During the last decades, photocathode rf guns have been proven to be successful for providing very high quality electron beams required for vacuum ultraviolet and x-ray free-electron lasers. Beam dynamics simulations show that the electron beam quality in a rf gun depends strongly on the beam dynamics in the vicinity of the cathode. Therefore, the injection process plays a significant role in the beam performance. Several codes are available to simulate the beam dynamics in the gun. They are able to track the beam under the influence of external fields and space charge forces, but details of the emission processes are still missing in these simulations. In photocathode rf guns, the electron beams have a high charge density. Especially during emission from the cathode, the electrons have a very low velocity and experience high longitudinal space charge forces counteracting the applied accelerating field. Because of the space charge field, some part of the electrons emitted from the cathode might move backward to the cathode where they can produce secondary electrons. A high electric field in the gun cavity, on the other hand, generates a large amount of dark current. If the field-emitted electrons from the cathode or any other surface inside the cavity hit the cathode, secondary electrons can be produced as well. For a detailed understanding of the electron beam and dark current in a rf gun, simulations including a model of the secondary electron emission are necessary. In this paper, a simple model is discussed with an application to the beam dynamics at high emission phases in rf guns. Detailed simulations have been done in comparison to measurements at the Photo Injector Test Facility at DESY in Zeuthen. The primary electrons which are photoemitted from the cathode and the secondary electrons which are produced by the primaries at the cathode could be clearly distinguished in measurements and simulations.

A formulation for the characterization of superradiant and stimulated-superradiant radiative emission from bunched electron beams is presented. The radiation is characterized in terms of power and spectral power per radiation mode, which provide a measure of the useful spatially coherent radiation power and spectral power emitted by a radiation source. When the bunched electron beam emits superradiantly, these parameters scale like the square of the number of electrons, orders of magnitude more than spontaneous emission. The formulation applies to emission from single bunches, a finite number of bunches in a macropulse, or periodic bunching. It can be employed on any kind of $e$-beam radiation scheme. Specific analytic expressions are derived for coherent synchrotron radiation and prebunched free-electron laser, providing a basis for comparing and understanding their connection.

Further enhancement of the intense coherent superradiant and stimulated-superradiant emission from prebunched electron beams is possible, in schemes of prebunched beam radiation devices, and particularly free electron laser (FEL). The enhancement of coherent power and spectral power by use of a waveguide, particularly at the zero-slippage condition, is evaluated. A special scheme of a stimulated-superradiance FEL oscillator is analyzed and is shown to feature ultimate radiative energy conversion efficiency (near 100%).

In this paper, we discuss the optics effects of the realistic detector solenoid field on beam size at the interaction point (IP) of a future linear collider and their compensation. It is shown that most of the adverse effects on the IP beam size arise only from the part of the solenoid field which overlaps and extends beyond the final focusing quadrupoles. It is demonstrated that the most efficient and local compensation can be achieved using the novel method of weak antisolenoids near the IP, while a correction scheme which employs only skew quadrupoles is less efficient, and compensation with strong antisolenoids is not appropriate. One of the advantages of the proposed antisolenoid scheme is that this compensation works well over a large range of the beam energy.

Recently, resonance driving terms were successfully measured in the CERN SPS and the BNL RHIC from the Fourier spectrum of beam position monitor (BPM) data. Based on these measurements a new analysis has been derived to extract truly local observables from BPM data. These local observables are called local resonance terms since they share some similarities with the global resonance terms. In this paper we derive these local terms analytically and present experimental measurements of sextupolar global and local resonance terms in RHIC. Nondestructive measurements of these terms using ac dipoles are also presented.

The High Current Experiment at Lawrence Berkeley National Laboratory is part of the U.S. program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high intensity (line charge density $\sim 0.2\mu \mathrm{C}/\mathrm{m}$) over long pulse durations ($4\mu \mathrm{s}$) in alternating gradient focusing lattices of electrostatic or magnetic quadrupoles. This experiment is testing transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and steering, envelope matching, image charges and focusing field nonlinearities, halo, and electron and gas cloud effects. We present the results for a coasting 1 MeV ${\mathrm{K}}^{+}$ ion beam transported through ten electrostatic quadrupoles. The measurements cover two different fill factor studies (60% and 80% of the clear aperture radius) for which the transverse phase space of the beam was characterized in detail, along with beam energy measurements and the first halo measurements. Electrostatic quadrupole transport at high beam fill factor ($\approx 80\%$) is achieved with acceptable emittance growth and beam loss, even though the initial beam distribution is not ideal (but the emittance is low) nor in thermal equilibrium. We achieved good envelope control, and rematching may only be needed every ten lattice periods (at 80% fill factor) in a longer lattice of similar design. We also show that understanding and controlling the time dependence of the envelope parameters is critical to achieving high fill factors, notably because of the injector and matching section dynamics.

