Accelerator Field Measurements at High Power

When a coupled chain of resonant cavities is driven at high peak or average power, it may not necessarily respond in the same way as it does at low power. Differences can be caused by such factors as thermal effects, glow discharge and multipactoring which are not normally present at the tuning power level. These factors may be strongly influenced by the geometry of the cavity chain. A continuing effort is being carried on at Los Alamos to make comparative measurements at both low and high power on the 805 MHz LAMPF accelerating structure. This structure and the procedures used in tuning it have been described elsewhere in these proceedings where it has been pointed out that tank-to-tank average fields must be tuned to ± 1-2% of each other for proper beam dynamics, and that the stopband of π/2 mode resonantly coupled structures must be properly set for stable high-power operation. The question naturally arises as to whether the field distribution and stop-band in tanks tuned at low power remain unchanged at high power. Some differences have been seen earlier in the distribution of electric probe readings which were set very close to each other at low power and tended to scatter rather badly at high power.