Zhaohan He

Table-Top Laser-Based Source of Femtosecond, Collimated, Ultrarelativistic Positron Beams

The generation of ultrarelativistic positron beams with short duration (τ(e+) ≃ 30  fs), small divergence (θ(e+) ≃ 3  mrad), and high density (n(e+) ≃ 10(14)-10(15)  cm(-3)) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Electron diffraction using ultrafast electron bunches from a laser-wakefield accelerator at kHz repetition rate

We show that electron bunches in the 50–100 keV range can be produced from a laser wakefield accelerator using 10 mJ, 35 fs laser pulses operating at 0.5 kHz. It is shown that using a solenoid magnetic lens, the electron bunch distribution can be shaped. The resulting transverse and longitudinal coherence is suitable for producing diffraction images from a polycrystalline 10 nm aluminum foil. The high repetition rate, the stability of the electron source, and the fact that its uncorrelated bunch duration is below 100 fs make this approach promising for the development of sub-100 fs ultrafast electron diffraction experiments.

Improvements to laser wakefield accelerated electron beam stability, divergence, and energy spread using three-dimensional printed two-stage gas cell targets

High intensity, short pulse lasers can be used to accelerate electrons to ultra-relativistic energies via laser wakefield acceleration (LWFA) [T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979)]. Recently, it was shown that separating the injection and acceleration processes into two distinct stages could prove beneficial in obtaining stable, high energy electron beams [Gonsalves et al., Nat. Phys. 7, 862 (2011); Liu et al., Phys. Rev. Lett. 107, 035001 (2011); Pollock et al., Phys. Rev. Lett. 107, 045001 (2011)]. Here, we use a stereolithography based 3D printer to produce two-stage gas targets for LWFA experiments on the HERCULES laser system at the University of Michigan. We demonstrate substantial improvements to the divergence, pointing stability, and energy spread of a laser wakefield accelerated electron beam compared with a single-stage gas cell or gas jet target.

Laser-driven generation of collimated ultra-relativistic positron beams

We report on recent experimental results concerning the generation of collimated (divergence of the order of a few mrad) ultra-relativistic positron beams using a fully optical system. The positron beams are generated exploiting a quantum-electrodynamic cascade initiated by the propagation of a laser-accelerated, ultra-relativistic electron beam through high-Z solid targets. As long as the target thickness is comparable to or smaller than the radiation length of the material, the divergence of the escaping positron beam is of the order of the inverse of its Lorentz factor. For thicker solid targets the divergence is seen to gradually increase, due to the increased number of fundamental steps in the cascade, but it is still kept of the order of few tens of mrad, depending on the spectral components in the beam. This high degree of collimation will be fundamental for further injection into plasma-wakefield afterburners.

Laser-ion acceleration through controlled surface contamination

In laser-plasma ion accelerators, control of target contamination layers can lead to selection of accelerated ion species and enhancement of acceleration. To demonstrate this, deuterons up to 75 keV are accelerated from an intense laser interaction with a glass target simply by placing 1 ml of heavy water inside the experimental chamber prior to pumping to generate a deuterated contamination layer on the target. Using the same technique with a deuterated-polystyrene-coated target also enhances deuteron yield by a factor of 3 to 5, while increasing the maximum energy of the generated deuterons to 140 keV.

Stereolithography based method of creating custom gas density profile targets for high intensity laser-plasma experiments

Laser based stereolithography methods are shown to be useful for production of gas targets for high intensity laser-plasma interaction experiments. A cylindrically symmetric nozzle with an opening of approximately 100 μm and a periodic attachment of variable periodicity are outlined in detail with associated density profile characterization. Both components are durable within the limits of relevant experiments.

Control of the configuration of multiple femtosecond filaments in air by adaptive wavefront manipulation

We demonstrate the ability to position single and multiple filaments arbitrarily within the energy reservoir of a high power femtosecond laser pulse. A deformable mirror controlled by a genetic algorithm finds the optimal phase profile for producing filaments at user-defined locations within the energy reservoir to within a quarter of the nominal filament size, on average. This proof-of-principle experiment demonstrates a potential technique for fast control of the configuration of the filaments.

Compressor optimization with compressor-based multiphoton intrapulse interference phase scan (MIIPS)

The multiphoton intrapulse interference phase scan (MIIPS) technique is modified to optimize the compressor settings of a chirped pulse amplification (CPA) laser system. Here, we use the compressor itself to perform the phase scan inherent in MIIPS measurement . A frequency-resolved optical gating measurement shows that the pulse duration of the compressor optimized using the modified MIIPS technique is 33.8 fs with a 2.24 rad temporal phase variation above 2% intensity. The measured time-bandwidth product is 0.60, which is close to that of transform-limited Gaussian pulse (0.44).

A high-repetition-rate laser-wakefield accelerator for studies of electron acceleration

We report on an experimental demonstration of laser wake field electron acceleration using few-milijoule laser pulses tightly focused on a 100 μm scale gas target. Using a comparatively low energy pulse has the benefit of a more compact system with a high repetition rate (typically kHz), which can prove useful for both practical applications and better statistical studies of laser plasma interactions. A proof-of-principle experiment was conducted to demonstrate the applicability of such electron sources from laser plasma wake field accelerator for ultrafast electron diffraction.

Properties and spectroscopic performance of semiconductor detectors under high-flux irradiation

In recent years, operation of wide band-gap semiconductor detectors under high-flux irradiation in medical and security applications has called the attention of the scientific commnity. However, under high-flux irradiation, these detectors are limited by poor hole transport properties and other factors. In our studies, the build up of space charge as a function of time has been systematically investigated through direct comparison between experiments and charge transport simulations. In order to benchmark and calibrate our simulations, charge transport properties of CdZnTe detectors used in high-flux experiments were measured. Our results show that the polarization effect, caused by the build up of positive space charge as a function of time, initially distorts and eventually causes complete breakdown of the operating electric field.