The electron cloud effect has been studied by means of detailed simulation codes that typically track the particles' evolution under the influence of the corresponding electromagnetic forces and fields. In this paper we show that, for the RHIC case, the electron cloud can be treated from an abstract point of view as a bunch to bunch evolution using simple maps. Secondly, we show how this treatment yields a useful conclusion, which is otherwise difficult to obtain: for a fixed number of bunches and total beam current in RHIC, it is possible to determine the best way to distribute the bunch pattern around the ring to minimize the electron cloud formation. This application is an example of how maps become a useful tool for exploring the electron cloud evolution in parameter space.

A formalism presented in a previous paper for the analysis of cumulative beam breakup (BBU) with arbitrary time dependence of the beam current and with misalignment of the cavities and focusing elements [J. R. Delayen, Phys. Rev. ST Accel. Beams 6, 084402 (2003)] is extended to include time dependence of the focusing and coupling between the beam and the dipole modes. Such time dependence, which could result from an energy chirp imposed on the beam or from rf focusing, is known to be effective in reducing BBU-induced instabilities and emittance growth. The analytical results are presented and applied to practical accelerator configurations and compared to numerical simulations.

ac dipoles in accelerators are used to excite coherent betatron oscillations at a drive frequency close to the tune. If the excitation amplitude is slowly increased to the desired value and slowly decreased back to zero there is no significant emittance growth. The aim of this article is to study the adiabaticity of the ramping process of an ac dipole as a function of the different parameters involved.

We examine the use of sextupole magnets to correct nonlinearities in the longitudinal phase space transformation of a relativistic beam of charged particles in a dispersionless translating section, or dogleg. Through heuristic analytical arguments and examples derived from recent experimental efforts, augmented by simulations using the particle tracking codes PARMELA and ELEGANT, sextupole corrections are found to be effective in optimizing the use of such structures for beam compression or for shaping the current profile of the beam, by manipulation of the second-order longitudinal dispersion. Recent experimental evidence of the use of sextupoles to manipulate second-order horizontal and longitudinal dispersion of the beam is presented. The theoretical and experimental results indicate that these manipulations can be used to create an electron bunch with a current profile having a long ramp followed by a sharp cutoff, which is optimal for driving large-amplitude wake fields in a plasma wake field accelerator.

We examine the effect of the collective force due to coherent synchrotron radiation (CSR) in an electron storage ring with small bending radius. In a computation based on time-domain integration of the nonlinear Vlasov equation, we find the threshold current for a longitudinal microwave instability induced by CSR alone. The model accounts for suppression of radiation at long wavelengths due to shielding by the vacuum chamber. In a calculation just above threshold, small ripples in the charge distribution build up over a fraction of a synchrotron period, but then die out to yield a relatively smooth but altered distribution with eventual oscillations in bunch length. The instability evolves from small noise on an initial smooth bunch of rms length much greater than the shielding cutoff.

We are concerned with coherent longitudinal motion in a storage ring, especially with situations in which coherent synchrotron radiation (CSR) can influence stability of the beam. The collective force from CSR is usually described by an impedance or a wake function in such a way that the force depends only on the charge distribution at the present time. This description is exact only for a rigid bunch, since causality demands that the force depend on the prior history of the bunch. We show how to treat a deforming bunch by applying the “complete impedance” $Z(n,\omega )$, a function of wave number and frequency. We derive this impedance and study its analytic properties for a special model: radiation from circular orbits shielded by parallel plates representing the metallic vacuum chamber. We analyze the corresponding collective force, obtaining the usual formula as a first approximation, plus easily computed corrections that depend on present and prior values of the time derivative of the charge density. In related papers we have applied these results in numerical simulations of instabilities induced by CSR.

A method of organizing electron-wave interaction at the multiplied frequency of the signal wave is proposed. This type of electron-wave interaction provides multiplied-frequency electron bunching, which leads to formation of an intense harmonic of the electron current at a selected multiplied frequency of the signal wave. This effect is attractive for the use in klystron-type cyclotron masers with frequency multiplication as a way to increase the output frequency and improve the selectivity.

High current and low emittance are principal requirements for heavy-ion injection into a linac driver for inertial fusion energy. An electrostatic quadrupole injector is capable of providing these high charge density and low emittance beams. We have modified the existing 2-MV injector to reduce beam emittance and to double the pulse length. We characterize the beam delivered by the modified injector to the High Current Transport Experiment and the effects of finite rise time of the extraction voltage pulse in the diode on the beam head. We demonstrate techniques for mitigating aberrations and reducing beam emittance growth in the injector.

