Generation of high energy electrons using laser plasma interactions have been of immense interest over the past few decades, owing to their application in various fields like ion acceleration and fast ignition for ICF. Till date relativistic electrons (> 1 MeV) are mostly obtained at intensities of 〖10〗^18 W/cm^2 and above. Such high intensities are generally achieved by using either high power lasers or specialized optics designed to reduce the focal spot size [1-6]. Both these procedures are financially cumbersome and are often limited by the number of shots that can be delivered at a stretch. The need for reducing the laser intensity and delivering high repetition rate system is therefore crucial for viable commercial applications. The other approach has been towards the use of structured targets especially mas-limited targets. Owing to their lack of a cold electron bath mass-limited targets are known to generate hotter electrons compared to bulk targets [7-9].
In addition, generation of high energy electrons has also been approached using target structure modification [6,8-9]. Structural modifications in nano or microscale dimensions result in local field enhancements leading to enhanced charged particle acceleration. However, these techniques too, are constrained by target quality and cannot be implemented for kHz systems. Deriving from these preliminary ideas of mass-limited targets and target structure modification, we demonstrate a novel technique for generation of high energy electrons at kHz repetition rate using laser driven dynamic structure formation. We use a controlled pre-pulse to generate hydro-dynamically induced micro-structures on a 15 um Methanol droplet. The generation of these dynamic structures being laser driven ensures a 100% reproducibility at each target laser interaction.
On arrival of the main pulse of intensity 4×〖10〗^16 W/cm^2 the generated structures lead to confinement and enhancement of the laser field resulting in generation of electrons with energy as high as 6MeV and associated Bremsstrahlung at 100 times lower incident intensity than used in other contemporary experiments. The measured electron and X-ray spectrum are observed to be comprised of two distinct temperature regimes one ranging from 200 keV - 1MeV and other from 1 MeV - 6 MeV having temperatures about 200 keV and 1MeV respectively. These electrons and X-rays seem to have beam like characteristics, where the yield and directionality can be controlled by modulating the laser parameters. The measurement of the electron angular distribution reveals that the electrons confined in the laser polarization plane and are emitted mostly at ± 50° with respect to the laser backward direction, showing a highly direction beam of high energy electrons.
Building on this, we explore the prospect of kHz rate X-ray and electron imaging, in particular for medical imaging applications. We demonstrate proof of principle experiments showing high resolution <20 um imaging using both X-rays and electrons of biological and metallic samples revealing the possibility of a table-top high resolution, kHz rate electron and X-ray sources obtained at a moderate laser intensity of 〖10〗^16 W/cm^2.
1. Modena A.,et al., “Electron acceleration from the breaking of relativistic plasma waves”, Nature, 377, 606–608 (1995).
2. Malka G., et al., “Experimental Observation of Electrons Accelerated in Vacuum to Relativistic
Energies by a High-Intensity Laser”, Phys. Rev. Lett., 78, 3314 (1997).
3. Gahn C., et al., ”Multi-MeV Electron Beam Generation by Direct Laser Acceleration in High- Density Plasma Channels”, Phys. Rev. Lett., 83, 4472 (1999).
4. Malka V.,et al., “Electron Acceleration by a Wake Field Forced by an Intense Ultrashort Laser Pulse”, Science, 298, Issue 5598, 1597–1600 (2002).
5. Albert O., et al., “Generation of relativistic intensity pulses at a kilohertz repetition rate”, Optics Lett.,25, Issue 15, 1125–1127 (2000).
6. Zieglar A.,et al.,“ Enhanced Proton Acceleration by an Ultrashort Laser Interaction with Structured Dynamic Plasma Targets”, Phys. Rev. Lett. 110, 215004 (2013)
7. M.Anand, et al, “Enhanced hard x-ray generation from micro-droplets in intense laser field”, Appl. Phys. Lett. 88, 181111 (2006).
8. M. Krishnamurthy, et al.,“Bright point source of ultrashort hard x-rays from laser bioplasmas”, Opt. Exp. 20, (2012) 5754.
9. Deep Sarkar,et al., “Silicon nanowire based high brightness, pulsed relativistic electron source”, APL Photonics (2017).
|Working group||Laser-driven electron acceleration|