Performance of the Cornell High- Brightness, High-Power Electron Injector Bruce Dunham and Adam Bartnik December 17, 2012 B. Dunham Cornell University

Overview Emittance Measurements High Power Operations and Results Conclusion B. Dunham Cornell University Outline

ERL Injector Prototype: Achievements to date:  65 mA average 4 MeV  0.9  m 77 pC, 5 MeV B. Dunham Cornell University Cornell ERL Injector

ParameterMetricStatusNotes Average Current100 mA* 50 mA at 5 MeV (1300 MHz) Bunch Charge77 pC Pulsed mode (50 MHz) Energy5 to 15 MeV 14 MeV max (due to cryo limits) Laser Power> 20 W > 60 W at 520 nm (1300 MHz) Laser Shapingbeer can dist. Adequate for now Gun Voltage kV Currently operating at 350 kV Emittance < 2  m (norm, rms) Ultimate ERL goal 0.3  m, with merger Operational Lifetime > 2000 C/cm 2 Recent improvements with new cathodes B. Dunham Cornell University Injector Requirements

Goals for 80 pC bunch charge Emittance < 2.0 mm-mrad (all emittance numbers are 100%, rms, normalized ) Bunch length < 2 ps Progress Individually, x and y emittance = 0.9 mm-mrad But x and y are not simultaneously small Machine settings for good x produce poor y, etc. What in our injector could cause asymmetric in x and y? Progress Towards Low Emittance

Injector Schematic Misalignment in beginning magnets can cause asymmetry  DC gun  Solenoids  Buncher Direction of beam

Alignment effort Align DC gun by scanning laser position, measuring pin-cushion downstream. Align buncher and SRF cavities using correctors, making sure beam does not move when turned on/off. Align solenoids by scanning current, using motors to move/tilt magnets. Slice emittance is a sensitive tool to verify good machine alignment, as all misalignments lead to an emittance change.

B. Dunham Cornell University Slice emittance at 0 pC After alignment, agrees well with simulation But x/y asymmetry remains

Beam Coupler antennas only in x-direction! SRF Cavity Couplers How big is the effect? Need an accurate simulation… The SRF cavities have a slight x/y asymmetry in their coupling

Step 1: Realistic Field Maps Existing software allows magnetic or electric boundary conditions Create traveling wave as superposition Normalize result to match real forward and reflected power

Step 2: Beam Dynamics Simulations use GPT Uses measured field maps for all magnets Now with (realistic) asymmetric cavity fields Can phase cavities using axially symmetric fields (fast), then use realistic fields (slow) Constructed a simple Matlab interface to load machine settings and simulate Load current Machine state Plot expected performance

Strong asymmetry after first cavity couplers Beam size asymmetry is accurately reproduced in the lab At minimum y emittance, the x emittance is 2x larger Results Emittance (mm-mrad) Distance (meters) Can we circumvent this? (without building a new cavity…) Cavity couplers

Emittance in merger section Thought: Can one asymmetry cancel another? Merger section: 6 quadrupole magnets 3 dipole magnets Highly x/y asymmetric! Can we produce good emittance in both planes in the merger?

Optimized Simulation Optimize emittance using a using a genetic algorithm Emittance asymmetry can be (mostly) removed Currently setting up to measure this effect… Simultaneous good emittance!

Emittance Summary We have reached our target low emittance individually in x/y planes SRF cavity couplers create aymmetry in beam size / emittance We developed an accurate model of the effect by incorporating a more accurate field map into GPT Using a genetic algorithm to optimize the beamline, we find that the asymmetry in emittance can be removed in the merger section

B. Dunham Cornell University High Current Operation What is important for running high currents? (more details tomorrow) Halo is a major problem (tuning, radiation shielding and machine protection) Beam dump monitoring and protection Fast shutdown – want to block the laser before anything else trips... Catching transients (due to FE, ions, scattering,...) for troubleshooting RF trips (mostly due to coupler arcs) Feedback for bunch charge, laser position and beam orbit Current measurement Measurements of RF response to the beam, HOM’s Monitoring HV power supply ripple and frequency response Vacuum monitoring, fast and slow Personnel protection Overall machine stability

B. Dunham Cornell University 1300 MHz laser After recent improvements, we increased the average power from < 15 W at 520 nm to over 60 Watts! Added a second pre-amp, compressed the pulse after the amplifier to reduce non-linearities instead of before it, and starting using commercial, high-power fiber terminators. Commercial fiber terminator Now, we have more headroom for dealing with cathode lifetime, and shaping and transport losses

Laser Stability Beam current fluctuations made the RF unstable during high current operations, due to laser intensity and position changes. A fast-feedback system was installed, using a BPM as the sensor. This dramatically reduced the RF trip level. Thanks to F. Loehl The same electro-optic modulator is used as a fast laser shutter, with 1  s response time B. Dunham Cornell University

~600 Coulombs delivered (same spot) QE before QE after L. Cultrera, et al., PRST-AB 14 (2011) Laser location Cathode QE maps showed little change over 8 hours B. Dunham Cornell University Cathode Stability High current operation using a CsK 2 Sb photocathode

Designed for 600 kW average power We burned a hole in the aluminum dump at 25 mA. This was due to the incorrect setup of the raster/defocusing system, and one shorted magnet. B. Dunham Cornell University Beam Dump Damage

The defocusing/rastering system is calibrated using the large viewscreen (shown above) before installing the dump. Large BeO viewscreen at the dump location Beam image beyond the raster during calibration A bpm located after the raster for continuous monitoring A large quadrant detector before the dump ensures that the beam cannot get too big Beam Dump Monitoring B. Dunham Cornell University

We added an array of 80 thermocouples around the dump, and read them out at 0.5 Hz using an 80 channel Keithley meter. Response time is surprisingly fast, making this an important tool for monitoring the dump temperature and temperature uniformity. In this view, the water flows from the center (end of dump) to the outside (front of dump) B. Dunham Cornell University Beam Dump

Key developments Expertise in several different photocathodes (both NEA and antimonides) Improvements to the laser (higher power) Feedback system on the laser Minimization of RF trips (mainly couplers) Minimizing radiation losses due to halo Improved beam dump diagnostics Previous record was 32 mA (Boeing) first 50mA!! GaAs cathode B. Dunham Cornell University New Average Current 5 MeV

B. Dunham Cornell University GaAs cathode before use. GaAs cathode after use. The small circle is the only activated area on the cathode Ion damage is mostly at the center Cathode Damage

B. Dunham Cornell University CsK 2 Sb cathode with offset active area. Lifetime at 60 mA was ~30 hours (1/e). Max current reached with this cathode -> 65 mA. CsK 2 Sb cathode, 60 mA

B. Dunham Cornell University Front surface of the cathode (CsK 2 Sb on Si) after use. Back surface of the cathode puck after use. Cathode Damage Active area is offset from the center Ion damage limited to the central area

B. Dunham Cornell University Cs 3 Sb cathode with offset active area. QE increases during this period! Cs 3 Sb cathode, 33 mA

B. Dunham Cornell University Overall Summary We have reached our target low emittance individually in x/y planes SRF cavity couplers create asymmetry in beam size / emittance Using a genetic algorithm to optimize the beamline, we find that the asymmetry in emittance can be removed in the merger section Broke the long-standing average current record for a photoemission injector! 65 mA Excellent lifetime at 33 mA with an alkali cathode, less good at 60 mA

This work is supported by the National Science Foundation grant DMR Support B. Dunham Cornell University

RF reflected power for each cavity RF forward power