Julius Huijts

Bio: Julius Huijts is an academic researcher from École Polytechnique. The author has contributed to research in topics: Laser & Plasma. The author has an hindex of 5, co-authored 11 publications receiving 56 citations.

Demonstration of stable long-term operation of a kilohertz laser-plasma accelerator

Abstract: We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetrically shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

Demonstration of stable long-term operation of a kilohertz laser-plasma accelerator

Abstract: We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a newly designed asymmetrically shocked gas nozzle. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

Carrier-envelope phase effects in Laser Wakefield Acceleration with near-single-cycle pulses

Abstract: Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realisation of an electron source that can operate at kHz-repetition rate while relying on modest laser energy It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam As a result, the beam pointing and electron energy spectrum become CEP-dependent For injection in a density gradient these effects are reduced, as electron injection is mostly longitudinal and mainly determined by the density gradient Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams Mitigation of CEP effects can nevertheless be achieved using density gradient injection

Identifying observable carrier-envelope phase effects in laser wakefield acceleration with near-single-cycle pulses

Abstract: Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realization of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum, and the direction of emitted betatron radiation become CEP dependent. For injection in a density gradient, the effect on beam pointing is reduced and the electron energy spectrum is CEP independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam.

Optimization and stabilization of a kilohertz laser-plasma accelerator

Abstract: Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than 18 × 10 6 consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates.

Injection and Trapping of Tunnel Ionized Electrons into Laser Produced Wakes

Abstract: A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

Plasma density gradient injection of low absolute momentum spread electron bunches - eScholarship

Abstract: Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76 +- 0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies.

Back to the Future: Very High-Energy Electrons (VHEEs) and Their Potential Application in Radiation Therapy

Abstract: The development of innovative approaches that would reduce the sensitivity of healthy tissues to irradiation while maintaining the efficacy of the treatment on the tumor is of crucial importance for the progress of the efficacy of radiotherapy. Recent methodological developments and innovations, such as scanned beams, ultra-high dose rates, and very high-energy electrons, which may be simultaneously available on new accelerators, would allow for possible radiobiological advantages of very short pulses of ultra-high dose rate (FLASH) therapy for radiation therapy to be considered. In particular, very high-energy electron (VHEE) radiotherapy, in the energy range of 100 to 250 MeV, first proposed in the 2000s, would be particularly interesting both from a ballistic and biological point of view for the establishment of this new type of irradiation technique. In this review, we examine and summarize the current knowledge on VHEE radiotherapy and provide a synthesis of the studies that have been published on various experimental and simulation works. We will also consider the potential for VHEE therapy to be translated into clinical contexts.

Laser-Accelerated, Low-Divergence 15-MeV Quasimonoenergetic Electron Bunches at 1 kHz

Abstract: We demonstrate laser wakefield acceleration of quasi-monoenergetic electron bunches up to 15 MeV at 1 kHz repetition rate with 2.5 pC charge per bunch and a core with < 7 mrad beam divergence. Acceleration is driven by 5 fs, < 2.7 mJ laser pulses incident on a thin, near-critical density hydrogen gas jet. Low beam divergence is attributed to reduced sensitivity to laser carrier envelope phase slip, achieved in two ways using laser polarization and gas jet control: (1) electron injection into the wake on the gas jet's plasma density downramp, and (2) use of circularly polarized drive pulses. Under conditions of mild wavebreaking in the downramp, electron beam profiles have a 2D Lorentzian shape consistent with a kappa electron energy distribution. Such distributions had previously been observed only in space or dusty plasmas. We attribute this shape to the strongly correlated collisionless bunch confined by the quadratic wakefield bubble potential, where transverse velocity space diffusion is imparted to the bunch by the red-shifted laser field in the bubble.

Laser-accelerated, low divergence 15 MeV quasi-monoenergetic electron bunches at 1 kHz

Abstract: We demonstrate laser wakefield acceleration of quasi-monoenergetic electron bunches up to 15 MeV at 1 kHz repetition rate with 2.5 pC charge per bunch and a core with < 7 mrad beam divergence. Acceleration is driven by 5 fs, < 2.7 mJ laser pulses incident on a thin, near-critical density hydrogen gas jet. Low beam divergence is attributed to reduced sensitivity to laser carrier envelope phase slip, achieved in two ways using laser polarization and gas jet control: (1) electron injection into the wake on the gas jet's plasma density downramp, and (2) use of circularly polarized drive pulses. Under conditions of mild wavebreaking in the downramp, electron beam profiles have a 2D Lorentzian shape consistent with a kappa electron energy distribution. Such distributions had previously been observed only in space or dusty plasmas. We attribute this shape to the strongly correlated collisionless bunch confined by the quadratic wakefield bubble potential, where transverse velocity space diffusion is imparted to the bunch by the red-shifted laser field in the bubble.