An experiment is underway to study the performance of several materials as field-emission cathodes for low voltage (<EQ 100 kV), repetitive (< 1 kHz) electron accelerators. A thyratron-switched blumlein line modulator with a 70 (Omega) characteristic impedance, and 1 microsecond(s) pulse width, is operated between 20 and 100 kV from single-shot to 300 Hz rep-rate. This provides a high average power (50 kW) test bed for the study. A comparison is made of cathodes fabricated from velvet, carbon, diamond coatings, niobium wire nanocomposite, and poly-crystalline tungsten. Surface emission is monitored by an array of Faraday cups. The `turn-on' time, uniformity of emission, and gap closure time are measured as a function of the spatially averaged, macroscopic electric field at the cathode. The carbon fiber cathode produces the largest current density and has the lowest threshold voltage for emission.