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We present the design of a rf undulator at 91.392 GHz that has a period of 1.75 mm, while the minimum beam aperture seen by the beam is 2.375 mm. To confine the fields inside the undulator a corrugated waveguide is connected through a matching section to a linear taper and a mirror. After the mirror, a Bragg reflector and a matching section are used to reflect back all the fields leaking out of the mirror opening. This undulator requires approximately 1.4 MW for submicrosecond pulses to generate an equivalent K value of 0.1. Transferring such amounts of power at mm-wave frequencies requires overmoded corrugated waveguides, and coupling through irises creates excessive pulsed heating. We have designed a coupling scheme that allows coupling power from a highly overmoded corrugated waveguide to the undulator cavity through the beam pipe without disturbing the undulator fields. This system of the rf undulator and its coupling scheme allows for fast dynamic control of the polarization of the emitted light.

Many numerical solvers model the general nonlinear problem of particle motion in self-generated fields through the computation of particle trajectories and fields in a Lagrangian and Eulerian framework, respectively. The particle-weighting step mapping the source terms between these frameworks is a primary source of noise and computation time. Herein we eliminate this costly step by treating the trajectories as bounds on the particle flow, applying charge conservation to interpolate the source distribution. The result is an order of magnitude improvement in accuracy and an 80 to 120 fold speed-up.

Vacuum radio-frequency (rf) breakdown is one of the major factors that limit operating accelerating gradients in rf particle accelerators. The occurrence of rf breakdowns was shown to be probabilistic, and can be characterized by a breakdown rate. Experiments with hard copper cavities showed that harder materials can reach larger accelerating gradients for the same breakdown rate. We study the effect of cavity material on rf breakdowns with short $X$-band standing wave accelerating structures. Here we report results from tests of a structure at cryogenic temperatures. At gradients greater than $150\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ we observed a degradation in the intrinsic cavity quality factor, ${Q}_{0}$. This decrease in ${Q}_{0}$ is consistent with rf power being absorbed by field emission currents, and is accounted for in the determination of accelerating gradients. The structure was conditioned up to an accelerating gradient of $250\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at 45 K with $1{0}^{8}$ rf pulses and a breakdown rate of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}/\mathrm{pulse}/\mathrm{m}$. For this breakdown rate, the cryogenic structure has the largest reported accelerating gradient. This improved performance over room temperatures structures supports the hypothesis that breakdown rate can be reduced by immobilizing crystal defects and decreasing thermally induced stresses.

This paper explores the physics of vacuum rf breakdowns in subterahertz high-gradient traveling-wave accelerating structures. We present the experimental results of rf tests of 200 GHz metallic accelerating structures, made of copper and copper-silver. These experiments were carried out at the Facility for Advanced Accelerator Experimental Tests (FACET) at the SLAC National Accelerator Laboratory. The rf fields were excited by the FACET ultrarelativistic electron beam. The traveling-wave structure is an open geometry, 10 cm long, composed of two halves separated by a gap. The rf frequency of the fundamental accelerating mode depends on the gap size and can be changed from 160 to 235 GHz. When the beam travels off axis, a deflecting field is induced in addition to the longitudinal field. We measure the deflecting forces by observing the displacement of the electron bunch and use this measurement to verify the expected accelerating gradient. Furthermore, we present the first quantitative measurement of rf breakdown rates in 200 GHz metallic accelerating structures. The breakdown rate of the copper structure is 10^{-2} per pulse, with a peak surface electric field of 500 MV/m and a rf pulse length of 0.3 ns, which at a relatively large gap of 1.5 mm, or one wavelength, corresponds to an accelerating gradient of 56 MV/m. For the same breakdown rate, the copper-silver structure has a peak electric field of 320 MV/m at a pulse length of 0.5 ns. For a gap of 1.1 mm, or 0.74 wavelengths, this corresponds to an accelerating gradient of 50 MV/m.

