Based on experience gained from present machines, a new generation of accelerators with high intensities and low losses is being designed. For example, the design for the Spallation Neutron Source storage ring specifies fractional beam losses of the order of ${10}^{-4\mathrm{}}$, so that even small instabilities or resonances can lead to the violation of this number. The purpose of this paper is to show that there exist potentially important beam fields, missing from standard analyses, that can lead either to instabilities or to large deviations of instability thresholds from their conventional values. Some of these fields and related effects, e.g., “fast damping,” were discovered earlier and are presented here in more standard form as an extension of the wake field's definition. In addition, nonrelativistic collective phenomena are analyzed. It is shown that the nonrelativistic case could be significantly different from the relativistic case.

Initial experiments which have explored the physics of the underdense (blowout) regime of the plasma wakefield accelerator (PWFA) at the Argonne Wakefield Accelerator facility are reported. In this regime, the relativistic electron beam is denser than the plasma, causing the beam channel to completely rarefy, and leaving a high quality accelerating region which also contains a uniform ion column. This ion column in turn allows the drive and accelerating beams to be well guided over many initial beam beta-function lengths. The results of these experiments, which have taken place over several years, are reviewed. Notable achievements in the course of these studies include the creation and measurement of drive and witness beam generated in an rf photoinjector, as well as previously published studies on drive beam guiding in the underdense regime. In addition, these experiments allowed measurement of both beam energy loss and gain, at a maximum average rate of 25 MeV/m in this regime of the PWFA, which is consistent with a peak acceleration gradient of 62 MeV/m in the excited waves. Difficulties associated with this type of experiment are discussed, as are prospects for mitigating these difficulties and achieving high gradient acceleration in planned future experiments.

High-speed digital filter systems for bunch-by-bunch feedback systems have been developed at KEK. A two-tap finite impulse response filter with simple hardware realizes the functions of a 90° phase shift, suppression of the static component and digital delay of up to a few hundred turns for the KEKB rings. Difficulties in the circuit board, such as the trimming of the timing skews or the problem of long-term reliability, have been solved using custom GaAs large-scale integrated circuits which demultiplex and multiplex fast parallel-digital data coming from the analog-to-digital converter and going to the digital-to-analog converter. Two major applications of the filter board, the bunch current monitor and the bunch oscillation recorder with 20 MB memories for transient-domain analysis of the instabilities, are also described.

We show that the effect of BNS damping for the fast ion instability found in the recent paper “Natural BNS damping of the fast ion instability” [Phys. Rev. ST Accel. Beams 2, 044403 (1999)] occurs only when initial perturbations in the beam are amplified by many orders of magnitude, and, most likely, the linear approximation breaks down. We also demonstrate that the asymptotic solution found by Raubenheimer and Zimmermann gives a good approximation for the linear regime of the instability contrary to the statement of Pestrikov.

A barrier bucket experiment with two dedicated barrier cavities was performed at the Brookhaven AGS. One of the barrier cavities was a magnetic alloy (MA)–loaded cavity and the other was a ferrite-loaded cavity. They generated a single sine wave with a peak voltage of 40 kV at a repetition rate of 351 kHz. A barrier rf system was established with these cavities and five bunches from the AGS booster were accumulated. A total of $3\times {10}^{13}$ protons were stored without beam loss, and were successfully rebunched and accelerated. The longitudinal emittance growth was observed during accumulation by the barrier bucket, the blowup factor of which was about 3. The longitudinal mismatch between the rf bucket and the beam bunch was the main reason for the emittance growth. The potential distortions by beam loading of the ferrite cavity and the overshooting voltage of the MA cavity disturbed the smooth debunching.

In this paper, we propose a new method to evaluate the longitudinal coupling impedance of a charged particle passing perpendicularly through the center of a plane conducting ring. It is shown that the solution of the problem can be expressed as a double Neumann series, whose expansion coefficients can be easily computed. Moreover, we show the causality of the wake potential for a particle traveling at the speed of light.

Optical transition radiation (OTR) has proven to be a versatile and effective diagnostic for measuring the profile, divergence, and emittance of relativistic electron beams with a wide range of parameters. Diagnosis of the divergence of modern high brightness beams is especially well suited to OTR interference (OTRI) techniques, where multiple dielectric or metal foils are used to generate a spatially coherent interference pattern. Theoretical analysis of measured OTR and OTRI patterns allows precise measurement of electron beam emittance characteristics. Here we describe an extension of this technique to allow mapping of divergence characteristics as a function of transverse coordinates within a measured beam. We present the first experimental analysis of the transverse phase space of an electron beam using all optical techniques. Comparing an optically masked portion of the beam to the entire beam, we measure different angular spread and average direction of the particles. Direct measurement of the phase-space ellipse tilt angle has been demonstrated using this optical masking technique.

