Injector for dielectric laser accelerators based on an ultracold electron source and a magnet design avoiding apparent emittance growth

TL;DR

The paper addresses the bottleneck of low injected charge in sub-relativistic dielectric laser accelerators by proposing an injector based on a ultracold electron source and a novel permanent-magnet focusing system that creates zero axial field at the source and the focus to suppress emittance growth. A multi-objective optimization framework guides the magnet design, yielding Pareto fronts that balance focal size and injected charge, with a two-magnet assembly chosen for practicality. Particle-tracking simulations show that a 0.5 fC bunch at 16.6 keV can be tightly focused into a DLA structure, yielding a final focal spot on the order of a micron and a modest energy spread, corresponding to about 3% injection efficiency into a 1 GV/m dielectric gradient (roughly 90 electrons per bunch). The approach provides a significant increase in injected charge over prior injectors and offers a general design methodology applicable to other sub-relativistic, high-brightness beam applications such as ultrafast electron diffraction and compact accelerators.

Abstract

Dielectric Laser Acceleration holds the promise of extremely high acceleration gradients in laser-driven miniaturized accelerator structures. However, sub-relativistic experimental demonstration has so far been limited by bunch charges well below 1 electron per bunch due to the low acceptance of these 'accelerator-on-a-chip' devices. Here, we propose a novel design for an injector tailor made for dielectric laser acceleration, based on just-above-threshold ionization of laser-cooled atoms. The key new feature is a an innovative magnet design that avoids apparent emittance growth by using the field-free regions of an axially polarised ring of permanent magnets as source point. We optimized this design using a multi-objective optimization approach that is also applicable to the development of other sub-relativistic, high-brightness/low-emittance electron beam setups (like setups for ultrafast electron diffraction). The expected injected bunch charge of our proposed injector is 90 (60) electrons for a dielectric laser acceleration gradient of 1 GV/m (100MV/m) operating at a 10 micron driver laser wavelength, increasing the expected bunch charge by about two orders of magnitude.

Figures (7)

• Figure 1: Field of an Axially Magnetized Ring Shaped Permanent Magnets. The field lines are plotted on a cross-section of the magnet through the symmetry axis. The plot shows the axial magnetic field $B_z$ as a function of axial coordinate $z$. The two zeros of the magnetic field are indicated at the intersection of dashed lines.
• Figure 2: Trajectories of Rb+ ions in the accelerator section. The color scheme represents the electric potential, with red representing ground and blue representing the highest negative potential. The trajectories of the Rb+ ions are overlaid in white.
• Figure 3: Illustration of the proposed injector for dielectric laser acceleration based on the ultracold electron source. The pink dot represents the laser cooled Rb atom cloud. The cylindrical negative electrode(s) are mechanically held by a corrugated HV insulator shown in blue. The grounded metallic mirror placed at 45° allows optical access as well as dumps the Rb+ beam off axis. The grating along with its support structure forms the ground reference for the accelerator. The first 'magnet' is discretized in a symmetrically patterned array of axially magnetized magnets. The second magnet is ring shaped with a rectangular cross-section. The green dot represents the focal point of the electron beam.
• Figure 4: Pareto fronts for different focal positions. Each Pareto front shows the trade-off between the two optimization criteria, i.e. focal spot size $<r>$ and bunch charge $Q$. The black dot indicates the selected design.
• Figure 5: Particle Tracking Results: The first plot shows the trajectories of a random sample of the electrons. The inset shows a smaller portion of the trajectories near the focal point. The second plot is the divergence angle of a random sampling of particle trajectories. The third plot shows the normalized emittance as calculated by the General Particle Tracer nemirrms routine.