Chaojie Zhang

Chaojie Zhang

he/him

Plasma/Accelerator Physicist at UCLA • Plasma Wakefield Acceleration • Ultrafast Laser-Matter Interaction

Talks and presentations

Latest results on PWFA experiments from FACET-II

August 01, 2024

Plenary Talk, Advanced Accelerator Concepts Workshop (AAC24), Naperville, IL, USA

Abstract: FACET-II is a national user facility that offers a unique capability for developing advanced acceleration and coherent radiation generation techniques using high-energy electron beams. In this talk, we will present the latest results from plasma wakefield acceleration (PWFA) experiments at FACET-II, focusing on the following topics. First, we provide evidence of energy depletion of the 10 GeV drive beam and efficient energy transfer from the beam to the wake, in both beam-ionized and laser-preionized plasmas, which is a crucial stepping stone towards achieving high energy transfer efficiency from the drive to the witness bunch in the ultimate two-bunch PWFA configuration. We will also show examples of machine-learning-enabled beam tuning to increase drive beam density, thereby enhancing energy transfer efficiency. Next, we present results on generating high-energy, low-emittance beams via downramp and ionization trapping in PWFA. Using density downramp injection, we achieve the generation of electron bunches exceeding 20 GeV with small energy spread and emittance. Additionally, we show the generation of multi-GeV, multi-color electron beams via ionization injection, resulting from periodic injection induced by betatron oscillations of the drive bunch. Finally, we will discuss the first experimental attempts at beam matching to a lithium density upramp and share preliminary results from the two-bunch PWFA experiment.

High-efficiency wake excitation in meter-scale beam-ionized hydrogen plasmas at FACET-II

June 01, 2024

Invited Talk, 51st IEEE International Conference on Plasma Science (ICOPS), Beijing, China

Abstract: Plasma wakefield acceleration has witnessed rapid progress in the past decades and is considered a promising route for building future linear colliders that demand both high repetition rates and high energy efficiency. In this talk, I will present the results that start to address the issues of repetition rate and energy transfer efficiency. These results were obtained from the Plasma Wakefield Acceleration Experiments (E300 Collaboration) at the newly commissioned FACET-II facility at SLAC. By self-ionizing a continuous hydrogen gas flow integrated into the accelerator beamline via a differential pumping system using time-structured 10 GeV electron bunches, we have generated plasmas with meter-scale lengths. This plasma source makes it possible to significantly increase the PWFA repetition rates by rapidly replenishing the gas between shots. Our experiments have also demonstrated high-gradient acceleration with high drive-bunch to wake energy transfer efficiency. At pressures ≥1.5 Torr, we observed the onset of pump depletion, which is an important first step towards achieving high overall energy transfer efficiency from the drive to the witness electron bunch in future two-bunch experiments. We also observed that the back of the drive bunch gains multi-GeV energy. These experimental findings are supported by particle-in-cell simulations, revealing a beam-to-wake energy transfer efficiency of approximately 60% at ~2 Torr. Our results not only demonstrate efficient wake excitation but also underscore the potential of continuous flow hydrogen plasmas in achieving the high energy transfer efficiencies required for PWFA-based linear colliders.

Measurements of Weibel magnetic fields in optical-field ionized plasmas

November 01, 2021

Invited Talk, 63rd Annual Meeting of the APS Division of Plasma Physics, Pittsburgh, PA, USA

Abstract: Generation and amplification of magnetic fields in plasmas is a long-standing topic that is of great interest to both fundamental and applied physics. Weibel instability is a well-known mechanism responsible for self-generating magnetic fields in anisotropic plasmas and has been extensively investigated in both theory and simulations, yet its experimental verification has proven challenging. Recently, we have demonstrated a new experimental platform that enables initialization of the plasma electron velocity distribution in a controllable manner and then measure the evolution of Weibel magnetic fields using an external ultrashort relativistic electron probe. Here we will first present experimental results on time-resolved measurements of Weibel magnetic fields in non-relativistic plasmas produced by optical field ionization using an ultrashort IR laser (0.8 µm). It was found that the Weibel magnetic fields self-organize into a quasistatic structure consistent with a helicoid topology within a few picoseconds and such a structure lasts for tens of picoseconds. The magnetic fields show a well-defined wave vector. The growth rate and saturated magnetic field magnitude were measured and agree well with kinetic theory predictions. We then discuss the feasibility of extending the study to quasi-relativistic regime by using intense CO2 (10 µm) lasers to produce much hotter anisotropic plasmas. The platform we have demonstrated is suitable for exploring a broad range of plasma phenomena such as magnetic reconnection, annihilation and island formation thereby opening a new avenue for studying astrophysical phenomena in the laboratory.

Probing plasma wakefield using femtosecond relativistic electron bunches

August 01, 2016

Plenary Talk, Advanced Accelerator Concepts Workshop (AAC16), National Harbor, MD, USA

Abstract: This plenary presentation introduced the groundbreaking technique of using femtosecond relativistic electron bunches to probe plasma wakefields with unprecedented temporal and spatial resolution. The talk described the inception and first experimental demonstration of femtosecond relativistic electron microscopy and its applications to wakefield visualization.