J. Hansknecht

Ion Back-Bombardment of GaAs Photocathodes Inside DC High Voltage Electron Guns

DC high voltage GaAs photoguns are key components at accelerator facilities worldwide. New experiments and new accelerator facilities demand improved performance from these guns, in particular higher current operation and longer photocathode operating lifetime. This conference submission explores bulk GaAs photocathode lifetime as a function of beam current, active photocathode area, laser spot size and the vacuum of the gun and beam line. Lifetime measurements were made at 100 μA, a beam current relevant for accelerators like CEBAF, and at beam currents of 1 mA and 5 mA, a regime that is interesting for high current Free Electron Laser (FEL) and Energy Recovery Linac (ERL) operation.

A Biased Anode to Suppress Ion Back‐Bombardment in a DC High Voltage Photoelectron Gun

Ion back‐bombardment is the dominant mechanism that limits the operating lifetime of DC high voltage GaAs photoelectron guns. In this work, an electrically isolated anode electrode was used to distinguish the QE damage contributions of ions produced within the cathode/anode gap and those produced downstream of the anode. This new anode design provides a means to suppress QE decay due to ionized gas in the beam line.

Measurements of Photocathode Operational Lifetime at Beam Currents up to 10 mA using an Improved DC High Voltage GaAs Photogun

This work extends past research at Jefferson Lab aimed at better appreciating the mechanisms that limit photocathode operational lifetime at high current (> 1 mA). Specifically, the performance of an improved 100 kV DC high voltage load locked photogun will be described. Although difficult to measure directly, we believe the new gun has better vacuum conditions compared to the original gun, as indicated by enhanced photocathode lifetimes exceeding 2000 C using a 1.55 mm diameter drive laser spot at the photocathode. In addition, the dependence of the lifetime on the laser spot size at the photocathode was measured and a charge density lifetime exceeding 106 C/cm2 was measured with a 0.32 mm laser spot diameter.

Polarized source performance and developments at Jefferson Lab

The polarized photoinjector at Jefferson Lab continues to provide high average current, high polarization, high quality beam to nuclear physics Users in as many as three endstations simultaneously. Long lifetime operation has been obtained from two identical polarized guns. A new high power modelocked ti-sapphire laser has been constructed to enhance the effective operating lifetime of the photoinjector. Efforts to enhance beam polarization and reduce helicity correlated beam systematic effects are underway.

Status of the Jefferson Lab Polarized Beam Physics Program and Preparations for Upcoming Parity Experiments

An ambitious nuclear physics research program continues at Jefferson Lab with Users at three experiment halls receiving reliable, highly polarized electrons at currents to 100 μA. The polarized photoguns and drive lasers that contribute to Jefferson Lab’s success will be described as well as significant events since PES2000. Typical of conditions at accelerators worldwide, success brings new challenges. Beam quality specifications continue to become more demanding as Users conduct more challenging experiments. In the months that follow this workshop, two parity violation experiments will begin at Jefferson Lab, G0 and HAPPEx2. The photogun requirements for these experiments will be discussed as well as our plans to eliminate/minimize systematic errors. Recent efforts to construct high power Ti‐Sapphire drive lasers for these experiments also will be discussed.

Evaluation of electropolished stainless steel electrodes for use in DC high voltage photoelectron guns

DC high voltage photoelectron guns are used to produce polarized electron beams for accelerator-based nuclear and high-energy physics research. Low-level field emission (∼nA) from the cathode electrode degrades the vacuum within the photogun and reduces the photoelectron yield of the delicate GaAs-based photocathode used to produce the electron beams. High-level field emission (>μA) can cause significant damage the photogun. To minimize field emission, stainless steel electrodes are typically diamond-paste polished, a labor-intensive process often yielding field emission performance with a high degree of variability, sample to sample. As an alternative approach and as comparative study, the performance of electrodes electropolished by conventional commercially available methods is presented. Our observations indicate the electropolished electrodes exhibited less field emission upon the initial application of high voltage, but showed less improvement with gas conditioning compared to the diamond-paste poli...

