Breakdown Studies in Room Temperature Electron Linac Structures

This paper is an overall review of studied carried out by the authors and some of their colleagues on RF breakdown, Field Emirrion and RF processing in room temperature electron linac structures. The motivation behind this work is twofold: in a fundamental way, to contribute to the understanding of the RF breakdown phenomenon, and as an application, to determine the maximum electric field gradient that can be obtained and used safely in future c* linear colliders. Indeed, the next generation of these machines will have to reach into the TeV (10’1 eV) energy range, and the accelerating gradient will be one of the crucial parameters affecting their design, construction and cost. For a specified total energy, the gradient sets the accelerator length, and once the RF structure, frequency and pulse repetition rate are selected, it also determines the peak and average power consumption. These three quantitien are at the heart of the ultimate realizability and cost of these accelerators. Recent overall parameter studies for a 1 TeV c* linear collider’ indicate that accelerating gradients on the order of 100 to 200 MV/m are desirable and that the range of frequencies wherein such gradients would be affordable is 10 to 20 GHz. On the basis of the work reported here, it now appears that such performance is possible, at least in short and simple disk-loaded structures. Considerable insight has been gained into the mechanisms surrounding the complex and still elusive breakdown phenomenon. As will be seen, however, more work is needed to gain further understanding into the underlying physics, to determine if the required gradients are achievable in long and complicated structures, and to verify that the accompanying field emitted currents which can absorb power, cause parasitic wakefields and spurious x-rays along the accelerator, are tolerable. Accelerating Structures Tested All experiments reported here, except for one that was started recently and is still incomplete, were performed on resonant standing-wave structures consisting of one or a few cavities. The use of such structures was made necessary by the present unavailability of the extremely high peak power sources that will eventually be required (500 to 1000 MW/m). The accelerating gradient in a short standing-wave section of length L, shunt impedance per unit length rsw , and peak input power P is given by