N.M. Kroll

Planar waveguide hybrids for very high power RF

Two basic designs have been developed for waveguide hybrids, or 3-dB couplers, capable of handling hundreds of megawatts at X-band. Coupling is provided by one or two connecting waveguides with h-plane junctions and matching elements. In the former case, the connecting waveguide supports two modes. Small apertures and field-enhancing e-bends are avoided to reduce the risk of RF breakdown. The h-plane symmetry also allows the use of over-moded rectangular waveguide in which the height has been increased to reduce field amplitudes without affecting the scattering matrix. The theory and designs are presented, along with the results of prototype tests of functionality and power-handling capability. Such a device is integral to the RF pulse compression or power distribution system of the Next Linear Collider (NLC) for combining, splitting, and directing power. This work was motivated by the observation of RF breakdown at power levels above 200 MW in conventional and modified magic-Ts.

Applications of Time Domain Simulation to Coupler Design for Periodic Structures

We present numerical procedures for analyzing the properties of periodic structures and associated couplers based upon time domain simulation. Simple post processing procedures are given for determining Brillouin diagrams and complex field distributions of the traveling wave solutions, and the reflection coefficient of the traveling waves by the input and output. The availability of the reflection coefficient information facilitates a systematic and efficient procedure for matching the input and output. The method has been extensively applied to coupler design for a wide variety of structures and to a study directed towards elimination of the surface field enhancement commonly experienced in coupler cells.

Numerical design and analysis of a compact TE10 to TE01 mode transducer

A high‐power low‐loss mode transducer design has been proposed to adapt the output of the X‐Band klystron, WR90 rectangular waveguide, to the input of the pulse compression system, SLED II, which utilizes overmoded circular waveguides operating in the low‐loss TE01 mode. This device is much more compact than the conventional Marie’ type mode converters. The device splits the incoming klystron output into two separate rectangular guides that are then fed into a circular guide through a four‐slot arrangement. We will use both MAFIA and HFSS to calculate the transmission properties of the three‐dimensional structure. We will also determine the extent of mode contamination and compare the numerical results with experiments.

Design parameters for the damped detuned accelerating structure

The advanced accelerating cavities for the NLCTA (and anticipated for NLC) will incorporate damping as well as detuning. The damping is provided by a set of four waveguides (which also serve as pumping manifolds) that run parallel to the structure, with strong iris coupling to each cavity cell and terminated at each end by absorbers. The previously reported equivalent circuit analysis has been refined and the dependence upon design parameters explored. We find that adequate damping can be provided by a single waveguide mode, leading to designs which are more compact than those initially considered. The design parameters and their rationale will be presented.

Room temperature accelerator structures for linear colliders

Early tests of short low group velocity and standing wave structures indicated the viability of operating X-band linacs with accelerating gradients in excess of 100 MeV/m. Conventional scaling of traveling wave traveling wave linacs with frequency scales the cell dimensions with /spl lambda/. Because Q scales as /spl lambda//sup 1/2/, the length of the structures scale not linearly but as /spl lambda//sup 3/2/ in order to preserve the attenuation through each structure. For the NLC we chose not to follow this scaling from the SLAC S-band linac to its fourth harmonic at the X-band. We wanted to increase the length of the structures to reduce the number of couplers and waveguide drives which can be a significant part of the cost of a microwave linac. Furthermore, scaling the iris size of the disk-loaded structures gave unacceptably high short range dipole wakefields. Consequently, we chose to go up a factor of about 5 in average group velocity and length of the structures, which increases the power fed to each structure by the same factor and decreases the short range dipole wakes by a similar factor. Unfortunately, these longer (1.8 m) structures have not performed nearly as well in high gradient tests as the short structures. We believe we have at least a partial understanding of the reason and will discuss it below. We are now studying two types of short structures with large apertures with moderately good efficiency including: 1) traveling wave structures with the group velocity lowered by going to large phase advance per period with bulges on the iris, 2) /spl pi/ mode standing wave structures.

A spectral function method applied to the calculation of the wake function for the NLCTA

The equivalent circuit representation of the dipole modes of the SLAC damped detuned structure (DDS) which is being fabricated at LAC has been analyzed by three different methods. The first two are based upon a modal analysis: in the first, damped modes are found by a first order perturbation in the cell to damping manifold coupling strength; while in the second, preferred when the coupling strength is large (as is the case for the SLAC structure) they are determined exactly (a time consuming procedure). The third method, which the authors report here, expresses the wake as a modal sum for modes whose frequencies place them outside the propagation bands of the manifolds (a minor contribution) plus a Fourier like integral of a spectral function over the propagation band of the manifolds (the major contribution). They will present comparisons to previous calculations, assessment of appropriate domains of applicability, and applications to the SLAC structure with matched and mismatched manifold terminations.

Some effects of the transverse stability requirement on the design of a grating linac

The transverse stability of the grating linac proposed by Palmer is analyzed. It is shown that an open structure such as a grating is always unstable transversly as long as it is uniform. The structure can be made stable by utilizing the strong focusing principle. This is achieved by periodically interrupting the grating shape. We analyze the strong focusing grating linac, and find that the stability requirement places a non‐trivial constraint on the phase acceptance of the system.

Absolute beam position measurement in an accelerator structure

Abstract High gradient acceleration requires exquisite control of beam position relative to the accelerating structure electrical center in order to avoid the excitation of transverse deflecting fields. We have measured such “absolute” beam position in a prototype X-band accelerator structure using beam induced microwave signals. We then verified our measurements independently by probing the beam excited field with a second witness bunch. Absolute beam positioning precision has been demonstrated at the level of 40 μm.

A high power cross-field amplifier at X-band

A high power cross-field amplifier (CFA) is under development at SLAC to provide sufficient peak power to feed a section of an X-band accelerator without the need for pulse compression. The CFA employs a conventional distributed secondary emission cathode and a novel anode structure which consists of an array of vane resonators alternatively coupled to a rectangular waveguide. The waveguide impedance is tapered linearly from input to output to provide a constant RF voltage at the vane tips, leading to uniform power generation along the structure. Nominal design for this tube calls for 300 MW output power, 20 dB gain, a DC voltage of 142 kV, a magnetic field of 5 kG, an anode-cathode gap of 3.6 mm, and a total interaction length of about 60 cm. The authors have used ARGUS to model the cold circuit properties and CONDOR to model the electronic power conversion. An efficiency of 60% is expected. The design effort is discussed.<<ETX>>

Analysis and application of microwave radiation from the damping manifolds of the SLAC damped detuned structures (DDS)

The power spectrum emerging from the damping manifolds of a DDS provides valuable quasi-local information on the displacement of a drive beam from the axis of individual cells, where the displacement may be due to beam offset, small cell misalignment, or a combination of the two. The degree of localization and the indexing of frequency to cell number is determined directly from the spectral function theory. Examples for specific DDS designs are presented. These relations can be used to determine geometrical misalignment patterns.