Browsing by Subject "Electron acceleration"

Now showing 1 - 4 of 4

Above-threshold ionization at intensity greater than 10²⁰ W/cm²
(2023-09-10) Yandow, Andrew James; Ditmire, Todd; Milner, Thomas E; Keto, John W; Shapiro, Paul R; Downer, Michael W
The ADK-PPT model of tunneling ionization is experimentally well-verified at intensity below 10¹⁹ W/cm². The relative simplicity of the tunneling model physics and the highly nonlinear ionization rate scaling with intensity make the detection of high ion charge states a promising method for directly measuring laser intensity. We present our progress toward the next generation of high-field atomic physics experiments, including modeling of the ion and electron dynamics in ultra-intense laser fields. We modeled the interaction of a highly-charged ion with a tightly-focused (f/1) laser pulse with a wavelength of 1057 nm and a pulse duration of 140 fs, characteristic of Nd:glass OPCPA laser facilities. We found that ponderomotive expulsion of the ions from the laser field necessitates new experimental methods for directly measuring or indirectly inferring the presence of high charge states. Our modeling showed ions will be accelerated to energies > 2 MeV/nucleon and ATI electron energies can exceed 1.4 GeV at the peak intensity of 3 x 10²³ W/cm² expected to be reached in the next few years. We also present our findings from two novel experimental campaigns on the Texas Petawatt Laser, an unsuccessful test of field-free ion time-of-flight and a successful detection of electrons produced by above-threshold ionization (ATI) of highly-charged neon states. We observed ATI electrons with energy greater than 10 MeV originating from the neon K-shell when laser intensity exceeds 2 x 10²⁰ W/cm².
Generation, measurement and application of x-rays from laser-plasma electron accelerators
(2021-12-03) Hannasch, Andrea Jeanne; Downer, Michael Coffin; Ditmire, Todd; Paban, Sonia; Ketcham, Richard; Cowan, Tom
This dissertation presents a comprehensive study of the generation mechanisms, diagnostic techniques and possible applications of few keV to 100 MeV x-rays generated by laser wakefield electron accelerators. Chapters 1-3 review the principles of x-ray science and laser wakefield acceleration, and 3 mechanisms by which laser wakefield accelerators produce x-rays: 1) betatron oscillations of the electrons while still accelerating; 2) inverse Compton scatter (ICS) x-rays involving electron oscillations induced when electrons collide with a counter-propagating laser pulse after exiting the accelerator; 3) bremsstrahlung from the impact of accelerated electrons with a solid target. Chapters 4-6 then present original, recently published work, starting in chapter 4 with experiments that characterized secondary x-rays from a laser wakefield accelerator at Helmholtz-Zentrum Dresden Rossendorf. In this work, a laser wakefield accelerator was driven by the 150 TW DRACO laser system and produced electrons tunable in energy from 250 to 350 MeV. I co-designed and built a compact calorimeter consisting of a stack of x-ray absorbers alternating with imaging plates. This single device enabled me to unfold spectra of all three major types of x-rays, both individually and in mixtures: 1) few-keV betatron x-rays, 2) ICS x-rays that were spectrally peaked at ~1 MeV photon energy, and 3) broadband bremsstrahlung with an average energy of ~30 MeV and a high energy tail extending beyond 100 MeV photon energy. Chapter 5 presents results obtained at The University of Texas in which I extended the work in chapter 4 and used a redesigned compact calorimeter to characterize secondary x-rays generated from a GeV-class accelerator. In this work, the accelerator was driven by the 1 PW Texas Petawatt Laser (TPW) which accelerated electrons to energies ranging from 500 MeV to 2 GeV. The compact calorimeter was redesigned for improved sensitivity to photons from 1 MeV to >100 MeV and enabled me to unfold ICS x-rays that were peaked at ~10 MeV photon energy, and broadband bremsstrahlung with average energies ~80 MeV. Chapter 6 then presents additional results obtained on the DRACO laser system in which I characterized the capabilities of a LPA and plasma mirror to generate ICS x-rays in both a linear and nonlinear regime. I used a CsI(Tl) scintillator to characterize the strength and divergence of ICS x-rays generated by retro-reflecting the accelerator’s spent drive laser pulse back onto the accelerated electrons using a plasma mirror. These measurements showed that the laser-electron interaction ranged from sub-relativistic to relativistic, depending on the plasma mirror distance from the accelerator exit. Finally, chapter 7 presents unpublished results from the TPW and presents unfolded spectra from a bremsstrahlung target scan in which a series of targets ranging from 25 μm-thick Kapton to 7.6 mm-thick Pb were used to produce Bremsstrahlung with average energies ranging from 60 MeV to >100 MeV. Chapter 7 also presents preliminary results from the application of bremsstrahlung x-rays to nuclear activation of copper. This dissertation concludes with a summary of the presented results and a discussion of future directions for laser plasma produced x-ray science.
Pulsed photoneutron source driven by electrons from laser wakefield acceleration
(2017-12) Jiao, Xuejing; Hegelich, Bjorn Manuel; Keto, John
Relativistic electron beams driven by laser wakefield acceleration (LWFA) were utilized to produce ultrashort neutron sources. The experiment was carried out on the 10 TW UT3, 38 f s, ~0.5 J, 800 nm Ti:Sapphire laser at the University of Texas at Austin. The target was a high-density pulsed gas jet composed of 90% Helium and 10% Nitrogen. The laser pulse with a peak intensity of 1.5E¹⁸ W/cm² interacted with the target to create a cylindrical plasma channel of 60 um radius (FWHM) and 1.5 mm length (FWHM). Electron beams of ~80 pC with Gaussian energy distribution centered at 37MeV and a width of 30MeV (FWHM) were produced via laser wakefield acceleration. A 2D particle in cell (PIC) simulation was performed to study the acceleration process. The LWFA was found to be running at a nonlinear broken-wave regime. The electron spectrum acquired from simulation quantitatively agree with the experimental observation. Neutron fluence of ~2.4x10⁶ per shot with ~300 ps temporal length was generated through bremsstrahlung and subsequent photoneutron reactions in a 26.6mm thick tungsten converter. Results were compared with simulations in GEANT4, showing agreement in neutron fluence, neutron angular distribution and conversion ratio.
Self-modulated laser wakefields driven by a CO₂ laser
(2019-08) Welch, James Roland; Downer, Michael Coffin; Keto, John; Ditmire , Todd; Belkin, Mikhail
Long-wavelength lasers are being explored for use in multi-stage laser wakefield accelerators with the eventual goal of creating a TeV-scale lepton collider. We use a high-power, ultrafast CO₂ laser (5 J, 4 ps, 10.6 μm) to generate self-modulated laser wakefields in a hydrogen gas jet, the first such experiment in this regime to our knowledge. Collective Thomson scattering (CTS) co-propagating probes were used to measure the plasma density in the gas jet and estimate the wakefield amplitudes. The measured wakefield have similar amplitudes to previous self-modulated laser wakefield experiments which measured accelerated electrons. The effects of gas jet geometry on the measured wavelength shift of the CTS probes were explored in depth, demonstrating a complex interplay of self-focusing and divergence effects based on the local plasma density and the pulse power.