Permanent magnet insertion devices (IDs), which are the main radiation generating devices in synchrotron light sources and free-electron lasers, use a time-invariant but space-periodic magnetic field to wiggle relativistic electrons for short-wavelength radiation generation. Recently, a high power microwave based undulator has also been successfully demonstrated at SLAC which promises the advantage of dynamic tunability of radiation spectrum and polarization. Such IDs employ transverse elecromagnetic fields which are periodic in both space and time to undulate the electrons. In this paper we develop a detailed theory of the principle of electromagnetic IDs from first principles for both linear and circular polarization modes. The electromagnetic equivalent definitions of undulator period (${\ensuremath{\lambda}}_{u}$) and undulator deflection parameter ($K$) are derived. In the inertial frame where the average momentum of the electron is zero, we obtain the figure-8-like trajectory for the linear polarization mode and the circular trajectory for the circular polarization mode. The corresponding radiation spectra and the intensity of harmonics is also calculated.

A new design of dual circular polarizer is presented in this paper. This innovative design converts radiofrequency (rf) energy from TE10 mode in a rectangular waveguide to two polarized TE11 modes in a circular waveguide. A reflection less than $\ensuremath{-}20\text{ }\text{ }\mathrm{db}$ is achieved and breakdown field is less than $42\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at input of 1 MW. Meanwhile, this polarizer has a megahertz bandwidth, and the thermal stability is also discussed. This device can be used for broadcasting and receiving the circular polarized signals.

We are exploring the physics and frequency-scaling of vacuum rf breakdowns at sub-THz frequencies. We present the experimental results of rf tests performed in metallic mm-wave accelerating structures. These experiments were carried out at the facility for advanced accelerator experimental tests (FACET) at the SLAC National Accelerator Laboratory. The rf fields were excited by the FACET ultrarelativistic electron beam. We compared the performances of metal structures made with copper and stainless steel. The rf frequency of the fundamental accelerating mode, propagating in the structures at the speed of light, varies from 115 to 140 GHz. The traveling wave structures are 0.1 m long and composed of 125 coupled cavities each. We determined the peak electric field and pulse length where the structures were not damaged by rf breakdowns. We calculated the electric and magnetic field correlated with the rf breakdowns using the FACET bunch parameters. The wakefields were calculated by a frequency domain method using periodic eigensolutions. Such a method takes into account wall losses and is applicable to a large variety of geometries. The maximum achieved accelerating gradient is $0.3\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ with a peak surface electric field of $1.5\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ and a pulse length of about 2.4 ns.

Silicon woodpile photonic crystals provide a base structure with which to build a three-dimensional dielectric waveguide system for high-gradient laser-driven acceleration. To realize an on-chip woodpile laser accelerator, a key component is the power coupler to deliver laser power to the fundamental accelerating mode. The woodpile waveguide is periodically loaded in the longitudinal direction; therefore simple cross-sectional mode profile matching is not sufficient to launch the accelerating mode appropriately and will result in significant scattering loss. Several traveling-wave coupler design schemes developed for multicell radio frequency cavity accelerators can be adapted to the woodpile accelerator coupler design. This paper presents design procedures and results using these methods. We present simulations indicating near 100% power transmission between the transverse electric mode of a silicon-guide side coupler and the transverse--magnetic-like accelerating mode of a woodpile waveguide. The coupler launches a full traveling-wave propagation of the accelerating mode, which maintains its propagation quality over long waveguide structures, and provides better tolerance on the structure fabrication uncertainty and material breakdown than standing-wave coupling.

As proposed by SLAC, the efficiency of a pulse compressor of the SLED-II type could be increased by changing both the phase of the microwave source and the coupling coefficient of the delay line. In the existing SLED-II system at frequency 11.4 GHz, the resonant delay line is coupled with the source via an iris with a constant reflection coefficient. Replacement of the iris with an active component makes it possible to create an active SLED-II system. In this paper, the use of plasma switches as the active elements is discussed. Plasma switches have been developed and tested at a high-power level for production of flattop compressed pulses. Active switching of SLED-II has demonstrated a marked increase in efficiency (by 20%) and power gain (by 37%) as compared with passive switching. The active compressor has produced 173 ns rf flattop output pulses with a power of about 112 MW.

