Fundamental Photoemission Brightness Limit from Disorder Induced Heating

TL;DR

This paper investigates the fundamental limit of electron temperature in photoemission caused by disorder induced heating, which constrains the brightness of dense electron beams in advanced photoinjectors.

Contribution

It introduces a novel N-body simulation approach to accurately model disorder induced heating, surpassing traditional Coulomb collision methods, and applies it to realistic photoemission scenarios.

Findings

Disorder induced heating limits electron temperature to a few meV.

Traditional Coulomb collision models fail for strongly coupled plasmas.

Correlated effects significantly influence temperature growth and cooling.

Abstract

We determine the limit of the lowest achievable photoemitted electron temperature, and therefore the maximum achievable electron brightness, due to heating just after emission into vacuum, applicable to dense relativistic or nonrelativistic photoelectron beams. This heating is due to poorly screened Coulomb interactions equivalent to disorder induced heating seen in ultracold neutral plasmas. We first show that traditional analytic methods of Coulomb collisions fail for the calculation of this strongly coupled heating. Instead, we employ an N-body tree algorithm to compute the universal scaling of the disorder induced heating in fully contained bunches, and show it to agree well with a simple model utilizing the tabulated correlated energy of one component plasmas. We also present simulations for beams undergoing Coulomb explosion at the photocathode, and demonstrate that both the temperature growth and subsequent cooling must be characterized by correlated effects, as well as correlation-frozen dynamics. In either case, the induced temperature is found to be of several meV for typical photoinjector beam densities, a significant fraction of the intrinsic beam temperature of the coldest semiconductor photocathodes. Thus, we expect disorder induced heating to become a major limiting factor in the next generation of photoemission sources delivering dense bunches and employing ultra-cold photoemitters.