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Field emitted electrons can seriously affect the operation of high-field, high-duty factor electron accelerators. Accelerated field emission can result in high average power beams which can radiation damage beamline components. In addition, localized losses generate thermal hot spots whose outgassing degrades the ultra-high vacuum required in photoinjectors and cryomodules. However, despite their importance, the effects of field emission are rarely included in the design and engineering of electron injectors. This work attempts to remedy this situation by combining two well-known and well-documented programs, GPT and Geant4, to track electrons and their losses in an injector beamline. This paper describes a system of programs which simulates electron paths and losses along the beamline. In addition, the tracking results can be zoomed and panned along and about the beampipe envelope using an open-source 3D CAD program. The scattering albedo calculations of the combined program, GPT-Geant4, are shown to be in good agreement with the literature albedos. The paper concludes with a dark current simulation for the LCLS-II injector from the cathode to the collimator at 1.5 m from the cathode.

The space charge limit (SCL) of emission from photocathodes sets an upper limit on the performance of both high- and low-field electron guns. Generally, one is forced to strike a compromise between the space charge limit and the cathode's intrinsic emittance (I. Bazarov et al., Phys. Rev. Lett., 102,104801(2009)). However, it is possible to nearly eliminate the SCL due to the image charge by engineering the topography of the cathode's surface. A cathode with a surface plasma frequency below the frequency spectrum of the accelerating electrons will greatly reduce the bunch's image charge or polarization of the cathode, resulting in a small image-charge field. Thereby mitigating the cathode's space charge limit. In the work presented here, a theory for the image-charge field produced by a disk of charge being accelerated from the cathode surface is developed to include the frequency-dependent behavior of surface dielectric function on the fields seen by the beam. The paper applies this theory to mitigate the SCL of a novel cathode based upon a wire-array metasurface. It is shown that such meta-cathodes should have negligible space charge limits for photoelectric, thermionic and field emission., Comment: 20 pages and 13 figures

This paper discusses the emittance growth produced by sextupole fields in conjunction with a solenoidal focusing. The sextupole emittance is derived and compared with numerical simulations using a measured sextupole field for a vacuum chamber in a solenoidal field. It is found that the sextupole emittance is dominated by the field's radial non-linearity and the growth due to the skew angle is much smaller. It is shown that a corrector sextupole placed after the sextupole-solenoid system is effective at cancelling this emittance growth. Leaving only the growth due to the solenoid's spherical aberration., Comment: 6 pages, 6 figures

Small, stray magnetic and RF fields in electron guns and injectors can perturb an electron beam and introduce correlations between the otherwise orthogonal transverse trajectories. These correlations couple the x and y dynamics which increases the transverse emittance. If the correlation becomes "diluted" or randomized in the beam transport then as the correlation disappears the 4D emittance increases. This paper discusses two important correlations common to most electron injectors. The first results from the coupling of a weak quadrupole field with beam rotation in a solenoid, and the second x-y coupling is generated by an asymmetric on-axis RF field due to a high-power RF coupler or cavity port. This paper shows that a small quadrupole field combined with solenoidal focusing can result in significant emittance growth due to coupled transverse dynamics. It also shows how adding a skewed quadrupole field can exactly cancel this correlation and its emittance. Similar emittance cancellation is demonstrated for asymmetric RF fields, with the degree of cancellation limited by the electron bunch length. Analytic expressions are derived and compared with simulations and experiments., Comment: 18 pages, 11 figures

Advances in electron beam technology have been central to creating the current generation of x-ray free electron lasers and ultra-fast electron microscopes. These once exotic devices have become essential tools for basic research and applied science. One important beam technology for both is the electron source which, for many of these instruments, is the photocathode gun. The invention of the photocathode gun and the concepts of emittance compensation and beam matching in the presence of space charge and RF forces have made these high-quality beams possible. Achieving even brighter beams requires taking a finer resolution view of the electron dynamics near the cathode during photoemission and the initial acceleration of the beam. In addition, the high-brightness beam is more sensitive to degradation by the optical aberrations of the gun's field and the magnetic lenses. This paper discusses these topics including the beam properties due to photoemission physics, space charge effects close to the cathode, and optical distortions introduced by the RF and solenoid fields. Analytic relations for these phenomena are derived and compared with numerical simulations., Comment: Added section with figure after Introduction. Edited text, equations and figures throughout manuscript

