Chuanxiang Tang

Review of recent theories and experiments for improving high-power microwave window breakdown thresholdsa)

Dielectric window breakdown is a serious challenge in high-power microwave (HPM) transmission and radiation. Breakdown at the vacuum/dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above the dielectric. Methods of improving breakdown thresholds are key challenges in HPM systems. First, the main theoretical and experimental progress is reviewed. Next, the mechanisms of multipactor suppression for periodic rectangular and triangular surface profiles by dynamic analysis and particle-in-cell simulations are surveyed. Improved HPM breakdown thresholds are demonstrated by proof-of-principle and multigigawatt experiments. The current theories and experiments of using dc magnetic field to resonantly accelerate electrons to suppress multipactor are also synthesized. These methods of periodic profiles and magnetic field may solve the key issues of HPM vacuum dielectric breakdown.

Note: Single-shot continuously time-resolved MeV ultrafast electron diffraction

We have demonstrated single-shot continuously time-resolved MeV ultrafast electron diffraction using a static single crystal gold sample. An MeV high density electron pulse was used to probe the sample and then streaked by an rf deflecting cavity. The single-shot, high quality, streaked diffraction pattern allowed structural information within several picoseconds to be continuously temporally resolved with an approximately 200 fs resolution. The temporal resolution can be straightforwardly improved to 100 fs by increasing the streaking strength. We foresee that this system would become a powerful tool for ultrafast structural dynamics studies.

Experimental demonstration of high quality MeV ultrafast electron diffraction.

The simulation optimization and an experimental demonstration of improved performances of mega-electron-volt ultrafast electron diffraction (MeV UED) are reported in this paper. Using ultrashort high quality electron pulses from an S-band photocathode rf gun and a polycrystalline aluminum foil as the sample, we experimentally demonstrated an improved spatial resolution of MeV UED, in which the Debye-Scherrer rings of the (111) and (200) planes were clearly resolved. This result showed that MeV UED is capable to achieve an atomic level spatial resolution and a approximately 100 fs temporal resolution simultaneously, and will be a unique tool for ultrafast structural dynamics studies.

Generation of first hard X-ray pulse at Tsinghua Thomson Scattering X-ray Source

Tsinghua Thomson Scattering X-ray Source (TTX) is the first-of-its-kind dedicated hard X-ray source in China based on the Thomson scattering between a terawatt ultrashort laser and relativistic electron beams. In this paper, we report the experimental generation and characterization of the first hard X-ray pulses (51.7 keV) via head-on collision of an 800 nm laser and 46.7 MeV electron beams. The measured yield is 1.0 × 10(6) per pulse with an electron bunch charge of 200 pC and laser pulse energy of 300 mJ. The angular intensity distribution and energy spectra of the X-ray pulse are measured with an electron-multiplying charge-coupled device using a CsI scintillator and silicon attenuators. These measurements agree well with theoretical and simulation predictions. An imaging test using the X-ray pulse at the TTX is also presented.

Generating High-Brightness Electron Beams via Ionization Injection by Transverse Colliding Lasers in a Plasma-Wakefield Accelerator

The production of ultrabright electron bunches using ionization injection triggered by two transversely colliding laser pulses inside a beam-driven plasma wake is examined via three-dimensional particle-in-cell simulations. The relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. The result is that the residual momentum of the ionized electrons in the transverse plane of the wake is reduced, and the injection is localized along the propagation axis of the wake. This minimizes both the initial thermal emittance and the emittance growth due to transverse phase mixing. Simulations show that ultrashort (~8 fs) high-current (0.4 kA) electron bunches with a normalized emittance of 8.5 and 6 nm in the two planes, respectively, and a brightness of 1.7×10(19) A rad(-2) m(-2) can be obtained for realistic parameters.

Experimental verification of improving high-power microwave window breakdown thresholds by resonant magnetic field

Recently, high-power microwave (HPM) dielectric multipactor is theoretically discovered to be suppressed by utilizing external resonant magnetic field. This paper gives the related experimental demonstration of increasing the vacuum window breakdown thresholds. In the S-band HPM experiment with 0.5 μs width, the magnetic field with gyrofrequency Ω close to rf frequency ω can triple the breakdown threshold. Besides, reducing or enhancing magnetic field resulted in a relatively lower threshold in comparison of Ω∼ω, agreeing with theoretical analysis. By HPM pulse compression to 14 ns width, the threshold was also demonstrated to be significantly enhanced by magnetic field.

High-efficiency coaxial relativistic backward wave oscillator

This paper studies the coaxial relativistic backward wave oscillator (CRBWO) through analytical, numerical, and experimental methods. This new type of device is remarked by its high efficiency of more than 35%, which is predicted by the theoretical calculation and the numerical simulation and validated by experiment. The two primary hindrances preventing CRBWO from achieving the expected high efficiency, the poor coaxiality and the power capacity, are discussed in detail and some advanced methods are developed. The theoretical and numerical conclusions agree with the experiment results, which are obtained from the electric probe and the calorimeter simultaneously for each shot of CRBWO. Employing the electron beam pulse of the full width at half maximum 28 ns, a microwave pulse of the width about 20 ns is generated in the experiment; the power is 710 MW and the efficiency is higher than 33%.

Phase-space dynamics of ionization injection in plasma-based accelerators.

The evolution of beam phase space in ionization injection into plasma wakefields is studied using theory and particle-in-cell simulations. The injection process involves both longitudinal and transverse phase mixing, leading initially to a rapid emittance growth followed by oscillation, decay, and a slow growth to saturation. An analytic theory for this evolution is presented and verified through particle-in-cell simulations. This theory includes the effects of injection distance (time), acceleration distance, wakefield structure, and nonlinear space charge forces, and it also shows how ultralow emittance beams can be produced using ionization injection methods.

Frequency-Tunable High-Power Mesoband Microwave Radiator

This paper presents a frequency-tunable high-power mesoband microwave radiator that uses a switched quarter-wavelength oscillator and a patch antenna. The oscillator is charged and then switched by an annular electrode between the inner and outer conductors. The length of the oscillator is adjustable by moving the electrode in order to generate different frequency resonances. A coupler with a quasi-constant coupling coefficient was employed to output a high-power mesoband microwave. A novel high-power wideband patch antenna with a bandwidth of 64% and a maximum gain of 8.1 dB was used to radiate the generated mesoband microwave. The experimental results of the radiation factor were as high as 182 kV at 333 MHz, and the measured center frequency of the radiated mesoband microwaves ranged from 227 to 345 MHz.

Analytic model for the breakup of a coasting beam with space charge in isochronous accelerators

An analytical model based on the negative mass instability is introduced in this paper to explain the formation of the breakup of a coasting beam into small clusters in isochronous machines such as the case observed by Pozdeyev and Rodriguez in a small isochronous ring. Solving Poisson’s equation in both charge and vacuum regions with the longitudinal beam density perturbation, the coherent radial space charge force which decreases the transition gamma is obtained. It is found that the modified transition gamma depends on the wave number of the density perturbation, longitudinal beam density distribution, beam intensity, and beam size. By combining the longitudinal space charge force caused by the perturbation and the modified transition gamma, a dispersion relation for a monoenergetic beam is derived and evaluated for the fastest-growing instability mode in terms of the beam parameters, such as energy, bunch length, intensity, and emittance. The fastest-growing negative mass mode number, which determines...