Abstract
A normal-conducting, X-band traveling wave structure operating in the dipole mode has been systematically high-gradient tested to gain insight into the maximum possible gradients in these types of structure. Measured structure conditioning, breakdown behavior, and achieved surface fields are reported as well as a postmortem analysis of the breakdown position and a scanning electron microscope analysis of the high-field surfaces. The results of these measurements are then compared to high-gradient results from monopole-mode cavities. Scaled to a breakdown rate of ${10}^{\ensuremath{-}6}$, the cavities were found to operate at a peak electric field of $154\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ and a peak modified Poynting vector ${S}_{c}$ of $5.48\text{ }\text{ }\mathrm{MW}/{\mathrm{mm}}^{2}$. The study provides important input for the further development of dipole-mode cavities for use in the Compact Linear Collider as a crab cavity and dipole-mode cavities for use in x-ray free-electron lasers as well as for studies of the fundamental processes in vacuum arcs. Of particular relevance are the unique field patterns in dipole cavities compared to monopole cavities, where the electric and magnetic fields peak in orthogonal planes, which allow the separation of the role of electric and magnetic fields in breakdown via postmortem damage observation. The azimuthal variation of breakdown crater density is measured and is fitted to sinusoidal functions. The best fit is a power law fit of exponent 6. This is significant, as it shows how breakdown probability varies over a surface area with a varying electric field after conditioning to a given peak field.
Highlights
Dipole-mode rf structures play an important role in many particle accelerators [1], but at present little is known about the limits of their operation, with all designs currently using results scaled from accelerating structures
While the difference in field is less than 10%, the breakdown rate should scale as E30; that would equate to a BDR that is 17.5 times higher in the first cell compared to the last
In dipole-mode cavities, the electric field varies as sinðφÞ, and the magnetic field varies with cosðφÞ; this fit can tell us something about field dependence of breakdown on a local scale after conditioning
Summary
Dipole-mode rf structures play an important role in many particle accelerators [1], but at present little is known about the limits of their operation, with all designs currently using results scaled from accelerating structures. This fact allows the role of each field component to be inferred from the damage location on postmortem inspection after a high-gradient test. A phase advance of 120°, with an aperture diameter of 10 mm and a disk thickness of 2 mm, was chosen as a compromise between group velocity
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