It is well-known that the electron beam quality required for applications such as FELs and ultra-fast electron diffraction can be degraded by the asymmetric fields introduced by the RF couplers of superconducting linacs. This effect is especially troublesome in the injector where the low energy beam from the gun is captured into the first high gradient accelerator section. Unfortunately modifying the established cavity design is expensive and time consuming, especially considering that only one or two sections are needed for an injector. Instead, it is important to analyze the coupler fields to understand their characteristics and help find less costly solutions for their cancellation and mitigation. This paper finds the RF coupler-induced emittance for short bunches is mostly due to the transverse spatial sloping or tilt of the field, rather than the field's time-dependence. It is shown that the distorting effects of the coupler can be canceled with a static (DC) quadrupole lens rotated about the z-axis.

The Linac Coherent Light Source (LCLS) is the first x-ray laser user facility based upon a free electron laser (FEL). In addition to many other stringent requirements, the LCLS XFEL requires extraordinary beam quality to saturate at 1.5 angstroms within a 100 meter undulator.[1] This new light source is using the last kilometer of the three kilometer linac at SLAC to accelerate the beam to an energy as high as 13.6 GeV and required a new electron gun and injector to produce a very bright beam for acceleration. At the outset of the project it was recognized that existing RF guns had the potential to produce the desired beam but none had demonstrated it. This paper describes the analysis and design improvements of the BNL/SLAC/UCLA s-band gun leading to achievement of the LCLS performance goals.

Since the discovery of electrons more than a century ago, the generation, transport and characterization of electron beams has been an active field of research. Breakthroughs in this field have led to applications as far reaching as cancer treatment, investigation of nanoscale material and dark matter. In this book, we present a snapshot of the photoinjector technology which has contributed to these advances by providing an introduction to the basic theory and engineering of state-of-the-art electron beam sources. Photoinjectors produce relativistic electrons for exciting new devices such as x-ray free electron lasers and the polarized beams for very high energy physics linear colliders. The chapters are written by renowned experts in the field who share their working knowledge of the technologies needed for designing and building photoinjectors. This 335 page book covers the following topics: Photoinjector Theory, Normal Conducting RF Injectors, Superconducting RF Photoinjectors, DC/RF Injectors, Photocathode Theory, Metal Cathodes, Semiconductor Photocathodes for Unpolarized Beams, Cathodes for Polarized Electron Beams, Laser Systems, RF Systems and Beam Diagnostics., Comment: This is a 335 page book

This paper examines the question of the hereditary nature of intelligence and the validity of some of the statistical procedures which have been used in measuring the degree of hereditability. The author feels that proof of the question lacks sufficient scientific rigor for the support of any conclusion, particulary for a question of such political and emotional importance. (CTM)

This paper deals with the evaluation of an out-of-school tutorial program, CROSSROADS, which was funded under the U.S. Department of Education's Basic Skills Improvement Program. Like many projects funded under the Basic Skills program, CROSSROADS was terminated after two years even though it had been funded with the expectation of four years for program development and demonstration. Subjects of the evaluation were 166 tutees from four junior high schools in Long Beach, California. Progress rates of the tutees were measured by pre- and posttests, standardized tests, school grades and criterion referenced measures. Evaluation results suggested that tutees learned some basic skills but not enough to impact upon standardized test scores or grades. Results also suggested that out-of-school basic skills programs will have a difficult time in producing substantial gains in basic skills in lower achieving children due to problems with tutee motivation and logistics of running out-of-school programs. Although the evaluation did not substantiate strongly the effectiveness of the CROSSROADS model for basic skill improvement, findings and issues are pertinent to efforts to improve basic skills and to educational policies and evaluation practice. (Author/JAZ)