Fig. 6. Theoretical mode chart based on Eq. (1). Compare this with the actual test results shown in Fig. 3. The output amplitude is in arbitrary units. mode, i.e. with an accelerating bias voltage on the grids used to boost the photoelectron energy by several electron volts. Our photoemitter has a quantum yield which peaks in the visible and uses a standard glass window. With our tungsten lamp, the measured photoelectron energy spectrum peaks at about 0.5 eV. After investigating the general properties of this photoklystron we began to investigate ways to lower the minimum accelerating bias voltage for which oscillations could be obtained in hopes that we could reach the point where oscillation could be sustained by the kinetic energy from photoemission alone, about 0.5 eV. The required accelerating voltge or electron energy can be lowered by lowering the resonant frequency. In our test model, the capacitance is that of the two parallel grids and is fixed. The inductor, however, is outside the vacuum seal and can be adjusted to a low or high inductance. We found that at a frequency of 5.2 MHz our test model photoklystron will oscillate using only photoemission electron kinetic energy, that is, no external electron acceleration bias voltage is required. Switching from a tungsten lamp to a xenon lamp (a good solar spectrum approximation) greatly increases the rf output amplitude even though the CsSb photocathode material employed does not have a strong blue light response. Photoelectron kinetic energy is thus shown to be important in enhancing the oscillation amplitude. A small negative bias voltage is still required on the reflector electrode, however, since the reflector draws no current there is no energy drain on this bias supply. It may be possible to provide this bias voltage by tapping off a portion of the rf output and rectifying it. A voltage supply would be necessary to initiate oscillations but could be then removed. We suspect but have not yet confirmed that space charge effects near the photoemitter play a role in shaping the photoelectron spectrum to a peaked spectrum suitable for interaction with the rf field (Cooke, D. private communication, 1979). A cloud of very low energy photoelectrons close to the photoemitter may repel other very low energy photoelectrons thus chopping off the low energy portion of the spectrum. Colson (private communication, 1979) has shown that a peaked spectrum
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