Jiaqi Qiu

Planar ultrananocrystalline diamond field emitter in accelerator radio frequency electron injector: Performance metrics

A case performance study of a planar field emission cathode (FEC) based on nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in an RF 1.3 GHz electron gun. The FEC was a 100 nm (N)UNCD film grown on a 20 mm diameter stainless steel disk with a Mo buffer layer. At surface gradients 45–65 MV/m, peak currents of 1–80 mA (equivalent to 0.3–25 mA/cm2) were achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD surface with (1) the beam emittance of 1.5 mm × mrad/mm-rms and (2) longitudinal FWHM and rms widths of non-Gaussian energy spread of 0.7% and 11% at an electron energy of 2 MeV. Current stability was tested over the course of 36 × 103 RF pulses (equivalent to 288 × 106 GHz oscillations).

GHz laser-free time-resolved transmission electron microscopy: A stroboscopic high-duty-cycle method

A device and a method for producing ultrashort electron pulses with GHz repetition rates via pulsing an input direct current (dc) electron beam are provided. The device and the method are based on an electromagnetic-mechanical pulser (EMMP) that consists of a series of transverse deflecting cavities and magnetic quadrupoles. The EMMP modulates and chops the incoming dc electron beam and converts it into pico- and sub-pico-second electron pulse sequences (pulse trains) at >1GHz repetition rates, as well as controllably manipulates the resulting pulses. Ultimately, it leads to negligible electron pulse phase-space degradation compared to the incoming dc beam parameters. The temporal pulse length and repetition rate for the EMMP can be continuously tunable over wide ranges. Applying the EMMP to a transmission electron microscope (TEM) with any dc electron source (e.g. thermionic, Schottky, or field-emission source), a GHz stroboscopic high-duty-cycle TEM can be realized. Unlike in many recent developments in time-resolved TEM that rely on a sample pumping laser paired with a laser launching electrons from a photocathode to probe the sample, there is no laser in the presented experimental set-up. This is expected to be a significant relief for electron microscopists who are not familiar with laser systems. The EMMP and the sample are externally driven by a radiofrequency (RF) source synchronized through a delay line. With no laser pumping the sample, the problem of the pump laser induced residual heating/damaging the sample is eliminated. As many RF-driven processes can be cycled indefinitely, sampling rates of 1-50GHz become accessible. Such a GHz stroboscopic TEM would open up a new paradigm for in situ and in operando experiments to study samples externally driven electromagnetically. Complementary to the lower (MHz) repetition rates experiments enabled by laser photocathode TEM, new experiments in the multi-GHz regime will be enabled by the proposed RF design. Because TEM is also a platform for various analytical methods, there are infinite application opportunities in energy and electronics to resolve charge (electronic and ionic) transport, and magnetic, plasmonic and excitonic dynamics in advanced functional materials. In addition, because the beam duty-cycle can be as high as ~10(-1) (or 10%), detection can be accomplished by commercially available detectors. In this article, we report an optimal design of the EMMP. The optimal design was found using an analytical generalized matrix approach in the thin lens approximation along with detailed beam dynamics taking actual realistic dc beam parameters in a TEM operating at 200keV.

Efficient extraction of high power THz radiation generated by an ultra-relativistic electron beam in a dielectric loaded waveguide

We have measured an intense THz radiation produced by a sub-picosecond, relativistic electron bunch in a dielectric loaded waveguide. For efficient THz pulse extraction, the dielectric loaded waveguide end was cut at an angle. For an appropriate choice of angle cut, such antenna converts the TM01 mode excited in the waveguide into a free-space fundamental Gauss-Hermite mode propagating at an angle with respect to the electron beam trajectory. Simulations show that more than 95% of energy can be extracted using such a simple approach. More than 40 oscillations of about 170 ps long 0.48 THz signal were explicitly measured with an interferometer and 10 μJ of energy per pulse, as determined with a calorimetric energy meter, were delivered outside the electron beamline to an area suitable for THz experiments.

