High-flux monochromatic ion and electron beams based on laser-cooled atoms

We propose a source for high-brightness ion and electron beams based on the ionization of an effusive atomic beam which is transversely laser cooled and compressed. The very low transverse temperature (mK range) and the relative low density of the starting atomic sample ensure excellent initial conditions for obtaining bright and monochromatic charge sources. In contrast to the standard photoionization techniques used by similar sources, we utilize field ionization of Rydberg atoms. This approach allows a substantial reduction of the required laser power and copes differently with the problems of the energy spread created during the ionization process and of the stochastic space-charge effect. Theoretical modeling and prospective ideas of this emerging technology are given.

Figure 1

Sketch of the ultracold electron-ion source. An intense effusive atomic beam is transversely cooled and compressed using laser-cooling techniques. Electrons or ions (depending on electrode polarities) are produced by laser excitation to Rydberg states that are then field ionized. The beam is finally focused and accelerated, for instance, in a FIB column.

Figure 2

Simulations of the collimation of the Cs atomic beam after being emitted from the recirculating oven.

Figure 3

Energy levels of the sodium Rydberg state $n=20,\ell >1$ versus the electric field [74]. The dotted line is the classical ionization limit. The position of two stabilized states are shown in the two boxes. The red dot indicates a possible excitation zone and the arrow a typical path toward ionization.

Figure 4

Electrodes (with cylindrical symmetry with internal diameter of 3 mm) and axial electric field (picture from Simion software). The voltages from left to right, 789, 493, 325, 45, and 45 V, have been chosen to illustrate the $n=30$ case where a 1 kV/cm field and a 10 kV/cm/cm gradient is needed. The two curved lines are equipotential.

Figure 5

Simulation of a 10 pA electron beam produced by laser ionization of a sample of cold atoms. Left (a): trajectory simulation with electrodes of cylindrical symmetry with internal diameter of 2 mm. Right (b): spot size (for 50% of the beam particles) for several initial electron temperatures with an acceleration field of 12.5 V/cm.

Figure 6

GPT simulations of the acceleration part for of an ion source of several different currents (1 pA–10 nA), with ${\sigma }_z=1\mu$m and $R=50\mu$m. The reduced brightness of the 50% “best” particles after an acceleration along $z$ created by uniform electric fields of 100 kV/cm (a), 10 kV/cm (b), and 1 kV/cm (c).