial simulations of the MI w/ Proton Driver Instigated study program at Fermilab

More calculations from LBL (other talks)

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First Simulation Input• Simulations suggested that MI might be near a threshold for electron cloud formation

4-5 orders or magnitude increase of cloud density with a doubling of bunch intensity

• Leads to a program of studies: Try to find evidence of a cloud with present MI

Expand simulations

Look at secondary emission in the MI

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

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Dynamic Pressure Rise

See fast rise over the course of a cycle (1s)

The control system induces delay

Occurs only at location of uncoated ceramic

Ion Pump Current

Ceramic beam pipes

Beam Intensity

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Dynamic Rises Around the Ring

Locations of vacuum rises

Rises observed at ~4% of pumps

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50 Hz Pump

• Higher bandwidth pump

• Saw more structure

Pressure increases with

injections

Increases and decreases during

cycle

0

20

40

60

80

100

120

140

160

180

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Time (s)

Pres

sure

Rise

(nto

rr)

Pressure Difference

Inferred Gas Load

20_02

0 84 168 252 336 420 504 588

1.0 × 1011 / bunch0.5 × 1011 / bunch

Type of pump installed at two locations

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Electron Probe• Retarding Field Analyzer

Borrowed from Argonne

Installed in drift region

• Have a lot of interferenceMagnet bus (grounds)

RF & beam signals (RF noise)

• Being used as an electron counterNot biasing retarder

Filtered output current

Collector

Retarder

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Cycle Measurement• DC signal seen to spike at middle of cycle

Around the time of transition

• Rapid increase of signal occurs into accelerationDip occurs at transitionMaximum occurs shortly after transitionElectron count decreases toward the end of the cycle

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0 0.2 0.4 0.6 0.8 1 1.2

Time (s)

Cu

rren

t (u

A)

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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

• Expect new measurements with 11-batch structure

More high intensity bunches

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Closer look at Transition• Cloud increases rapidly as size decreases

• Decreases temporarily at minimum bunch lengthWasn’t expected, but repeatable

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.50 0.60 0.70 0.80 0.90

Time (s)

Cu

rren

t (u

A)

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Another 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

• Perhaps due to electron energy or beam pipe geometry (Furman)

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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

POSINST needs more like 1.3-1.5 for

reasonable simulation results

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Future Measurements• Improve MI detector installation

Better shielded cables and groundsBetter electronics/DAQ

• Real-time bunch-by-bunch tune measurementAchieved by manipulating damper system

• Running sums of beam oscillations

• Well suited for FPGA

May see coherent shifts

• Plan to install new detectors in Booster & RR

• Enameled coatings for high resistivity electrodesFritz Caspers ideaLooking into getting MI pipe coated and installed

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Summary• Measurements of electron cloud formation in MI

Vacuum pressure rise & Direct electron detection• Suggest few % neutralization

• No Instabilities from electrons

• Dip of electron current at transitionPerhaps due to SEY/geometry effects

• Simulations suggest possibility of thresholdSomewhat consistent with observed turn-onHowever, disconnect on SEYMain Injector upgrades may push us past threshold

• However, none of the approved upgrades do so

• Planning to continue measurementsTest new, higher intensity beamsNew instrumentation

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Electron Cloud Measurements

at the Fermilab Main Injector

Bob Zwaska

Fermilab

ECloud07 Workshop

April 9, 2007