Sensitive Ion Pump Current Monitoring Using an In‐House Built Ion Pump Power Supply

Ion pumps are common vacuum pumps on DC high voltage photoguns and baked‐accelerator beamlines. Commercial ion pump power supplies provide a measure of the electrical current drawn by the pump, but typically have resolution to only ∼0.1 uA, which corresponds to pressure ∼10−9 Torr, a value considerably higher than the minimum pressure required by photoguns and nearby beamline. This submission describes a very sensitive in‐house‐built ion pump power supply with current monitoring capability good to less than 1 nA, and corresponding pressure in the low‐minus;10−11 Torr range. Besides providing “free” pressure monitoring on a scale equivalent to the best available commercial pressure gauges, the ion pump power supply also serves as a sensitive diagnostic for detecting field emission from the photogun cathode electrode and bad electron beam orbits that could diminish photogun operating lifetime. Since its inception, this ion pump power supply has become an invaluable tool for operating the CEBAF polarized ele...

High Intensity Polarized Electron Sources

During the 1990s, at numerous facilities world wide, extensive R&D devoted to constructing reliable GaAs photoguns helped ensure successful accelerator‐based nuclear and high‐energy physics programs using spin polarized electron beams. Today, polarized electron source technology is considered mature, with most GaAs photoguns meeting accelerator and experiment beam specifications in a relatively trouble‐free manner. Proposals for new collider facilities however, require electron beams with parameters beyond todays state‐of‐the‐art and serve to renew interest in conducting polarized electron source R&D. And at CEBAF/Jefferson Lab, there is an immediate pressing need to prepare for new experiments that require considerably more beam current than before. One experiment in particular—Q‐weak, a parity violation experiment that will look for physics beyond the Standard Model—requires 180 uA average current at polarization >80% for a duration of one year, with run‐averaged helicity correlated current asymmetry l...

Magnetized electron beam for the JLEIC re-circulator cooler ring

The ion beams of the proposed Jefferson Lab Electron Ion Collider (JLEIC) must be cooled to achieve the required collision luminosity. In general, cooling is accomplished when an electron beam co-propagates with an ion beam moving at the same average velocity, but with different temperature, where the energy of chaotic motion of the ion beam is transferred to the cold electron beam. The cooling rate can be improved by about two orders of magnitude if the process occurs inside a solenoidal magnetic field – so-called magnetized cooling - that forces the electrons to follow small helical trajectories thereby increasing the interaction time with ions and improving the cooling efficiency. However, one of the challenges associated with implementing this cooling technique relates to the fringe field of the cooling solenoid which imparts a large unwanted azimuthal kick onto the electron beam that prevents the electron beam from traveling in the desired tight, well-defined volume within the solenoid. As proposed by Derbenev, the ill-effect of this fringe field can be cancelled if the electron beam is born in a similar field and encountering a fringe field upon exiting the electron gun that produces an azimuthal kick in the opposite direction, such that the two kicks cancel. Besides requiring magnetized beam, the JLEIC re-circulator cooler design requires an electron beam with very high average current and high bunch charge: 140 mA and with nanoCoulomb bunch charge. This contribution describes the latest milestones of a multiyear program to build a magnetized electron beam source based on a 350 kV DC high voltage photogun with inverted insulator geometry.

Measurement of Electron Beam Polarization from Unstrained Bulk GaAs via Two Photon Photoemission

This paper describes measurements of the beam polarization and quantum efficiency for photoemission using two-photon excitation from unstrained bulk GaAs illuminated with pulsed, high intensity 1560nm laser light. Quantum efficiency is linearly proportional to 1560nm peak laser intensity, which was varied in three independent ways, indicating that the emitted electrons are promoted from the valence to the conduction band via two-photon absorption. Beam polarization was measured using a microMott polarimeter, with a value of 16.8(4)% polarization at 1560nm, which is roughly half the measured value of 33.4(8)% using 778 nm light.