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We consider an axisymmetric microwave cavity for an accelerator structure whose eigenfrequency for its second lowest TM-like axisymmetric mode is twice that of the lowest such mode, and for which the fields are asymmetric along its axis. In this cavity, the peak amplitude of the rf electric field that points into either longitudinal face can be smaller than the peak field which points out. Computations show that a structure using such cavities might support an accelerating gradient about 47% greater than that for a structure using similar single-mode cavities, without an increase in breakdown probability.

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Fast phase shifters are described that use a novel barium strontium titanate ceramic that can rapidly change its dielectric constant as an external bias voltage is changed. These phase shifters promise to reduce by $10 times the power requirements for the rf source needed to drive an energy recovery linac (ERL). Such phase shifters will be coupled with superconducting radiofrequency cavities so as to tune them to compensate for phase instabilities, whether beam-driven or those caused by microphonics. The most promising design is presented, which was successfully cold tested and demonstrated a switching speed of $30 ns for 77 deg, corresponding to <0:5 ns per deg of rf phase. Other crucial issues (losses, phase shift values, etc.) are discussed.

A novel cavity structure is described that could be the basis for a two-beam, high-gradient, accelerator. Versions of the structure could be used for acceleration of beams of electrons, positrons, muons, protons, or heavier ions; with either electron or proton drive beams. The structure embodies cavities that are excited in several harmonically related eigenmodes, such that rf fields reach their peak values only during small portions of each basic rf period. This feature could help raise breakdown and pulse heating thresholds. The two-beam accelerator structure comprises chains of these cavities. In this configuration, no transfer elements are needed to couple rf energy from the drive beam to the accelerated beam, since both beams traverse the same cavities. Purposeful cavity detuning is used to provide much smaller deceleration for a high-current drive beam, than acceleration for a low-current accelerated beam, i.e., to provide a high transformer ratio. A self-consistent theory is presented to calculate idealized acceleration gradient, transformer ratio, and efficiency for energy transfer from the drive beam to the accelerated beam, for either parallel or antiparallel motion of the beams. The theory has been cast in dimensionless quantities so as to facilitate optimization with respect to efficiency, acceleration gradient, or transformer ratio, and to illuminate the interdependence of these parameters. Means for dramatically shortening the structure fill time are also described. However, no beam dynamics analysis is presented, so the range of parameters within which this new acceleration concept can be used will remain uncertain until it is established that stable beam transport along the structure using an appropriate focusing system is possible.

The most severe problem of the international linear collider (ILC-type) is its high cost, resulting in part from the enormous length of the collider. This length is determined mainly by the achievable accelerating gradient in the RF system of the collider. In current technology, the maximum acceleration gradient in superconducting (SC) structures is determined mainly by the value of the surface RF magnetic field. In order to increase the gradient, a superconducting traveling wave accelerating (STWA) structure is suggested. Utilization of STWA structure with small phase advance per cell for future high energy linear colliders such as ILCs may provide an accelerating gradient 1.2–1.4 times larger [] than a standing wave structure. However, STWA structure requires a feedback waveguide for power redirecting from the end of the structure back to the front end of accelerating structure. Recent tests of a 1.3 GHz model of a single-cell cavity with waveguide feedback demonstrated an accelerating gradient comparable to the gradient of a single-cell ILC-type cavity from the same manufacturer []. In the present paper, high gradient test results are presented.

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