Velocity bunching has been recently proposed as a tool for compressing electron beam pulses in modern high brightness photoinjector sources. This tool is familiar from earlier schemes implemented for bunching dc electron sources, but presents peculiar challenges when applied to high current, low emittance beams from photoinjectors. The main difficulty foreseen is control of emittance oscillations in the beam in this scheme, which can be naturally considered as an extension of the emittance compensation process at moderate energies. This paper presents two scenarios in which velocity bunching, combined with emittance control, is to play a role in nascent projects. The first is termed ballistic bunching, where the changing of relative particle velocities and positions occur in distinct regions, a short high gradient linac, and a drift length. This scenario is discussed in the context of the proposed ORION photoinjector. Simulations are used to explore the relationship between the degree of bunching, and the emittance compensation process. Experimental measurements performed at the UCLA Neptune Laboratory of the surprisingly robust bunching process, as well as accompanying deleterious transverse effects, are presented. An unanticipated mechanism for emittance growth in bends for highly momentum chirped beam was identified and studied in these experiments. The second scenario may be designated as phase space rotation, and corresponds closely to the recent proposal of Ferrario and Serafini. Its implementation for the compression of the electron beam pulse length in the PLEIADES inverse Compton scattering (ICS) experiment at LLNL is discussed. It is shown in simulations that optimum compression may be obtained by manipulation of the phases in low gradient traveling wave accelerator sections. Measurements of the bunching and emittance control achieved in such an implementation at PLEIADES, as well as aspects of the use of velocity-bunched beam directly in ICS experiments, are presented.

Measurement of the residual betatron coupling with skew quadrupole modulation is a new diagnostics technique. It was developed and tested at the Relativistic Heavy Ion Collider (RHIC) as a promising method for measuring coupling on the ramp. By modulating the strengths of skew quadrupole families, the two tunes' responses are precisely measured with the phase lock loop system. The projection ratio of the residual coupling coefficient onto the coupling modulation direction can be determined. In this article, the analytical solution to the skew quadrupole modulation is given. Dedicated beam studies were carried out in RHIC Run'04 and the results are presented. The ability to measure coupling on the ramp opens the possibility of continuously correcting coupling during acceleration.

Commissioning of two large coherent light facilities (XFELs) at SLAC and DESY should begin in 2008 and 2011, respectively. In this paper we look further into the future, hoping to answer, in a very preliminary way, two questions. First: What will the next generation of XFEL facilities look like? Believing that superconducting technology offers advantages such as high quality beams with highly populated bunches, the possibility of energy recovery and higher overall efficiency than warm technology, we focus this preliminary study on the superconducting option. From this belief the second question arises: What modifications in superconducting technology and in the machine design are needed, as compared to the present DESY XFEL, and what kind of research and development program should be proposed to arrive in the next few years at a technically feasible solution with even higher brilliance and increased overall conversion of ac power to photon beam power? In this paper we will very often refer to and profit from the DESY XFEL design, acknowledging its many technically innovative solutions.

A theory of nonrelativistic, laminar, elliptic-beam formation is presented. The beam is shown to have the one-dimensional Child-Langmuir flow form. An analytic expression for the electrostatic potential outside the beam is derived. Equipotentials corresponding to electrode surfaces are computed numerically. The effectiveness of the electric field formed by the electrodes in focusing and preserving the elliptic, 1D Child-Langmuir flow beam is verified via 3D ray-tracing simulations.

For magnetic alloy (MA) loaded rf accelerating cavity, we have developed an indirect cooling system, which is effective for retaining its shunt impedance by cooling only one side of the MA cores. Because of its low-$Q$ high-permeability property, MA cores are suitable for constructing untuned broadband accelerating systems. Since these same cores are made of wound thin tape, the key to establishing an untuned broadband cavity with effective indirect cooling method is to suppress the reduction of core impedance when attaching metallic cooling plates to the core and keeping a good thermal contact between them. We have employed indium bonding to thermally connect both cores and cooling plates. Both cooling and endurance tests have demonstrated its successful results.

Using a simple field evaporation model and molecular dynamics simulations of nanoscale copper tip evolution in a high electric field gradient typical for linacs, we have studied a new mechanism for rf-field evaporation. The mechanism consists of simultaneous (collective) field evaporation of a large group of tip atoms in high-gradient fields. Thus, evaporation of large clusters is energetically more favorable when compared with the conventional, “one-by-one” mechanism. The studied mechanism could also be considered a new mechanism for the triggering of rf-vacuum breakdown. This paper discusses the mechanism and the experimental data available for electric field evaporation of field-emission microscopy tips.