This project addresses the development of two techniques to extend the reach of compact x-ray sources, producing bright, ultrafast x-rays in the multi-kilovolt regime from sources based on intense laser-plasma interaction. The research involves the experimental generation, measurement and modeling of ultrafast (femtosecond) x-rays using the mechanisms of betatron radiation and high harmonic generation (HHG) in the relativistic limit. The first mechanism uses the betatron oscillation of accelerated electrons in the focusing field of a plasma-based accelerator to generate intense broad-bandwidth incoherent x-rays spanning the kilovolt hundred kilovolt window. In the first phase of the work this betatron radiation source will be used to test, calibrate, and field x-ray diagnostics required to characterize the radiation in this range. This knowledge and expertise will be applied to HHG in the second phase of this program. Secondly, a new method for extending coherent x-ray HHG into the many-kilovolt regime by using multiple laser pulses of relativistic intensity will be developed. HHG occurs when an electron is ionized from a host atom and is then driven back to recombine with the atom by the laser field. Photon energy is limited in conventional single-pulse schemes by plasma-induced decoherence, which limits the effective interaction length. For relativistic intensities, the magnetic interaction of the ionized electron with the laser field further suppresses radiation generation by preventing the electron from recombining with the atom. An intense laser will be used to expel plasma electrons creating an ion channel, and, by using two counterpropagating pulses, the magnetic-field induced drift of the electron will be removed. This will extend the reach of compact coherent HHG sources from 0.5 kilovolt (current state-of-the-art) to potentially 100 kilovolt and beyond.
This project aims at the development of compact, short pulse (femtosecond), x-ray sources and we have explored four methods to achieve this goal. (1) The first is a theoretical study on a novel method for extending coherent high-harmonic generation (HHG) in gases into the hard x-ray (many-keV photon energy) regime by using relativistic laser pulses in cavitated plasmas. (2) The second is a theoretical study on a method to enhance betatron emission from a relativistic electron beam produced by a laser plasma accelerator (LPA) by using a tailored plasma density. (3) The third method involves radiation obtained by passing the LPA electron beam through an undulator magnet. Initially, spontaneous radiation is studied, with the eventual goal of a laser-accelerator-driven free electron laser (FEL) provided sufficiently high electron beam quality can be obtained (the FEL study is beyond the scope of this project). (4) The fourth method is an experimental study of betatron radiation emitted from a laser-plasma accelerator which is used to infer some aspects of the electron beam quality. Techniques for measuring x-rays have also been developed. These techniques for producing x-ray beams may benefit all fields that relyon the use of x-rays to probe and characterize matter. A general challenge in the field of ultrafast x-ray science is to develop high energy x-ray sources that are compact and inexpensive. A laser-plasma accelerator (LPA) is a new and compact technology for producing high-quality, high-energy electron beams that can subsequently be used to produce ultra-short x-ray pulses by a variety of methods. Previous LPA experiments at Lawrence Berkeley National Laboratory (LBNL) have generated high energy (1 GeV), high quality (small energy spread and small divergence) electron beams using a high power (60 TeraWatt), short pulse (40 femtosecond) laser beam in a 3 cm long plasma (ionized helium gas). A source based on the betatron emission from a LPA can deliver incoherent broadband hard x-rays. High harmonic generation by lasers in plasmas can provide narrow bandwidth, coherent x-rays. Undulators can provided narrow bandwidth incoherent (spontaneous) or coherent (via the free electron laser mechanism) radiation, depending on the electron beam quality. Depending on the electron beam energy, these x-rays can be tuned to cover the region from a few keV to few MeV. Sources driven by LPAs have the additional advantage in providing intrinsically short-pulse radiation, since the electron bunch producing the radiation is of femtosecond duration. Future applications of LPAs will be as a driven for an x-ray free electron laser (FEL). The FEL requires very good electron beam quality, and the ongoing experiments atL BNL will diagnose and improve the LPA electron beam quality by measuring the properties of undulator radiation.
