A novel phase focusing mechanism in multipactor discharge

In spite of the mutual repulsion among the space charges, a new phase‐focusing mechanism is discovered whereby the leading edge of the multipactor discharge in an rf circuit grows at the expense of the trailing edge. This effect arises from the different impact energies, and hence different secondary electron yields, experienced by different portions of the discharge. This phase focusing mechanism may shape the steady‐state multipactor discharge in the form of a very tight bunch of electrons. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69854/2/PHPAEN-3-5-1481-1.pd

Steady state multipactor and dependence on material properties

The interaction of multipactor discharge and an rf circuit is analyzed with the use of a simple model, in which the multipactor electrons are in the form of a single sheet that is released from the surface with a monoenergetic velocity. An explicit formula is derived for the saturation level of multipactor current in steady state. This formula is given in terms of the secondary electron yield properties of the multipactoring surfaces and the level of the external rf drive. It is valid when the quality factor QQ of the rf circuit is higher than 10, in which case the space charge effects do not contribute significantly to the saturation level. When it occurs, the steady state multipactor may consume tens of percents of the external rf power that is needed to sustain the gap voltage. Numerical computations determine the accessibility to steady state from the transient buildup. In particular, they suggest various conditions for the multipactor to exhibit in a burst mode or in a steady state mode. The dynamic linkage of the rf circuit and material properties allows the construction of the susceptibility diagram for various materials, within the limitations imposed by the present model. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70121/2/PHPAEN-4-3-863-1.pd

Beam Halo Imaging with a Digital Optical Mask

Beam halo is an important factor in any high intensity accelerator. It can cause difficulties in the control of the beam, emittance growth, particle loss and even damage to the accelerator. It is therefore essential to understand the mechanisms of halo formation and its dynamics in order to control and minimize its effects. Experimental measurement of the halo distribution is an important tool for such studies. In this paper, we present a new adaptive masking method that we have developed to image beam halo, which uses a digital micro-mirror-array device (DMD). This method has been thoroughly investigated in the laboratory using laser and white light sources, and with real beams produced by the University of Maryland Electron Ring (UMER). A high dynamic range ~10(5) has been demonstrated with this new method and recent studies indicate that this number can be exceeded for more intense beams by at least an order of magnitude. The method is flexible, easy to setup and can be used at any accelerator or light source. We present the results of our measurements of the performance of the method and images of beam halos produced under various experimental conditions.Comment: 44 pgs.; submitted to Phys. Rev. ST Accel. and Beams, 3/9/201

Multipactor discharge on metals and dielectrics: Historical review and recent theories

This paper reviews the history of multipactor discharge theory, focusing on recent models of multipactor accessibility and saturation. Two cases are treated in detail: That of a first-order, two-surface multipactor, and that of a single-surface multipactor on a dielectric. In both cases, susceptibility curves are constructed to indicate the regions of external parameter space where multipactor is likely to occur, taking into account the dependence on surface materials, and the effects of space charge and cavity loading. In the case of a dielectric, multipactor is found to deliver about 1% of the rf power to the surface. The two cases are contrasted in light of experimental observations. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71019/2/PHPAEN-5-5-2120-1.pd

Measurement and Simulation of Source-Generated Halos in the University of Maryland Electron Ring (Umer)

One of the areas of fundamental beam physics that have served as the rationale for recent research on UMER is the study of the generation and evolution of beam halos. Recent experiments and simulations have identified imperfections in the source geometry, particularly in the region near the emitter edge, as a significant potential source of halo particles. The edge-generated halo particles, both in the experiments and the simulations are found to pass through the center of the beam a short distance downstream of the anode plane. Understanding the detailed evolution of these particle orbits is therefore important to designing any aperture to remove the beam halo

Analytical methods for describing charged particle dynamics in general focusing lattices using generalized Courant-Snyder theory

The dynamics of charged particles in general linear focusing lattices with quadrupole, skew-quadrupole, dipole, and solenoidal components, as well as torsion of the fiducial orbit and variation of beam energy is parametrized using a generalized Courant-Snyder (CS) theory, which extends the original CS theory for one degree of freedom to higher dimensions. The envelope function is generalized into an envelope matrix, and the phase advance is generalized into a 4D symplectic rotation, or a U(2) element. The 1D envelope equation, also known as the Ermakov-Milne-Pinney equation in quantum mechanics, is generalized to an envelope matrix equation in higher dimensions. Other components of the original CS theory, such as the transfer matrix, Twiss functions, and CS invariant (also known as the Lewis invariant) all have their counterparts, with remarkably similar expressions, in the generalized theory. The gauge group structure of the generalized theory is analyzed. By fixing the gauge freedom with a desired symmetry, the generalized CS parametrization assumes the form of the modified Iwasawa decomposition, whose importance in phase space optics and phase space quantum mechanics has been recently realized. This gauge fixing also symmetrizes the generalized envelope equation and expresses the theory using only the generalized Twiss function beta. The generalized phase advance completely determines the spectral and structural stability properties of a general focusing lattice. For structural stability, the generalized CS theory enables application of the Krein-Moser theory to greatly simplify the stability analysis. The generalized CS theory provides an effective tool to study coupled dynamics and to discover more optimized lattice designs in the larger parameter space of general focusing lattices.open3