ACHIP: Electron acceleration with laser light
Accelerators on a chip: Dielectric laser accelerators
Particle accelerators are exciting research tools that provide energetic high brightness charged particle beams that can be used to probe physical phenomena otherwise inaccessible. However, the enormous cost and user demand of many of the radio-frequency based accelerators limits their availability. To provide an accelerator accessible on the university-lab scale, a novel accelerator design must be developed.
One such design, whose development is funded by the Gordon and Betty Moore Foundation (press release), is the Dielectric Laser Accelerator or DLA for short. DLAs leverage both the GV/m electromagnetic fields of commercially available lasers and the advanced nanofabrication techniques of dielectric materials developed in the semiconductor industry. The large available electromagnetic fields are used to create acceleration gradients that exceed those in radio frequency accelerators by a factor of 100. Even though DLAs are orders of magnitude smaller than their RF brethren, their accelerating gradients allow for DLAs to impart similar energy gains to charged particles. However, instead of imparting these energy gains over meters, DLAs impart these energy gains over millimeters, potentially enabling a university lab-scale particle accelerator.
An example of a dielectric laser acceleration element, which sits on the thin strip in the center of the silicon piece shown here. The silicon piece is upon a cent for scale. (Image: FAU/Joshua McNeur)
A dielectric laser acceleration element imaged at high magnification, with a strand of hair for scale. (Image: FAU/Joshua McNeur)
The DLAs tested at FAU operate in a phase-synchronous scheme, as follows. An incident laser pulse impinges upon a nanofabricated accelerating structure, exciting travelling near-field modes. Electrons, propagating in close proximity to the structure, are accelerated when their velocity matches the phase velocity of one of the traveling modes. This synchronicity condition and general principle is similar to that employed in large-scale RF accelerators, and the structures are designed to satisfy this condition (see video produced by our SLAC partners on the dual-grating approach).
A laser impinges upon the silicon grating (with the structural period and grating height d indictated) from above, exciting the travelling wave mode depicted by the blue and red regions in the bottom image that shows a cross section of two periods of the accelerator. An electron, depicted by the green circle and travelling to the right, surfs the travelling wave, also moving to the right. (Image: FAU/Joshua McNeur)
Already there have been repeated confirmations that this principle of acceleration works over a wide range of electron velocities (from 15% to 100% of the speed of light) and lasers [1,2,3,4]. The ACHIP collaboration, see below, aims to extend the success of DLAs towards the realization of the analogy of an accelerator beamline. Multiple stages of dielectric-laser based acceleration, focusing, and diagnostics are being developed and tested. Specifically, auxiliary dielectric elements can be integrated [4,5] to additionally provide spatial focusing, steering, and bunching of electron pulses, important elements making up an essential toolbox in any particle accelerator.
(a) Scanning electron microscope image of the focusing element fabricated from Si. The radius of curvature at the vertex of the parabola is 2.5 μm. (b) Accelerated electrons that traverse the lens structure above the parabolic vertex are deflected downwards, and those that traverse below the vertex are deflected upwards. The grating curvature angle and electron beam deflection angle are shown in the inset in (d). (c) Spatial profile of the accelerated electrons measured with a knife edge scan. (d) Position of the centroid of the accelerated spatial distribution at the location of the knife edge as a function of electron beam position on the structure, with linear fits for each minimum energy gain setting (Image: FAU/Joshua McNeur, from )
A laser-triggered electron cathode appropriate for operation with DLAs will eventually be incorporated with the multiple stages, resulting in a beamline where electrons are generated via laser-triggered emission, and then alternatingly accelerated, collimated, and diagnosed with sequential DLA-based devices. A hypothetical schematic of such a device is shown below.
A DLA-based Linac, consisting of a single drive laser, a laser-triggered electron source (A), subrelativistic electron accelerating sections (B1-B3), focusing/collimating dielectric laser elements (F1-F3), a speed of light accelerator (C) and a dielectric-laser based undulator (U) capable of generating XUV light. (Image: FAU)
The resulting beam may be used as a high brightness light source via the incorporation of an element that wiggles the electron beam transverse to its direction of motion, creating photons as the beam alternatingly curves upwards and downwards. The compact size of such a beamline and its various components allows for many exciting applications, ranging from handheld MeV electron sources for tumor irradiation to table-top Free Electron Lasers .
Ponderomotive interaction of laser fields and electron and attosecond electron pulse train generation
An alternative solution to electron pulse bunching using nanostructures involves the second-order all-optical ponderomotive scheme, in which two crossed laser beams generate an optical wave, either standing or co-propagating with the electron pulse. Free electrons impinging on such a transvers intensity grating will predominantly diffract as in the Kapitza-Dirac effect, or strongly disperse in a longitudinal grating . In the latter case, a relatively long (~100 fs) electron pulse may be structured into a train of attosecond bunches with lengths below 300 as . Such highly-bunched electron trains may be interesting not only for acceleration, but also for the ultrafast time-resolved probing of structural and chemical changes in atoms, molecules, and solids.
Very recently, we have achieved very similar results also in a nearfield scheme, so with the help of the photonic accelerator structures.
Accelerator on a Chip International Program — ACHIP
ACHIP, generously funded by the Gordon and Betty Moore Foundation and commenced on November 2015, is headed by Stanford and our group at FAU. See here for the Stanford ACHIP web site. Next to Stanford and FAU, the following groups are members of ACHIP:
DESY (R. Assmann, I. Hartl)
EPFL (L. Rivkin)
Hamburg University (F. Kaertner)
PSI (R. Ischebeck)
Purdue University (M. Qi)
SLAC (J. England, S. Tantawi)
Stanford University (B. Byer, S. Fan, J. Harris, O. Solgaard, J. Vuckovic)
Tech-X (B. Cowan)
TU Darmstadt (O. Boine-Frankenheim)
UCLA (P. Musumeci)
Industrial affiliate: Hamamatsu Photonics
The members of the Accelerator on a Chip International Program, ACHIP (Erlangen, 2016)
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 E. A. Peralta, K. Soong, R. J. England, E. R. Colby, Z. Wu, B. Montazeri, C. McGuinness, J. McNeur, K. J. Leedle, Walz, E. B. Sozer, B. Cowan, B. Schwartz, G. Travish R. L. Byer, “Demonstration of Electron Acceleration in a Laser-Driven Dielectric Micro-Structure,” Nature 503, 7474 (2013).
 K. J. Leedle, A. Ceballos, H. Deng, O. Solgaard, R. Pease, R.L. Byer, J. Harris, “Dielectric Laser Acceleration of sub-100 keV Electrons with Silicon Dual Pillar grating Structures,” Optics Letters 40 18 (2015).
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 M. Kozák, T. Eckstein, N. Schönenberger & P. Hommelhoff „Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum,“ Nat. Phys. 14, 121 (2017).
 M. Kozák, N. Schönenberger, and P. Hommelhoff, “Ponderomotive Generation and Detection of Attosecond Free-Electron Pulse Trains,”, Phys. Rev. Lett. 120, 103203 (2018).
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