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Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
The performance and operational flexibility of superconducting recirculating linacs can be limited by a variety of collective phenomena which are grouped under the name beam breakup. In this note the various beam breakup phenomena found in recirculating superconducting radio frequency linacs are described and appraised relative to beam performance.
Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
The performance and operational flexibility of superconducting recirculating linacs can be limited by a variety of collective phenomena which are grouped under the name beam breakup. In this note the various beam breakup phenomena found in recirculating superconducting radio frequency linacs are described and appraised relative to beam performance.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
As the intensity of a particle beam passing through a linear accelerator is raised, interactions between particles play an increasingly prominent role in determining the overall dynamics of the beam. These many body effects, known collectively as beam breakup, tend to degrade the quality of the transported beam, and hence they must be calculated to accurately predict the evolution of the beam as it traverses the accelerator. Several codes which compute various collective effects have been developed and used to simulate the dynamics of beams passing through superconducting accelerator structures. All the codes use the same basic algorithm: the beam is tracked through elements giving the focusing forces on the particles, and at the appropriate locations in the linac, localized forces are impressed on the particles which model the electromagnetic interactions. Here, a difficulty is that the usual ''Coulomb'' interaction between particles is changed by the electromagnetic environment of the accelerator. By such calculations it has been shown that recirculating linear accelerators such as the one being built at the Continuous Electron Beam Accelerator Facility (CEBAF) should remain stable against multipass beam breakup instability as long as the average current does not exceed about 20 mA, that the beam quality at CEBAF will be degraded when the single bunch charge approaches 109 electrons, and that the beam quality of superconducting linacs that are optimized for high current transport begins to decrease at around 101° electrons per bunch. The latter result is of interest to individuals who would use superconducting linacs as beam sources for free electron lasers or for superconducting colliders for high energy physics research.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Following the successful demonstration of energy recovery in a recirculating linac with superconducting cavities at the Jefferson Lab FEL[1], several ambitious electron accelerator projects have recently been proposed or are in study for either a light source or a collider based on this novel technology. These projects all intend to utilize a high quality linac electron beam generated with an average beam current typically in the range of 100's of mA. As is well known, a recirculating linac suffers from a beam breakup instability of a regenerative type caused by a Higher Order Mode (HOM) in an accelerating cavity, which can have a very high Q (quality factor) when superconducting. The instability can degrade the beam quality significantly. We investigate one such beam breakup instability in a recirculating linac that could arise as a result of high Q quadrupole modes excited in the cavity. In a simple model we derive a relation which allows one to deduce the upper limit on tolerable Q ...
Author: Publisher: ISBN: Category : Languages : en Pages : 2060
Book Description
Multipass beam breakup, a transverse beam instability generated by coherent excitation of cavity higher order modes in a recirculating linac, is a particular concern in designs utilizing superconducting technology. In this paper, an analytic model is presented that includes a description of this effect for a distribution of cavities along the linac with several recirculations in an impulse approximation. For N passes and M cvities, solution of the resulting equations reduces in general to finding M zeros of a 2(N-1) dimensional determinant or equivalently the M eigenvalues of an M-dimensional matrix. The parametric dependence of this system on higher order mode frequencies and Q, lattice functions, and transit times is discussed. Numerical examples are presented to clarify these issues and to model the CEBAF superconducting linac, where threshold currents are found to exceed design currents by more than an order of magnitude.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Beam breakup (BBU) instabilities in superconducting linacs are a significant issue due to the potentially high Q values of the cavity higher order modes (HOMs). The CEBAF accelerator, which employs high CW current and 5- pass recirculation through two superconducting linacs, poses unique instability problems. An experimental investigation of multipass BBU along with energy recovery has been completed using a single recirculation through the CEBAF injector linac. Experimental results are compared with computer simulation of multipass BBU.
Author: Publisher: ISBN: Category : Languages : en Pages : 15
Book Description
We describe here a study of a single bunch Beam Break Up (BBU) instability for a proposed Berkeley fast x-ray facility based on a recirculating linac [1]. The recirculating linac employs a 600 MeV superconducting RF linear accelerator and the electron beam energy of H"2.5 GeV is reached over four beam passes through the linac. A 120 MeV superconducting RF linear accelerator is used as an injector to the recirculating linac. The machine parameters are listed in Appendix A. The equation describing the transverse displacement x(s, z) of the electrons in an accelerated bunch, as a function of their longitudinal position within the bunch z, can be written in the form [2]: d/ds[[gamma](s) dx/ds] + k2(s)[gamma](s)x(s, z) = r0+"sub z}{sup {infinity}}[rho](z')W{sub {perpendicular}}(z'-z)x(s, z')dz' (1) where [gamma] is the relativistic factor, k the focusing strength, r0 the classical electron radius, [rho] the bunch density, W{sub {perpendicular}} the transverse wake function per unit length and s indicates the position along the linac. We assume infinitesimally small transverse beam dimensions (a good approximation, when the bunch dimensions are much less than the size of the beam pipe), so that x has to be interpreted as the displacement of the centre of a bunch slice. We also assume a bunch length much less than the betatron wavelength, therefore the displacement in the RHS of Eq.(1) is not retarded and, finally, we use an average transverse wake function obtained averaging the calculated short range wake of a single cavity [2] over the linac length. At first we consider the effect associated with a displacement of the electron bunch at the injection into the perfectly aligned linac. Later we extend the analysis to the case of misalignments of the linac RF cavities and cryomodules. Throughout the paper we compare the results obtained to the output of a simple tracking code, written as a Mathematical notebook. In order to keep the analytical expressions reasonably simple, we model the linac length as entirely filled with RF cavities (thus neglecting all the drift spaces, accounting for as much as one third of the total length). This causes the wakefield intensity used in the analysis to be bigger than the actual value and the average accelerating gradient to be smaller. It is shown in the paper that this corresponds to a conservative estimate of the BBU growth.
Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
Beam breakup (BBU) instabilities in superconducting linacs are a significant issue due to the potentially high Q values of the cavity higher order modes (HOMs). The CEBAF accelerator, which employs high CW current and 5-pass recirculation through two superconducting linacs, poses unique instability problems. An experimental investigation of multipass BBU along with energy recovery has been completed using a single recirculation through the CEBAF injector linac. Experimental results are compared with computer simulation of multipass BBU.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Measurements were performed of the frequencies and external Q's of the first two Higher-Order Mode (HOM) dipole passbands in the eight superconducting cavities of the cryomodule used in the recirculating linac of the driver accelerator of Jefferson Lab's IR FEL. Anomalous high-Q resonances were found, which could lead to beam instabilities at currents close to the operating current of the machine. These results led to more detailed simulations of its beam breakup (BBU) behavior. The analysis indicates that a few modes are responsible for the relatively low value of 27 mA of the threshold current. A simple modification of the superconducting cavities could lead to an increase of the threshold current by an order of magnitude.
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Author: Joseph J. Bisognano
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