05/20/ High-Current Polarized Source Developments Evgeni Tsentalovich MIT

05/20/ OUTLINE Introduction, motivation EIC requirements Developments at MIT-Bates RF gun developments Conclusion

05/20/ eRHIC (Linac-ring version) Polarized electron source with an extremely high current – crucial element of the project Luminosity ~  I(average) ~ 250 mA I(peak) ~ 100 A High polarization → strained GaAs → QE ~ 0.5% Average laser power ~ 80 W (fresh crystal) Hundreds Watts might be needed as crystal loses QE

05/20/ Main challenges High average current – cathode damage by ion bombardment High heat load on the cathode – tens of Watts of laser power Solution: Cathode with very large area High peak current – surface charge saturation (QE drops at high light intensity); space charge saturation

05/20/ High Average Current Average current of tens or even hundreds of mA is required Modern state-of-the-art guns produce ~  A Average current of ~ 1 mA achieved in tests at JLab and Mainz; lifetime ~ 20 h Average current of up to 10 mA achieved at Mainz with very short lifetime (needs active cathode cooling) Main problem – ion backbombardment

05/20/ Ion Damage Ion damage is inversely proportional to emitting area residual gas cathode Ionized residual gas strikes photocathode anode Ion damage distributed over larger area

05/20/ Damage location Electrons and ions follow different trajectories. Usually, ions tend to damage central area of the cathode. Laser spot Cathode Damage groove JLAB data Ring-like cathodes ?

05/20/ 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.

05/20/ JLAB results for anode biasing

05/20/ High Intensity Gun Studies at MIT/Bates The project investigates the feasibility of extracting very high (tens, perhaps hundreds of mA) 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.

05/20/ 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.

05/20/ Ring-shaped Laser Beam FiberL1L2AxiconCathode Axicon (conical lens) in combination with a converging lens (L2) produces ring-shaped beam in the focal plane of L2. Lens L1 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 L2

05/20/ Axicon-based System Simulations L1L2Axicon

05/20/ Axicon-based System Simulations

05/20/ Beam Profile (no axicon) FWHM<.5 mm

05/20/ Axicon Beam Profile

05/20/ Axicon Beam Profile

05/20/ Axicon Beam Profile

05/20/ QE map of the Fresh Crystal QE, %

05/20/ QE change (small spot in the center) Run C

05/20/ QE change (run with axicon) Run C

05/20/ QE change (axicon, anode biased 1 kV) Run C

05/20/ QE change (large spot in the center) Run C

05/20/ QE change (small spot in the corner) Run C

05/20/ Radial distribution

05/20/ Lifetime

05/20/ High Intensity Run (1 mA) Achieved.5 mA with laser power of.25 W (QE=.34 %) Achieved 1 mA 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 ~ W/degree

05/20/ 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.

05/20/ New optics Old optics: Small spot Axicon New optics

05/20/ Gun Simulation Large emitting area produces large emittance Although emittance is less important for eRHIC, 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

05/20/ Gun Simulations - Ions

05/20/ Gun Simulations - Ions

05/20/ Gun Simulations - Ions

05/20/ 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.

05/20/ Cathode Cooling Water in Water out HV Laser Manipulator Cathode Crystal

05/20/ DBR – Equipped Crystal “Normal” cathode Cathode with Distributed Bragg Reflector (DBR) In “normal” cathode, only 30% of light is reflected. In DBR- equipped cathode 99% of light is reflected.

05/20/ Polarized RF guns No positive results as yet, and very few attempts. Very high average current is not expected. Main advantages – high brightness, low emittance, high electron energy. High peak current could be achieved. Several normal-conducting RF gun projects are under way (SLAC, JLAB). High Order Mode (HOM) and Plane Wave Transformer (PWT) concepts are used. These concepts allow improved vacuum conditions. BNL develops Superconducting RF gun. Two versions are under way.

05/20/ SRF gun

05/20/ Conclusion Very significant results are expected in the next couple of years !