1 Electron Cloud Measurements at the Fermilab Main Injector Bob Zwaska Fermilab ECloud07 Workshop April 9, 2007

2 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 15 Hz  4-5 × protons per Booster batch Total cycle time ≥ 1.4 s + batches/15 NuMI

3 Main Injector Operation 53 MHz beam H= bunches  6-10 x protons/bunch Bunch length: 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

4 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

5 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)

6 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 AD

7 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

8 Dynamic Rises Around the Ring Locations of vacuum rises Rises observed at ~4% of pumps

9 50 Hz Pump Higher bandwidth pump Saw more structure  Pressure increases with injections  Increases and decreases during cycle × / bunch 0.5 × / bunch Type of pump installed at two locations

10 Electron Probe Retarding Field Analyzer  Borrowed from Argonne  Installed in drift region Have a lot of interference  Magnet bus (grounds)  RF & beam signals (RF noise) Being used as an electron counter  Not biasing retarder  Filtered output current Collector Retarder

11 Cycle Measurement DC signal seen to spike at middle of cycle  Around the time of transition 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

12 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

13 Closer look at Transition Cloud increases rapidly as size decreases Decreases temporarily at minimum bunch length  Wasn’t expected, but repeatable

14 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)

15 Secondary Emission Measurement Measured at SLAC’s facility with actual MI beam pipe (Bob Kirby) On average, 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 for reasonable simulation results

16 Future Measurements Improve MI detector installation  Better shielded cables and grounds  Better electronics/DAQ Real-time bunch-by-bunch tune measurement  Achieved 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 electrodes  Fritz Caspers idea  Looking into getting MI pipe coated and installed

17 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 transition  Perhaps due to SEY/geometry effects Simulations suggest possibility of threshold  Somewhat consistent with observed turn-on  However, disconnect on SEY  Main Injector upgrades may push us past threshold However, none of the approved upgrades do so Planning to continue measurements  Test new, higher intensity beams  New instrumentation

18 Electron Cloud Measurements at the Fermilab Main Injector Bob Zwaska Fermilab ECloud07 Workshop April 9, 2007