Intrinsic Electron Beam Emittance from Metal Photocathodes: The Effect of the Electron Effective Mass

A theoretical development of prior analyses, together with our solenoid scan measurements on eight planar metal photocathodes (Ag, Be, Cr, Cu, Mo, Sn, Ta, and W) and previous data on Mg [X. J. Wang, M. Babzien, R. Malone, and Z. Wu, in Proceedings of LINAC2002, Gyeongju, Korea, 2002 (Pohang Accelerator Laboratory, Pohang, Korea, 2002), pp. 142–144.] indicate that the transverse momentum (and hence intrinsic emittance) of an electron beam is fundamentally dependent on the electron effective mass in the metal.

Figure 1

The 20 kV dc electron gun and coupled magnetic lens geometry for $\Delta p_T$ measurements.

Figure 2

The measured $\mathrm{HW}{e}^{-1}\mathrm{M}$ spot size of the electron beam generated by a Mo photocathode at different lens strengths fitted using our AG model (solid curve). Also shown is the theoretical result based on Eq. (3) (shaded region) and two images of the electron beam spot captured by our CCD array at 1.45 and 2.05 A.

Figure 3

Normalized $\Delta p_{T,\mathrm{expt}}$ vs $\sqrt{m^{*}/m_0}$ for nine metal photocathodes and the $T\to 0\mathrm{K}$ theoretical curve based on Eq. (4). Data for Mg taken from Ref. 39.