A computer model for the ion charge-state distribution (CSD) in an electron cyclotron resonance ion source (ECRIS) plasma is presented that incorporates non-Maxwellian distribution functions, multiple atomic species, and ion confinement due to the ambipolar potential well that arises from confinement of the electron cyclotron resonance (ECR) heated electrons. Atomic processes incorporated into the model include multiple ionization and multiple charge exchange with rate coefficients calculated for non-Maxwellian electron distributions. The electron distribution function is calculated using a Fokker-Planck code with an ECR heating term. This eliminates the electron temperature as an arbitrary user input. The model produces results that are a good match to CSD data from the ANL-ECRII ECRIS. Extending the model to 1D axial will also allow the model to determine the plasma and electrostatic potential profiles, further eliminating arbitrary user input to the model.

A simple particle-core model for circular accelerators has been constructed assuming that dispersion effects are relatively weak. This model is applicable to a large class of high-intensity rings designed for modest density applications such as spallation neutron sources. Applying this model to isotropic beams in a smooth ring, halo formation processes in the presence of dispersion are investigated. In the analysis, it is found that dispersion matching is essential to suppress horizontal beam widening in injection if momentum spread is larger than a certain threshold. Even if the beam widening due to dispersion mismatch is suppressed, a halo can be formed in the same mechanism as in a linac, namely, parametric resonance between oscillating core and single particles. The width of halos formed by the particle-core resonance in modest-density rings is found to be a little narrower than that in a straight channel, and typically 1.6 to 1.8 times the maximum core width.

First results are reported from time resolved measurements of ion currents extracted from the Frankfurt 14 GHz Electron Cyclotron Resonance Ion Source with pulsed biased-disk voltage. It was found that the ion currents react promptly to changes of the bias. From the experimental results it is concluded that the biased disk effect is mainly due to improvements of the extraction conditions for the source and/or an enhanced transport of ions into the extraction area. By pulsing the disk voltage, short current pulses of highly charged ions can be generated with amplitudes significantly higher than the currents obtained in continuous mode.

A free electron can temporarily gain a very significant amount of energy if it is overrun by an intense electromagnetic wave. In principle, this process would permit large enhancements in the center-of-mass energy of electron-electron, electron-positron, and electron-photon interactions if these take place in the presence of an intense laser beam. Practical considerations severely limit the utility of this concept for contemporary lasers incident on relativistic electrons. A more accessible laboratory phenomenon is electron-positron production via an intense laser beam incident on a gas. Intense electromagnetic pulses of astrophysical origin can lead to very energetic photons via bremsstrahlung of temporarily accelerated electrons.

A method that measures the transverse profile (emittance) of the bunch by detecting radiation arising at the scattering of the bunch on scanning wire is widely used. In this work information about bunch scattering is obtained by measuring the oscillation frequency of the tightened scanning wire. In such a way, the system of radiation (or secondary particles) extraction and measurement can be removed. The entire unit consists of a compact fork with tightened wire and a scanning system. Normal oscillation frequency of a wire depends on wire tension, its geometric parameters, and, in a second approximation, its elastic characteristics. Normal oscillations are generated by interaction of an alternating current through the wire with magnetic field of a permanent magnet. In this case, it is suggested that the magnetic field of the accelerator (field of dipole magnets or quadrupole magnets) be used for excitation of oscillations. The dependence of oscillation frequency on beam scattering is determined by several factors, including changes of wire tension caused by transverse force of the beam and influence of beam self-field. Preliminary calculations show that the influence of wire heating will dominate. We have studied strain gauges on the basis of vibrating wire from various materials (tungsten, beryl bronze, and niobium zirconium alloys). A scheme of normal oscillation generation by alternating current in autogeneration circuit with automatic frequency adjustment was selected. A special method of wire fixation and elimination of transverse degrees of freedom allows us to achieve relative stability better than ${10}^{-5\mathrm{}}$ during several days at a relative resolution of ${10}^{-6\mathrm{}}$. Experimental results and estimates of wire heating of existing scanners show that the wire heats up to a few hundred grades, which is enough for measurements. The usage of wire of micrometer thickness diminishes the problem of wire thermalization speed during the scanning of the bunch.

In the framework of the modified Bethe's diffraction theory, we study the energy lost by a relativistic particle beam traveling in a coaxial liner with many holes, including the effect of attenuation in the coaxial region. The interference among the holes is the main source of losses and is affected by the attenuation in the coaxial only over sufficiently long distances. We derive analytical formulas for all the interesting quantities and particular attention is given to clarifying the physical meaning of the results; numerical examples are considered using LHC-like parameters.

Uncontrolled beam losses due to space-charge-induced halo generation are a concern in high intensity rings, which are characterized by high beam intensities and low uncontrolled beam loss requirements. It is therefore important to investigate the dynamics of space charge in high intensity rings. We report here the results of extensive calculations using a particle-tracking approach with a self-consistent particle-in-cell model and alternatively with a particle core model. We find that the inclusion of space charge forces provides agreement between calculated and experimentally observed beam profile shapes in the high intensity proton storage ring. We also confirm computationally the extension to rings of the accepted dynamics of halo generation with rms beam mismatch exciting the parametric resonance. In addition, we propose a new two-stage mechanism for halo production in rings in which space-charge-driven lattice resonances generate beam mismatch that excites the parametric resonance. Because of its dependence on lattice resonances, this mechanism is peculiar to rings and is capable of generating halo even from initially matched beams. It is also very sensitive to the operating point in tune space, as we show in the results of a vertical tune scan simulating injection into the Spallation Neutron Source accumulator ring. Our results extend and enhance the understanding of fundamental space charge physics, which has been developed for linear accelerators, to rings.

