We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current–voltage (I–V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications.

Phase and power control methods that satisfy the requirements of superconducting accelerators to magnetron RF sources were considered by a simplified kinetic model of a magnetron driven by a resonant injected signal. The model predicting and explaining stable, low noise operation of the tube below the threshold of self-excitation (the Hatrree voltage in free run mode) at a highest efficiency, a wide range of power control and a wide-band phase control was well verified in experiments demonstrating capabilities of the magnetron transmitters for powering of state of the art superconducting accelerators. Descriptions of the kinetic model, the experimental verification and a conceptual scheme of the highly-efficient magnetron RF transmitter for the accelerators are presented and discussed.

A simplified analytical model based on the charge drift approximation has been developed. It considers the resonant interaction of the synchronous wave with the flow of Larmor electrons in a magnetron. The model predicts stable coherent generation of the tube above and below the threshold of self-excitation. This occurs if the magnetron is driven by a sufficient resonant injected signal (up to -10 dB). The model substantiates precise stability, high efficiency and low noise at the range of the magnetron power control over 10 dB by variation of the magnetron current. The model and the verifying experiments with 2.45 GHz, 1 kW magnetrons are discussed.

Electron beams are required for many applications, from diagnosis and treatment in medicine to energy frontier discovery machines. Novel electron gun features are required to enhance the peak and/or average beam brightness by reducing the emittances of electron bunches while allowing the guns to operate at a high repetition rate using laser excitation of the photocathode. The generation of surface acoustical waves (SAWs) on piezoelectric substrates produces strong piezoelectric fields that propagate on the surface of the material. These fields can significantly suppress electron-hole recombination effects that result in enhanced quantum efficiency of photoemission. This STTR project with MuPlus, Inc. and George Washington University is developing the novel use of SAWs on photocathodes to control their emittances and improve their quantum efficiency, so that lower-power lasers can generate more intense electron beams having smaller emittances. State-of-the-art computer simulations were developed for a device that uses Interdigital Transducers (IDT) placed on GaAs photocathode wafers to generate SAW with the required wave properties to demonstrate control of the electron-hole recombination rate for polarized electron beams. The simulations indicate that the photocathode quantum efficiency can be improved by a factor of 14 by the use of SAWs relative to usual GaAs performance. Experimentally, several SAW devices were built, tested, and improved using the NIST Center for Nanoscale Science & Technology and the George Washington University Nanofabrication and Imaging Center. Plans for Phase II of this project include the continuation at GWU of the development of SAW photocathodes for polarized electron beams using GaAs and other materials for unpolarized applications. We will design and build drop-in replacement SAW photocathodes with RF feeds for the IDTs to be used with devices to measure the QE, polarization, and beam emittances at the Thomas Jefferson National Accelerator Facility Center for Injectors and Sources.

In many applications dipole magnets with coils having significant curvature are needed. This is particularly challenging for high temperature superconductors (HTSs) as they are brittle. One possible application for curved HTS coils was the fragment separator dipole magnets for the Facility for Rare Isotope Beams (FRIB). For this application these magnets would operate in a high radiation environment and would be subject to a high heat load. Removal of heat generated in magnets in this environment using conventional Ni–Ti and Nb 3 Sn superconductors, which generally operate at ∼4.5 K, is difficult. However, an HTS conductor can be used to permit operation at 40 K where heat removal is significantly more efficient. As these coils are curved, one side of the coils has a reverse curvature requiring the development of special technology to wind the coils. As part of an STTR grant to develop and demonstrate a super-ferric design for a 2.2 T magnet, two curved coils were fabricated with a 12-mm-wide SuperPower ReBCO conductor and first tested in liquid N 2 at 77 K. Afterwards the coils were installed into a cryostat and cooled to the design temperature of 48 K with cryocoolers. This paper presents the construction details and test results for these coils.

Various methods of phase and power control in magnetron RF sources of superconducting accelerators intended for ADS-class projects were recently developed and studied with conventional 2.45 GHz, 1 kW, CW magnetrons operating in pulsed and CW regimes. Magnetron transmitters excited by a resonant (injection-locking) phasemodulated signal can provide phase and power control with the rates required for precise stabilization of phase and amplitude of the accelerating field in Superconducting RF (SRF) cavities of the intensity-frontier accelerators. An innovative technique that can significantly increase the magnetron transmitter efficiency at the widerange power control required for superconducting accelerators was developed and verified with the 2.45 GHz magnetrons operating in CW and pulsed regimes. High efficiency magnetron transmitters of this type can significantly reduce the capital and operation costs of the ADSclass accelerator projects.

A new, efficient method to produce cold negative muon ions is proposed. The muonium atom is made up of an antimuon and an electron and is given the chemical symbol Mu. A second electron with binding energy or electron affinity of 0.75 eV makes the Mu- ion, which is in many ways almost identical to the H- ion that is used for charge-exchange injection into most proton particle accelerators. Muonium negative ions were observed in 1987 by interaction of muons with a foil. Using the foil charge-exchange approach, the efficiency of transformation of muons to negative muonium ions has been very low similar to 10(-4). However, by using a hot tungsten or palladium single crystal foil treated by cesium deposition, the production efficiency can be improved up to 50%. The process described here has surface muons focused onto a tungsten or palladium single crystal foil (that can be heated up to 2000 Celsius) and partially covered by a cesium layer up to minimal work function. The negative muon ions can be extracted by a DC electric field and further accelerated by a line and stripped m a thin foil.

State of the art high-current superconducting accelerators require efficient RF sources with a fast dynamic phase and power control. This allows for compensation of the phase and amplitude deviations of the accelerating voltage in the Superconducting RF (SRF) cavities caused by microphonics, etc. Efficient magnetron transmitters with fast phase and power control are attractive RF sources for this application. They are more cost effective than traditional RF sources such as klystrons, IOTs and solid-state amplifiers used with large scale accelerator projects. However, unlike traditional RF sources, controlled magnetrons operate as forced oscillators. Study of the impact of the controlling signal on magnetron stability, noise and efficiency is therefore important. This paper discusses experiments with 2.45 GHz, 1 kW tubes and verifies our analytical model which is based on the charge drift approximation.

We discuss the GEM*STAR reactor concept, which addresses all historical reactor failures. This system includes a superconducting RF (SRF) proton accelerator, an internal spallation neutron target and high temperature molten salt fuel with continuous purging of volatile radioactive fission products such that the reactor contains less than a critical mass and almost a million times fewer volatile radioactive fission products than conventional reactors like those at Fukushima.. The GEM*STAR [1] reactor can, without redesign, burn spent nuclear fuel, natural uranium, thorium, or surplus weapons material. . It will operate without the need for a critical core, fuel enrichment, or reprocessing making it an excellent candidate for export. While conventional nuclear reactors are becoming more and more difficult to license and expensive to build, SRF technology development is on a steep learning curve and the simplicity implied by subcritical operation will lead to reductions in regulatory hurdles and construction complexity.

Skew Parametric-resonance Ionization Cooling (Skew PIC) is designed for the final 6D cooling of a high-luminosity muon collider. Tracking of muons in such a channel has been modeled in MADX and matter-dominated simulation tool G4beanline in previous studies. In this work, we developed an optimization code based on Genetic Algorithm (GA). We optimized the cooling channel and increased the acceptance of the channel by using the GA code.