Minimum intrinsic emittance of photocathodes

3 problems

• Fundamental trade-off between quantum efficiency and mean transverse energy

  Open

  Cathodes that emit many electrons per photon tend to emit them with more sideways energy. Whether this trade-off is a law of nature or a limitation of current materials is unknown.

• Is a mean transverse energy below the thermal value achievable and usable

  Contested

  The sideways energy of emitted electrons was thought to bottom out at the temperature of the cathode. Some proposals and data suggest it can go lower, but this is not established.

• How surface roughness and work-function patchiness raise intrinsic emittance

  Open

  Real cathode surfaces are bumpy and patchy at the nanometer scale, which deflects electrons as they leave. The size of this effect for real materials is not well quantified.