Measurements of Ultimate Accelerating Gradients in the SLAC Disk-Loaded Structure
This paper is a status report on an on-going program at SLAC to study accelerator structures under high-gradient electric field conditions. The study is a part of a much broader program dealing with future linear colliders. The accelerating gradient that might be achievable in such machines is a crucial parameter because once the beam energy is selected, the gradient determines the length of the linac and directly or indirectly affects the choice of many other parameters such as frequency, energy consumption, size and number of RF sources, alignment, wakefield control, etc. The experiments done so far at SLAC deal with a conventional disk-loaded accelerator section of the constant-gradient type running at 2856 MHz. The original objective was to reach an accelerating gradient of at least 100 MV/m. The accelerating gradient at which the SLAC disk-loaded waveguide operates routinely is about 9 MV/m (with 36 MW klystrons feeding four 3 m sections without SLED) and about 12 MV/m with SLED I (2.5 sec pulse). With 50 MW tubes and SLED II (5 sec pulse) the gradient is presently being increased to over 20 MV/m. To reach 100 MV/m in a conventional 3 m constantgradient section without SLED, one would need amore » klystron with a peak power output of 900 MW. Since such a tube is not available, we decided to use a short standing-wave section in which the resonant fields can build up. The design criteria for this section, the fabrication, matching and tuning, the experimental set-up and the results are described.« less