Observation of Field-Emission Dependence on Stored Energy

Field emission from a solid metal surface has been continuously studied for a century over macroscopic to atomic scales. It is general knowledge that, other than the surface properties, the emitted current is governed solely by the applied electric field. A pin cathode has been used to study the dependence of field emission on stored energy in an L-band rf gun. The stored energy was changed by adjusting the axial position (distance between the cathode base and the gun back surface) of the cathode while the applied electric field on the cathode tip is kept constant. A very strong correlation of the field-emission current with the stored energy has been observed. While eliminating all possible interfering sources, an enhancement of the current by a factor of 5 was obtained as the stored energy was increased by a factor of 3. It implies that under certain circumstances a localized field emission may be significantly altered by the global parameters in a system.

Field emission study using an L-band photocathode gun

Field emission is strongly coupled to the breakdown problem. A series of experiments is being carried out at Argonne Wakefield Accelerator Facility (AWA) using an L-band photocathode gun. Cathodes with different shapes have been tested and a dark current imaging system has been set up. Initial experiment results are presented.

Stroboscopic High-Duty-Cycle GHz Time-Resolved Microscope: Toward Hardware Implementation and Commissioning

1. Euclid TechLabs, 365 Remington Blvd., Bolingbrook, IL 60440, USA 2. Integrated Dynamic Electron Solutions, 5653 Stoneridge Dr., Suite 117, Pleasanton, CA 94588, USA 3. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 4. Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY 11973, USA * s.baryshev@euclidtechlabs.com

Developing field emission electron sources based on ultrananocrystalline diamond for accelerators

Radiofrequency (RF) electron guns work by establishing an RF electromagnetic field inside a cavity having conducting walls. Electrons from a cathode are generated in the injector and immediately become accelerated by the RF electric field, and exit the gun as a series of electron bunches. Finding simple solutions for electron injection is a long standing problem. While energies of 30-50 MeV are achievable in linear accelerators (linacs), finding an electron source able to survive under MW electric loads and provide an average current of 1-10 mA is important. Meeting these requirements would open various linac applications for industry. The natural way to simplify and integrate RF injector architectures with the electron source would be to place the source directly into the RF cavity with no need for additional heaters/lasers. Euclid TechLabs in collaboration with Argonne National Lab are prototyping a family of highly effective field emission electron sources based on a nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) platform. Determined metrics suggest that our emitters are emissive enough to meet requirements for magnetized cooling at electron-ion colliders, linac-based radioisotope production and X-ray sterilization, and others.

Multipactor Discharge in a Resonator as an Active Switch for RF Pulse Compression

Pulse compression is a method of increasing the peak power of the microwave pulse at the expense of its length. Over the years a number of pulse compressors had been demonstrated with some being bulky but efficient, like the binary pulse compressor and other being compact but less efficient, like SLED-II. An active pulse compressor had been proposed to increase the efficiency and compression ratio which relies on a high power active switch. Currently there are no practical switches that can work reliably with 100 s of megawatts of power. Most of the switches (ferroelectric, plasma-based, semiconductor) are limited by the breakdown strength of various dielectric inserts. In this paper we report on an active switch development which is based on a pure copper resonator and controlled by a single-side multipactor discharge at a metallic wall in the presence of a resonant DC magnetic field and a normal to metal rf field. The discharge is ignited by external rf power produced by inexpensive 2.45 GHz, 1-5 kW magnetrons.

Electron imaging system for ultrafast diagnostics of HEDLP

A high energy electron beam is proposed to be used for time resolved imaging measurements of hydrodynamic processes in High Energy Density Laboratory Plasma (HEDLP). Generation of a high quality sub-picosecond electron beam with present RF photocathode technologies is technologically mature and cost effective. An electron bunch train with a flexible time structure is used to penetrate a time varying high density target. By imaging the scattered electron beam, the detailed target profile and its density evolution can be accurately determined. To illustrate the concept design, an experiment is proposed based on Argonne Wakefield Accelerator (AWA) beamline. An imaging lattice design and particle tracking simulation is finished.