All-optical method and system for generating ultrashort charged particle beam

#The present invention relates to the generation of ultrashort charged particle beam. More specifically, the present invention is concerned with an all-optical method and system for generating ultrashort charged particle beam.

#Production of ultrashort electron bunches is needed in a number of applications, ranging from time-resolved electron microscopy to free-electron laser injection.

#An ultrashort electron beam can be generated by many methods, including for example direct field vacuum acceleration, Wakefield acceleration and high power longitudinal field acceleration.

#In the appended drawings:

#FIG. 1 is a flowchart of a method according to an embodiment of an aspect of the present invention;

#FIG. 2 is a diagrammatic view of a system according to an embodiment of an aspect of the present invention;

#As illustrated in FIG. 1, a method according to an embodiment of an aspect of the present invention comprises creating a high intensity E-field by selecting optical elements to substantially radially polarize (step 10), shape and optimize (step 20) the beam from a high peak power laser, then tightly focus the beam in an on-axis geometry (step 30). Steps 10, 20 and 30 may be performed in any order, i.e. step 10 may be performed after of prior to step 20 and coatings of the focusing optics used in step 30 may be provided for selectively reflecting target parts of the beam for example. The high intensity E-field thus produced by superposition of the different field components of the radially polarized laser beam in a constructive way at the focus (step 40) is used for interaction with a medium (step 50) to accelerate charged particles and generate an ultrashort charged particle beam (step 60).

#The method uses all optical elements, and therefore does not require using anode or cathode. The optical elements are selected to create a high intensity E-field able to accelerate charged particles. The E-field is created by tightly focusing a substantially radially polarized laser beam (TM01 mode) with a numerical aperture (NA) optic of at least 0.5. Contrary to a linearly polarised beam (TEM00 mode), in which the total longitudinal component of the electric field is close to zero at the focus, the different electric field components of the radially polarized laser beam (TM01 beam) superpose in a constructive way along the propagation axis. At high intensities, the longitudinal electric field component can accelerate the electrons located in the focal region along the propagation axis, while the transverse field components (electric and magnetic) help maintain the particles close to the beam propagation axis. The intensity of the resulting longitudinal field is dependent upon the peak power of the pulse, and the numerical aperture (NA) of the focusing optics.

#It was shown by theoretical work that under tight focusing conditions such as those obtained with a high-aperture parabolic mirror, charged particles initially at rest on the optical axis may be accelerated to mega-electron-volt energies at peak powers as low as 10 GW with few-cycle pulses. It was shown that at higher peak powers, the use of shorter pulses yields a more efficient acceleration.