R.M. Jones

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.

Circuit and scattering matrix analysis of the wire measurement method of beam impedance in accelerating structures

In order to measure the wake field left behind multiple bunches of energetic electrons we have previously used the ASSET facility in the SLC. However, in order to produce a more rapid and cost-effective determination of the wakefields we have designed a wire experimental method to measure the beam impedance and from the Fourier transform thereof, the wakefields. In this paper we present studies of the wire effect on the properties of X-band structures in study for the JLC/NLC (Japanese Linear Collider/Next Linear Collider) project. Simulations are made on infinite and finite periodical structures. The results are discussed.

The transverse long-range wakefield in RDDS1 for the JLC/NLC X-band linacs

The re-designed RDDS (Rounded Damped Detuned Structure) consists of 206 cells with a rounded cell profile formed by a number of circular arcs and a straight section. In the previous analyses of the present structure all cells have been assumed to be coupled to the manifold via slots cut into the cells and, a perfect match to the HOM (Higher Order Mode) couplers has been used. In the structure being fabricated, four cells on either end of the structure are decoupled to accommodate the HOM couplers and the HOM couplers have finite, frequency dependent, reflection coefficients. The wakefield is calculated incorporating both effects.

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.

Wakefield Band Partitioning in LINAC Structures

In the NLC project multiple bunches of electrons and positrons will be accelerated initially to a centre of mass of 500 GeV and later to 1 TeV or more. In the process of accelerating 192 bunches within a pulse train, wakefields are excited which kick the trailing bunches off axis and can cause luminosity dilution and BBU (Beam Break Up). Several structures to damp the wakefield have been designed and tested at SLAC and KEK and these have been found to successfully damp the wakefield [1]. However, these 2{pi}/3 structures suffered from electrical breakdown and this has prompted us to explore lower group velocity structures operating at higher fundamental mode phase advances. The wakefield partitioning amongst the bands has been found to change markedly with increased phase advance. Here we report on general trends in the kick factor and associated wakefield band partitioning in dipole bands as a function of phase advance of the synchronous mode in linacs. These results are applicable to both TW (travelling wave) and SW (standing wave) structures.

Dipole wakefield suppression in high phase advance detuned linear accelerators for the JLC/NLC designed to minimise electrical breakdown and cumulative BBU

Recent experiments at SLAC and CERN have revealed evidence of significant deformation in the form of pitting of the cells of the 1.8 m series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned Structure). This pitting occurs in the high group velocity (v/sub g//c = 0.012) end of the accelerating structure and little evidence of breakdown has been found in the lower group velocity end of the structure. Additional, albeit preliminary experimental evidence , suggests that shorter and lower group velocity structures have reduced breakdown events with increasing accelerating field strengths. Two designs are presented here, firstly a 90 cm structure consisting of 83 cells with an initial v/sub g//c = 0.0506 (known as H90VG5) and secondly, an even shorter structure of length 60 cm consisting of 55 cells with an initial v/sub g//c = 0.03 (known as H60VG3). The feasibility of using these structures to accelerate a charged beam over 10 km is investigated. The particular issue focussed upon is suppression of the dipole wakefields via detuning of the cell frequencies and by locally damping individual cells in order to avoid BBU (Beam Break Up). Results are presented on beam-induced dipole wakefields and on the beam dynamics encountered on tracking the progress of the beam through several thousand accelerating structures.

Manifold damping of transverse wakefields in high phase advance traveling wave structures and local damping of dipole wakefields in standing wave accelerators

Operating the SLAC/KEK DDS (damped detuned structure) X-band linacs at high gradients (in excess of 70 MV/m) has recently been found to be limited by the accelerator structures breaking down and as a consequence severe damage occurs to the cells which makes the structures inoperable. A series of recent experiments at SLAC indicates that arcing in the structures is significantly reduced if the group velocity of the accelerating mode is reduced and additionally it has been discovered that reducing the length of the accelerating structure also limits the number and intensity of breakdown events. However, in designing new accelerating structures care must be taken to ensure that the beam-induced transverse wakefields do not cause the beam to become unstable. Here, we report on damping transverse wakefields in two different short structures: a 90 cm traveling wave structure in which the wakefield is coupled out to four attached manifolds and secondly, in a standing wave structure in which a limited number of cells heavily damp down the wakefield.

Including internal losses in the equivalent circuit model of the SLAC damped detuned structure (DDS)

In the equivalent circuit model for the DDS originally presented no losses were explicitly included in the cell circuits or the manifold circuits. Damping via the manifolds was effected by imposing matching conditions (including the possibility of reflection) on the ends of the manifolds. In this paper we extend the circuit theory to include lossy circuit elements. We discuss and compare shunt conductance and series resistance models for the cells. Manifold damping is modeled by introducing a shunt conductance per unit length in the transmission line elements of the manifolds. We apply the theory to the mitigation of performance degradation associated with fabricationally desirable decoupling of several cells at the ends of the structure from the manifolds.

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.

Energy dispersion compensation and beam loading in X-band linacs for the JLC/NLC

The shape of an RF pulse is distorted upon propagating through an X-band accelerator structure due to dispersive effects. This distortion together with beam loading introduce energy spread between 192 bunches. In order to minimize this energy spread we modify the input RF pulse shape. The pulse propagation, energy gain, and beam loading are modelled with a mode-matching computer code and a circuit model. A 2D model and a circuit model of a complete 60 cm structure, consisting of 55 cells and input and output couplers is analyzed. This structure operates with a 5/spl pi//6 phase advance per cell. Dispersive effects for this structure are more significant than for previously studied 2/spl pi//3 phase advance accelerating structures. Experimental results are compared with the theoretical model and excellent agreement is obtained for the propagation of an RF pulse through the structure.