Laser Plasma Electron Acceleration with kHz Lasers – HighRep

The French team:
The LOA team will use the laser facility in the Black Room. Currently, the laser delivers high contrast pulses at 800 nm, in 25 fs with an energy of about 10 mJ at one kHz. The pulses are then post-compressed to durations of 3.5 fs. The pulses can be focused close to the diffraction limit and relativistic intensities of 5x1018 W/cm2 are commonly produced.
We list below the main experimental equipment that will be used during the project:
- 1 TW kHz laser system with 3.5 fs pulses
- Vacuum chamber dedicated to the acceleration of electrons at kHz
- Pumping system allowing operation at kHz with high Z gases (operation with H2 or He is currently not possible at kHz due to the pumping load)
- Electronic diagnostics package: beam angular profile, charge, electron energy spectrum...
Within the framework of the project, two additional instrumental developments will be carried out:
- The development of a differential pumping scheme around the gas jet in order to be able to use lighter gases like helium or hydrogen. This will greatly increase the parameter space that the experiment will be able to cover and should increase the electron energy by a factor of 3 to 5.
- Development of a transverse emittance diagnostic to provide more quantitative estimates of the electron beam quality. For the moment, the beam quality is only evaluated on the basis of its divergence. The diagnosis will be based on the quadrupole scan technique and will require the use of focusing magnets. The implementation of this diagnosis will require investments in opto-mechanical elements and vacuum motorization.

German team:
The HHUD team has full access to the JuSPARC_VEGA Ti:Sa laser facility, located at Forschungszentrum Jülich. JuSPARC_VEGA was commissioned in early 2019, including measurements of operating parameters that are 37 mJ in 29 fs at 800 nm wavelength. This laser system is complementary to the one available at LOA: it delivers 10 times more energy but in 10 times longer laser pulses. Our experiments on electron production will be performed in a new vacuum chamber and a new electron beam line that will be developed within the project.

Notable results at this stage of the project are:
- Demonstration of stable operation for 5 hours of a laser-plasma gas accelerator (published in Physics of Plasmas 2021)
- First experimental demonstration of electric field shape effects (CEP effects) in a laser-plasma gas accelerator (published in Phys. Rev. X in 2022, theoretical study published in Physics of Plasmas 2021)
These results have been the subject of several invited presentations in international conferences.

The expected results for the continuation of the project are:
- Increase of the electron energy beyond 10 MeV
- Complete measurement and characterization of the X-ray radiation produced in the laser-plasma acceleration
- To reach a stabilization such that application experiments can be set up in the following fields
- Time-resolved electron diffraction with resolutions below 100 fs.
- Irradiation of cancer cells for radiobiology. Study of the effects of dose fractionation.

1. “Waveform control of relativistic electron dynamics in laser-plasma acceleration”; J. Huijts, L. Rovige, I.A. Andriyash, A. Vernier, M. Ouillé, J. Kaur, Z. Cheng, R. Lopez-Martens and J. Faure, Phys. Rev. X 12, 01136 (2022)
2. “Symmetric and asymmetric shocked gas jets for laser-plasma experiments”; L. Rovige, J. Huijts, A. Vernier, I.A. Andriyash, F. Sylla, V. Tomkus, V. Girdauskas, G. Raciukaitis, J. Dudutis, V. Stankevic, P. Gecys and J. Faure, Rev. Sci. Inst. 92, 083302 (2021)
3. “Identifying observable carrier-envelope phase effects in laser wakefield acceleration with near-single-cycle pulses”; J. Huijts, I. A. Andriyash, L. Rovige, A. Vernier and J. Faure, Phys. Plasmas 28, 043101 (2021)
4. “Optimization and stabilization of a kilohertz laser-plasma accelerator”; L. Rovige, J. Huijts, I.A. Andriyash, A. Vernier, M. Ouillé, Z. Cheng, T. Asai, Y. Fukuda, V. Tomkus, V. Girdauskas, G. Raciukaitis, J. Dudutis, V. Stankevic, P. Gecys, R. Lopez-Martens and J. Faure, Phys. Plasmas 28, 033105 (2021)

Laser-plasma acceleration has emerged as a promising technique for producing electron beams in a compact manner. In the majority of cases, 100 TeraWatt to PetaWatt scale laser systems, with rather low repetition rates (Hz or less), are used to drive such accelerators and to produce electron beams up to the GeV scale. However, many applications require higher repetition rates up to the kHz level and beyond. Very recently, first proof-of-principle experiments have demonstrated laser-plasma acceleration of few-MeV electrons at kHz repetition rates. These first results are encouraging but the observed beams are not useful for applications yet as they lack reliability and stability. The current project, HighRep, aims to bring kHz laser-plasma acceleration to a higher level of maturity. To do so, we will develop innovative targets to better control the injection into the accelerating plasma waves, which should, in turn, increase the stability, reliability and robustness of the electron source. The target developments will follow two main research paths: (i) micro-nozzles for producing controlled tailored gas-jet distributions at the micrometer level, permitting for instance to trigger electron injection in density transitions, (ii) cluster targets with micrometer positioning of single clusters, for controlling injection and increasing the electron energy to 10 MeV and beyond. Laser-plasma accelerators have the potential of producing femtosecond X-ray bursts via betatron radiation of the electrons in the plasma wakefield. Within HighRep, we will carry out first measurements of the emitted X-UV light at kHz repetition rate. The project is crucially based on the complementarity of the French and German groups, both in terms of expertise and equipment. In particular, the collaboration between the two groups will permit to perform experiments on two different 1 TW state-of-the-art laser systems that have very different parameters. This will allow us to study kHz laser-plasma acceleration in different regimes and to estimate which laser technology is most promising.