ACHIP (Accelerator on a Chip International Program)

Since October 2015, our group has also been working on miniaturised laser accelerators for electrons, so-called DLAs (Dielectric Laser Accelerators).

ACHIP in the university newspaper hoch3 Forschen

The winter 2020 issue of hoch3 Forschen published the article “Particle accelerator in pocket format” about the ACHIP programme (german only).

Dielectric structure for the non-resonant acceleration of electrons with a laser, assumed here to be a plane wave.

These structures function similarly to conventional Wideroe linear accelerators, except that the frequency is scaled from several hundred megahertz to several hundred terahertz (visible or infrared spectral range). This results in a corresponding reduction of the structure into the micrometre range (see picture). With the help of the lattice teeth in the structure, the electric (near) fields are shaped in such a way that a particle arriving at the right time experiences a strong acceleration at the tooth. In the space between the teeth, the particle is only very slightly slowed down, which corresponds to a drift tube. All in all, you get a net acceleration. A very clear description can be found in this YouTube video.

Dielectric grid structure for focusing or transporting an electron beam. Top: Particle tracking, middle: Electric laser field in beam direction

The advantage of DLAs is that field strengths of >1GV/m can be generated with modern laser systems, which are already commercially available. Furthermore, dielectrics can also withstand significantly higher field strengths (breakdown-limit) than metals. Modern microfabrication technology, known from semiconductor technology, allows accelerator structures to be produced on a chip.

This development led to the measurement of record gradients of ~300MV/m at SLAC for relativistic electrons in 2013. Record gradients for the acceleration of non-relativistic electrons were also achieved at FAU Erlangen and Stanford University.

In order to develop a practical accelerator system, however, there are still some hurdles to overcome, which are defined in particular by the electron beam parameters. Due to the fact that only the near fields contribute to the acceleration, the aperture must be very small (~1µm). Furthermore, the time length of the accelerating edge is also only about 1fs, i.e. for an effective acceleration process, the beam must be bunched on this time scale. Our research is now focused on increasing the beam intensities in DLA experiments, which are still very low due to these limitations.

Since October 2015, our research group has been part of an international collaboration funded by the Gordon and Betty Moore Foundation (see press release) with the aim of realising an accelerator on the size of a chip or table. To mark the launch of the new project “Accelerator on a Chip” (ACHIP for short), an article was also written for the homepage of the ETIT department. The website of the ACHIP collaboration can be found here.

The task of our group in the collaboration is both the field simulation and the simulation of particle dynamics in dielectric accelerator structures. This includes pure tracking simulations to determine the usable aperture as well as self-consistent particle-in-cell (PIC) simulations to quantify intensity effects. Furthermore, it is also about the development of laser-driven focusing structures, which are crucial for the transport of non-relativistic electron beams. (see the following picture)