Low Emittance High Brightness Electron Source for Wakefield Accelerators

The modern electron sources for a Dielectric Wakefield Accelerator (DWFA) requires high peak current and significant average current, with a low emittance and a very short bunch length. The bunch length of the accelerated beam needs to be compatible with accelerating frequencies in the THz; thus the bunch should be significantly less than picosecond at the point of injection. A prompt emitter such as a metal lack sufficient quantum efficiency to achieve the average currents required in the final collider applications for these machines. Semiconductor cathodes can achieve the average current required, but typically produce longer temporal tails due to longer response time of the cathode. RMD proposes to address this by developing a thin, ultrasmooth bialkali antimonide cathode that measures less than 10 nm in thickness and that will have a response time of ~100 fs, allowing it to meet the temporal requirements for the main beam of a THz DWFA. This cathode will respond to green or blue light, dramatically simplifying the realization of ultrashort optical pulses to drive the cathode as compared to UV light required for metals and Cs2Te. The sub-nm surface roughness of this cathode will allow it to achieve intrinsic emittance governed solely by the excess photon energy (0.27 µm/mm for 532 nm light). While the cathode will be kept optically thin to minimize the electron escape time, we anticipate a QE in excess of 1% at 532 nm, and 10% at 400 nm. The goal of the proposed Phase I is to demonstrate feasibility of sputter deposition technique to grow ultrathin (<10 nm) and smooth K2CsSb photocathodes. The properties of the thin photocathode, such as spectral emission, surface roughness and crystal structure will be evaluated as a function of its thickness and deposition conditions. It is anticipated that such a ultra-thin, smooth bialkali antimonide cathode will have a response time of ~100 fs, allowing it to meet the temporal requirements for the main beam of a THz DWFA. The proposed program will transition photocathode-growth technology from the beamline research lab-space to a commercial tool, where end users such as DWFA can grow their own cathodes reliably. Other benefits are cost-effective photocathode deposition for MCP-based large area detectors for the key science drivers in particle physics, such as the search for dark matter and the studies of the nature of neutrinos. Availability of such detectors with improved timing and position performance will also have a transformational impact on other critical fields including medical imaging, through advances in positron emission tomography (PET) detectors, and nuclear detection for homeland security applications.