DC-to-RF energy conversion in ungridded klystron gaps

The interaction between a density modulated electron beam and an ungridded RF gap is studied theoretically, using numerical methods and a digital computer. The efficiency of conversion and the optimum load conductance are calculated as a function of the normalized gap dimensions γa and γd. It is found that with commonly used gap dimensions, γa = γd = 1 radian, the efficiency is on the average 20 percentage points below the value of I
1
/2I
0
for the incident beam. To achieve higher efficiencies, γa and γd should be reduced to a value in the vicinity of 0.5. The resulting efficiency improvement is approximately 10 percentage points.