Abstract
The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.
Highlights
We first review some basic properties of the ICS process
We note that the phenomenon of energy loss to wakefields has an analogy in the IFEL – synchrotron radiation losses. These effects limit the maximum practical beam energy in an IFEL to the 10’s of GeV range, which is does not present an obstacle to light source applications. This merging of the IFEL accelerator and the ICS X-ray interaction point (IP) to yield a unique source of X-ray photons was performed on a high brightness electron beamline at the BNL ATF
There are three points of laser-electron interaction in this experimental scenario: a short IFEL energy modulator is first encountered, which combined with a downstream chicane forms a pre-bunching system; a subsequent tapered IFEL undulator, known as the Rubicon undulator, is where the laser provides ponderomotive acceleration; and, the ICS interaction point yields X-ray production
Summary
We first review some basic properties of the ICS process. Photons generated by ICS are localized in angle to a θ ~ 1/γ cone about the electron propagation direction – a directionality characteristic of radiation by relativistic cenhearrggye,dUpe,atrotiictlserse. sFtomr auslst,rma-erce2l.aNtievgisleticctienlgectthreornesc,otihleofLtohreeneltezcftarocnto(rthγe=ThmoUemce 2so n1, is the ratio limit, which is of the electron valid when the laser photon’s momentum in the electron rest frame is smaller than the electron’s rest energy), the single-particle angular dependence of the distribution in θ is confinedX-ray spectrum is: hkxray to within this 1/γ hkL angle; = 4γ 2 + γ 2θWhile there is an inherent off-axis redshift, these higher energy components correspond to the the highest flux density of X-rays.In practice, the scattering takes place in the context of a highly focused, short pulse beam of electrons colliding with a laser pulse of similar spatio-temporal characteristics. Photons generated by ICS are localized in angle to a θ ~ 1/γ cone about the electron propagation direction – a directionality characteristic of radiation by relativistic cenhearrggye,dUpe,atrotiictlserse. There are a number of aspects of the interaction arising from the distribution of electron and photon angles in the beams, as well as the influence of the finite time of laser-electron interaction, that affect the flux, bandwidth, and divergence of the X-ray photon distribution generated[21]. The most basic of these considerations is that the total number of generated X-rays is proportional to the number of electrons and laser photons available for interaction as well as the cross-sectional overlap of the two beams, i.e the luminosity L of the collision.
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