K. Ko

An RF cavity for the B-factory

The authors describe the proposed design for the 476 MHz accelerating cavity for the Stanford linear Accelerator Center/Lawrence Berkeley Laboratory/Lawrence Livermore National Laboratory B-factory. Use of conventional construction in copper means that careful consideration has to be paid to the problem of cooling. The need for a high shunt impedance for the accelerating mode dictated the use of a reentrant shape. This maximized the impedance of the fundamental mode with respect to the troublesome longitudinal and deflecting higher order modes, when compared to open or bell-shaped designs. A specialized damping scheme was employed to reduce the higher-order mode impedances while sacrificing as little of the fundamental mode power as possible. This was required to suppress the growth of coupled bunch beam instabilities and minimize the workload of the feedback system needed to control them. A window design capable of handling the high power was also required.<<ETX>>

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.

Manifold damping of the NLC detuned accelerating structure

In order to mitigate the reappearance of the HOM wakefield of a detuned accelerator structure and relax tolerance requirements, we propose to provide low level damping by coupling all cavities to several identical and symmetrically located waveguides (manifolds) which run parallel to each accelerator structure and are terminated at each end by matched loads. The waveguides are designed such that all modes which couple to the acceleration mode are non‐propagating at the acceleration mode frequency. Hence the coupling irises can be designed to provide large coupling to higher frequency modes without damping the acceleration mode. Because the higher order modes are detuned, they are localized and have a broad spectrum of phase velocities of both signs. They are therefore capable of coupling effectively to all propagating modes in the waveguides. Methods of analyzing and results obtained for the very complex system of modes in the accelerating structure and manifolds are presented.

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.

PEP-II B-factory prototype higher order mode load design

To reduce the impedance of the cavity higher order modes, (HOMs), a compact broad-band, low-reflection, waveguide load is required with a VSWR less than 2:1 in the frequency range 714 MHz to 2500 MHz. The load must also work in the high vacuum of the cavity, and be capable of dissipating up to 10 kW of power which is generated by the the interaction of the beam with the cavity HOMs and which is directed to each load assembly. A prototype load assembly is being fabricated which uses the lossy ceramic Al-N with 7% by weight glassy carbon to absorb the microwave power.

Impedance spectrum for the PEP-II RF cavity

The impedance spectrum presented by the PEP-II RF cavity to the beam is calculated using a 3D MAFIA model which includes the damping waveguides and the input coupler. The simulation assumes that all the ports leading out of the cavity, including the beam pipes, are terminated in matched loads. The effect of the external loading on the longitudinal impedances will be examined. This study takes into account the input coupler damping which has not been considered in previous calculations.

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>>

Recent results & plans for the future on SLAC Damped Detuned Structures (DDS)

The cells in the SLAC DDS are designed in such a way that the transverse modes excited by the beam are detuned in a Gaussian fashion so that destructive interference causes the wake function to decrease rapidly and smoothly. Moderate damping provided by four waveguide manifolds running along the outer wall of the accelerator is utilised to suppress the reappearance of the wake function at long ranges where the interference becomes constructive again. The newly developed spectral function method, involving a continuum of frequencies, is applied to analyze the wake function of the DDS 1 design and to study the dependence of the wake function on manifold termination. The wake function obtained with the actually realized manifold terminations is presented and compared to wake function measurements recently carried out at the ASSET facility installed in the SLAC LINAC.

Analysis and application of manifold radiation in DDS 1: First experiences

The cells in the SLAC DDS are designed in such a way that the transverse modes excited by the beam are detuned in a Gaussian fashion so that destructive interference causes the wake function to decrease rapidly and smoothly. Moderate damping provided by four waveguide manifolds running along the outer wall of the accelerator is utilized to suppress the reappearance of the wake function at long ranges where the interference becomes constructive again. The newly developed spectral function method, involving a continuum of frequencies, is applied to analyze the wake function of the DDS 1 design and to study the dependence of the wake function on manifold termination. The wake function obtained with the actually realized manifold terminations is presented and compared to wake function measurements recently carried out at the ASSET facility installed in the SLAC LINAC.

X-Band high power dry load for NLCTA

The specifications for the RF load to be used for absorbing the traveling wave at the end of an NLCTA accelerator section are that it be well matched over a broad band of frequencies (10% minimum), centered at 11.424 GHz, and capable of handling up to 63 MW peak and 3 kW average power. We present a ceramic (Aluminum Nitride+7% glassy carbon) load design which satisfies these requirements. It consists of a thin strip of dielectric material with a tapered surface placed inside each narrow wall of the waveguide. The complex dielectric constant of the ceramic at X-Band frequencies was first determined by measurement. Based on the measured values, a MAFIA model is used to evaluate the matching and the power dissipation. Results of the simulations indicate an optimal thickness of 0.045 in and overall length of 10 in.