In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV/m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV/m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV/m and a surface magnetic field of 890 kA/m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electricmore » field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV/m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.« less

In this paper, we study the RF feeding system for a set of standing-wave accelerator structures. To avoid the initial reflections produced by the structures, sometimes these structures are fed in pairs through a four-port 3-dB Hybrid. We present an extension to this system for an arbitrary number of accelerator structures and show it is always possible to cancel the reflection back to the source. The necessary and sufficient condition for this to happen depends only on the spacing between accelerator structures. In this system, the structures are not fed in a binary hierarchal system, rather in series with a set of directional couplers designed to bleed off an equal amount of power to each accelerator structure in the set. We study the sensitivity of such a system to errors resulting from the differences in accelerator structures spacing. We also study the sensitivity of the system to component imperfections, such as the finite directivity of the directional couplers, and the residual reflections from the loads that are attached to these couplers. We also study the system under fault conditions, such as a breakdown in an accelerator structure or a feed waveguide.

We present a multimode $X$-band rf pulse compression system suitable for a TeV-scale electron-positron linear collider such as the Next Linear Collider (NLC). The NLC main linac operating frequency is 11.424 GHz. A single NLC rf unit is required to produce 400 ns pulses with 475 MW of peak power. Each rf unit should power approximately 5 m of accelerator structures. The rf unit design consists of two 75 MW klystrons and a dual-moded resonant-delay-line pulse compression system that produces a flat output pulse. The pulse compression system components are all overmoded, and most components are designed to operate with two modes. This approach allows high-power-handling capability while maintaining a compact, inexpensive system. We detail the design of this system and present experimental cold test results. We describe the design and performance of various components. The high-power testing of the system is verified using four 50 MW solenoid-focused klystrons run off a common 400 kV solid-state modulator. The system has produced 400 ns rf pulses of greater than 500 MW. We present the layout of our system, which includes a dual-moded transmission waveguide system and a dual-moded resonant line (SLED-II) pulse compression system. We also present data on the processing and operation of this system, which has set high-power records in coherent and phase controlled pulsed rf.

Recent experience with $X$-band accelerator structure development has shown the rf input coupler to be the region most prone to rf breakdown and degradation, effectively limiting the operating gradient. A major factor in this appears to be high magnetic fields at the sharp edges of the coupling irises. As a first response to this problem, couplers with rounded and thickened iris horns have been employed and successfully tested at high power. To further reduce fields for higher power flow, conceptually new coupler designs have been developed, in which power is coupled through the broad wall of the feed waveguide, rather than through terminating irises. A ``mode-launcher'' coupler, which launches the ${\mathrm{TM}}_{01}$ mode in circular waveguide before coupling through a matching cell into the main structure, has been tested with great success. With peak surface fields below those in the body of the structure, this coupler represented a breakthrough in the Next Linear Collider structure program. The design of this coupler and of variations which use beam line space more efficiently are described here. The latter include a coupler in which power passes directly through an iris in the broad wall of the rectangular waveguide into a matching cell, also successfully implemented, and a variation which makes the waveguide itself an accelerating cell. We also discuss in some detail a couple of techniques for matching such couplers to traveling-wave structures using a field solver. The first exploits the cell number independence of a traveling-wave match, and the second optimizes using the fields of an internally driven structure.

Pulse compression systems for future linear colliders, such as NLC and JLC, involve hundreds of kilometers of waveguide runs. These waveguides are highly overmoded to reduce the rf losses. In this paper we present a novel idea for utilizing these waveguides several times by using different modes. This idea is suitable for reflective delay lines. All the modes being used have low-loss characteristics. We describe mechanically simple mode transducers that switch the propagation mode from one configuration to another with no observable dispersion. We apply this technique to a resonant delay line pulse compression system. We also present experimental results that verify these theoretical developments.

We describe development of semiconductor X-band high-power RF switches. The target applications are high-power RF pulse compression systems for future linear colliders. We describe the design methodology of the architecture of the whole switch systems. We present the scaling law that governs the relation between power handling capability and number of elements. We designed and built several active waveguide windows for the active element. The waveguide window is a silicon wafer with an array of four hundred PIN/NIP diodes covering the surface of the window. This waveguide window is located in an over-moded TE01 circular waveguide. The results of high power RF measurements of the active waveguide window are presented. The experiment is performed at power levels of a few megawatts at X-band.