The present analysis considers an intense non-neutral ion beam with characteristic axial velocity $V_b={\beta }_{b}c$ and directed kinetic energy $({\gamma }_b-1\mathrm{})m_b{c}^2$ propagating in the $z$ direction through an applied focusing field which produces a transverse focusing force ${\mathbf{F}}_{\mathrm{foc}}=-{\gamma }_b{m}_b{\omega }_{\beta b}^2(x{\hat{\mathbf{e}}}_x+y{\hat{\mathbf{e}}}_y)$ on a beam ion (smooth focusing approximation). For a thermal equilibrium distribution function $F_b\left(H_{\perp }^0\right)=\mathrm{const}\times \exp (-H_{\perp }^0/T_b)$, it is shown that the normalized radial density profile $\pi r_b^2{n}_b^0\left(r\right)/N_b$, plotted as a function of $r/r_b$, depends on a single dimensionless parameter, ${\delta }_b=N_b{Z}_b^2{e}^2/2{\gamma }_b^2{T}_b$, which is a measure of the normalized beam intensity. Here, $N_b\equiv \int \mathrm{dxdy}{n}_b^0\left(r\right)$ is the number of beam ions per unit axial length and $r_b^2\equiv N_b^{-1\mathrm{}}\int \mathrm{dxdy}{r}^2{n}_b^0\left(r\right)$ is the mean-square beam radius. “Universal” profiles for the beam density are presented for a wide range of system parameters. The present results are readily extended to the case of a cylindrical non-neutral plasma column confined by a uniform axial magnetic field $B_0{\hat{\mathbf{e}}}_z$.

So far, polarized proton beams have never been accelerated to energies higher than 25 GeV. During the acceleration process, the beam polarization is quite undisturbed, when the accelerator is well adjusted, except at first-order depolarizing spin orbit resonances. At some accelerators other effects have been observed but first-order resonances have always been dominant. At these resonances the spin tune plus or minus one of the orbit tunes is an integer. These beams have usually been investigated by theories which correspondingly lead to an undisturbed polarization during acceleration, except at such resonances. Therefore we speak of “first-order theories.” The first frequently used first-order theory is the single resonance model, which is usually used for simulating the acceleration process. Here the equation of spin motion is simplified drastically by dropping all but the dominant Fourier component of the driving term of that differential equation. The second frequently used first-order theory, the linearized spin-orbit motion theory, is also quite crude. It is based on a linearization of the spin and orbit equation of motion with respect to the phase space coordinates and two suitably chosen spin coordinates. Because of linearization this method cannot be used close to resonances but at fixed energies it is a useful tool. It will be shown that the validity of these first-order theories is restricted at Hadron Electron Ring Accelerator (HERA) energies of up to 820 GeV. An overview of the available theories which go beyond the first-order resonances is given and we explain which of these approaches are applicable for the analysis of polarization in the HERA proton ring. Since these theories include more than one Fourier harmonic in the driving term of the equation of motion, we refer to them as “non-first-order” or “higher-order” theories. Finally, the higher-order effects observed while simulating polarized beams in HERA with these advanced methods are illustrated.

The DARHT-2 accelerator under construction at Los Alamos National Laboratory requires a long flattop ($2\mu \mathrm{s}$) 2–4 kA, 3.5 MV, low-emittance electron beam source. The injector is being constructed at Lawrence Berkeley National Laboratory and consists of a large-area thermionic cathode mounted atop a vertical column. The 90° bend between the horizontally emitted beam and the column produces dipole and higher-pole fields which must be corrected. In addition, the fast rise of the current flowing into the vacuum tank excites rf modes which cause transverse oscillations of the beam centroid. We have modeled these effects with the 3D electromagnetic code LSP. The code has models for pulsed power transmission lines, space-charge-limited emission and transport of charged particles, externally applied magnetic fields, and frequency-dependent absorption of rf. We calculate the transverse displacement of the beam as a function of time during the current pulse, and the positioning and thickness of ferrite absorber needed to damp the rf modes. The numerical results are compared to analytic calculations.

A method is described that provides a solution to the long-standing problem of stray radiation-induced signals on photoemission-based x-ray beam position monitors (BPMs) located on insertion device x-ray beam lines. The method involves the introduction of a chicane into the accelerator lattice that directs unwanted x radiation away from the photosensitive x-ray BPM blades. This technique has been implemented at the Advanced Photon Source, and experimental confirmation of the technique is provided.

A method is presented to simulate the transport of a dc charged particle beam strongly influenced by its self-fields. If the initial phase space of the beam is chosen to be a Kapchinskij-Vladimirskij distribution, the simulation is in agreement with the Kapchinskij-Vladimirskij envelope equations. The simulation was initialized with a Gaussian transverse momentum distribution to investigate the origin of current profiles measured in a 130 keV, 635 mA plasma-focused electron beam.