High Intensity Polarized Electron Gun Studies at MITBates

High Intensity Polarized Electron Gun Studies at MIT-Bates 10/01/2008 PESP 2008 Evgeni Tsentalovich MIT 1

OUTLINE • • Introduction, motivation Major challenges Developments at MIT-Bates Conclusion 10/01/2008 PESP 2008 2

e. RHIC (Linac-ring version) Requires a polarized electron source with an extremely high current Currently achieved Luminosity ~ I(average) ~ 250 m. A I(peak) ~ 100 A 200 µA 10 A High polarization → strained Ga. As → QE ~ 0. 5% Average laser power ~ 80 W (fresh crystal) 1 W Hundreds Watts might be needed as crystal loses QE 10/01/2008 PESP 2008 3

Main challenges High average current – cathode damage by ion bombardment High peak current – surface charge saturation (QE drops at high light intensity); space charge saturation Solution: Cathode with very large area High heat load on the cathode – tens of Watts of laser power 10/01/2008 PESP 2008 4

Ion Damage Ion damage is inversely proportional to emitting area anode residual gas Ionized residual gas strikes photocathode 10/01/2008 PESP 2008 cathode Ion damage distributed over larger area 5

Damage location Electrons and ions follow different trajectories. Usually, ions tend to damage central area of the cathode. JLAB data Ring-like cathodes ? 10/01/2008 PESP 2008 6

Ion Trapping in CW Beam Cathode Anode Beam line Ions produced below the anode are trapped in the electron beam. Half of them will drift toward the gun and get accelerated in the cathode-anode gap toward the crystal. 10/01/2008 PESP 2008 7

JLAB results for anode biasing Anode = 0 V Support = 0 V Charge = 172 C Anode = 2 k. V Support = 300 V Charge 175 C 10/01/2008 PESP 2008 DQE 8

High Intensity Gun Studies at MIT/Bates • The project investigates the feasibility of extracting very high (tens, perhaps hundreds of m. A) current from the gun. • The project addresses issues of high average current and high heat load on the cathode. • Phase I – studies of ion damage, design and construction of the cathode cooler, gun simulations. • Phase II – design and construction of the gun and the beam line, beam tests. 10/01/2008 PESP 2008 9

Ion Damage Studies - Apparatus • Existing gun. • New diode array laser ( ~808 nm, P up to 45 W). • Existing test beam line. This beam line was not designed for high current and beam losses of 5 -10% are typical. These losses produce out-gassing, and reduce the lifetime by both poisoning the cathode and ion bombardment. Relatively low lifetime and significant ion damage allowed to conduct the measurements fast. • CW current – one can expect ion trapping. 10/01/2008 PESP 2008 10

Ring-shaped Laser Beam Fiber L 1 Axicon L 2 Cathode Axicon (conical lens) in combination with a converging lens (L 2) produces ring-shaped beam in the focal plane of L 2. Lens L 1 reduces the laser beam divergence (25 from the fiber). Without axicon, a very small beam spot will be produced. QE could be mapped by moving the L 2 10/01/2008 PESP 2008 11

Axicon-based System Simulations L 1 10/01/2008 PESP 2008 Axicon L 2 12

Axicon-based System Simulations 10/01/2008 PESP 2008 13

Beam Profile (no axicon) Measured using razor blade technique and inverse Abel transformation FWHM<. 5 mm 10/01/2008 PESP 2008 14

Axicon Beam Profile 10/01/2008 PESP 2008 15

Axicon Beam Profile 10/01/2008 PESP 2008 16

Axicon Beam Profile 10/01/2008 PESP 2008 17

QE map of the Fresh Crystal QE, % 10/01/2008 PESP 2008 18

QE change (small spot in the center) Run 12. 32 C 10/01/2008 PESP 2008 19

QE change (run with axicon) Run 17. 35 C 10/01/2008 PESP 2008 20

QE change (axicon, anode biased 1 k. V) Run 17. 62 C 10/01/2008 PESP 2008 21

QE change (large spot in the center) Run 17. 46 C 10/01/2008 PESP 2008 22

QE change (small spot in the corner) Run 16. 84 C 10/01/2008 PESP 2008 23

Radial distribution 10/01/2008 PESP 2008 24

Lifetime 10/01/2008 PESP 2008 25

High Intensity Run (1 m. A) • • • Achieved. 5 m. A with laser power of. 25 W (QE=. 34 %) Achieved 1 m. A with laser power of 1. 16 W (QE=. 15%) Gun vacuum pressure rise (factor of 10) Current dropped to 132 A in 1 hour At laser power of 1. 16 W, QE degrades even without HV ! – Overheating. • Thermal estimate (thermal conductivity through the stalk only ~. 01 -. 025 W/degree 10/01/2008 PESP 2008 26

Conclusion • Ion damage is concentrated near the center of the cathode in every configuration. • Ring-shaped beam allows to improve the lifetime significantly. • Biasing the anode improves the lifetime of the CW beam. • Active cooling is a “must” for laser powers exceeding 1 W. 10/01/2008 PESP 2008 27

New optics Old optics: Small spot Axicon New optics 10/01/2008 PESP 2008 28

Gun Simulation • Large emitting area produces large emittance • Although emittance is less important for e. RHIC, large beam could result in beam losses near the gun. • The main purpose of the simulations is to minimize the beam losses in the gun and beam line. • The second goal – ion distribution optimization 10/01/2008 PESP 2008 29

Gun Simulation 10/01/2008 PESP 2008 30

Gun Simulations - Ions 10/01/2008 PESP 2008 31

Gun Simulations - Ions 10/01/2008 PESP 2008 32

Gun Simulations - Ions 10/01/2008 PESP 2008 33

Cathode Cooling • The conceptual design of the test chamber is completed. • The test chamber will validate the adequacy of the cooling power, HV and high vacuum compatibility and vacuum cathode handling with manipulators. 10/01/2008 PESP 2008 34

Cathode Cooling HV Water in Water out Manipulator Crystal Cathode 10/01/2008 PESP 2008 Laser 35

DBR – Equipped Crystal For instance, talk by L. Gerchikov, St. Petersburg, at PESP 2007 “Normal” cathode Cathode with Distributed Bragg Reflector (DBR) In “normal” cathode, only 30% of light is reflected. In DBRequipped cathode 99% of light is reflected. 10/01/2008 PESP 2008 36