In the Brazilian synchrotron light source (LNLS—Laboratório Nacional de Luz Síncrotron), we observed that modulating the phase of the accelerating fields at approximately twice the synchrotron frequency suppressed remarkably well a longitudinal coupled-bunch mode of the beam driven by a higher order mode in one of the radiofrequency (rf) cavities. In this work, we present the results of a set of systematic measurements, in single and multibunch mode, aimed at characterizing the effects of rf phase modulation on the beam. We compare those experiments with the results of tracking simulations and of a theoretical model in which Landau damping is the stabilizing mechanism that explains the suppression of the longitudinal coupled-bunch instability. We also measure the frequency of the stable islands created in longitudinal phase space by phase modulation and the longitudinal beam transfer function as a function of the modulation frequency and amplitude. The experimental results are in good agreement with theoretical expectations.

Generating a multidimensional crystalline beam in a storage ring has been known to be difficult without a special cooling force, i.e., tapered cooling, because of the momentum dispersion induced by bending magnets. It is, however, possible to eliminate the dispersion all around the ring by adding an electric dipole field in each magnetic bending region. A storage ring with such unique deflectors should enable us to reach multidimensional crystalline states with an ordinary untapered cooling force. In order to verify this expectation, molecular dynamics simulations are performed to study beam crystallization in several dispersion-free storage rings including the S-LSR at Kyoto University. The present results show that various crystalline states can be established without relying on the tapered force.

Specific requirements for the electron beam in harmonic cascade-free electron lasers, together with means to produce such beams, are presented. All results are illustrated with simulations and particle tracking studies.

The vertical and horizontal impedance budgets of the European Synchrotron Radiation Facility (ESRF) storage ring are calculated by element-by-element wake potential calculation. Resistive wall wakes are calculated analytically; the short range geometrical wakes are calculated by a 3D electromagnetic field solver. The effect of the quadrupolar wakes due to the flatness of most ESRF vacuum chambers is included in the model. It can well explain the sensitivity of the horizontal single bunch threshold on vacuum chamber changes, in particular, in low-gap sections of the ESRF storage ring. The values of the current thresholds on the transverse planes could be predicted correctly by the model within a factor of 2.

Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60 MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12 MeV.

This paper describes a concept for superconducting resonators for the acceleration of ions in the velocity range $\beta =v/c=0.015–0.04$. Such a resonator operates in $\lambda /4$ mode with three loading elements and so can be thought of as a triple quarter wave resonator (3-QWR) providing 4 accelerating gaps. The use of a column to support the three stubs provides a benefit beyond those of the two-stub design (2-QWR). In the 3-QWR, the rf mirror currents in the walls surrounding the stubs need only travel through $45°$ instead of the $90°$ in the 2-QWR thus further reducing the current in the demountable joints. As in the 2-QWR, the shape of the column allows control of the frequency splitting between the accelerating and other modes. The copper structure is designed to be coated by a thin superconducting film of niobium or lead for operation at 4.3 K. The particular device reported here operates at 150 MHz with an optimum $\beta$ of 0.04. Its outer cylinder is the same size and shape as for the 2-QWR structure reported previously, in order to minimize construction and cryostat costs. A simple transmission line model is presented and the results of microwave studio and other numerical analyses are discussed. The 3-QWR resonators are appropriate for the upgrade of the low-velocity sections of the ANU Heavy Ion Accelerator Facility and other heavy ion accelerator boosters.

We describe a transparent dielectric grating accelerator structure that is designed for ultrashort laser pulse operation. The structure is based on the principle of periodic field reversal to achieve phase synchronicity for relativistic particles; however, to preserve ultrashort pulse operation it does not resonate the laser field in the vacuum channel. The geometry of the structure appears well suited for application with high average power lasers and high thermal loading. Finally, it shows potential for an unloaded gradient of $10\mathrm{GeV}/\mathrm{m}$ with 10 fs laser pulses and the possibility to accelerate ${10}^6$ electrons per bunch at an efficiency of 8%. The fabrication procedure and a proposed near term experiment with this accelerator structure are presented.

Experiments at the FLASH facility at DESY have demonstrated that the higher order modes induced in superconducting cavities can be used to provide a variety of beam and cavity diagnostics. The axes of the modes can be determined from the beam orbit that produces minimum power in the dipole HOM modes. The phase and amplitude of the dipole modes can be used to obtain high resolution beam position information, and the phase of the monopole modes to measure the beam phase relative to the accelerator rf. For most superconducting accelerators, the existing higher order mode couplers provide the necessary signals, and the downmix and digitizing electronics are straightforward, similar to those for a conventional beam position monitor.

In this paper, we study the rf feeding system for a set of standing-wave accelerator structures. To avoid the initial reflections produced by the structures, sometimes these structures are fed in pairs through a four-port 3-dB hybrid. We present an extension to this system for an arbitrary number of accelerator structures and show it is always possible to cancel the reflection back to the source. The necessary and sufficient condition for this to happen depends only on the spacing between accelerator structures. In this system, the structures are not fed in a binary hierarchal system, rather in series with a set of directional couplers designed to extract an equal amount of power to each accelerator structure in the set. We study the sensitivity of such a system to errors resulting from the differences in accelerator structure spacing. We also study the sensitivity of the system to component imperfections, such as the finite directivity of the directional couplers, and the residual reflections from the loads that are attached to these couplers. We also study the system under fault conditions, such as a breakdown in an accelerator structure or a feed waveguide.

Two prototypes of superconducting, 352 MHz coaxial half-wave resonators with $\beta =0.17$ and $\beta =0.31$ have been designed, constructed, and tested at INFN-LNL, in the framework of the SPES and EURISOL DS projects. Main features of these double-wall, 2-gap structures are compactness, mechanical stability, and easy installation in different kinds of cryostats. Their acceleration capabilities are similar to the ones of Spoke resonators with similar $\beta$. These cavities are being developed for acceleration of high power hadron beams in the $5–100\mathrm{MeV}/u$ energy range.

We have developed a diagnostic system that measures the spectrally integrated (i.e. the total) energy and power radiated by a pulsed blackbody x-ray source. The total-energy-and-power (TEP) diagnostic system is optimized for blackbody temperatures between 50 and 350 eV. The system can view apertured sources that radiate energies and powers as high as 2 MJ and 200 TW, respectively, and has been successfully tested at 0.84 MJ and 73 TW on the $Z$ pulsed-power accelerator. The TEP system consists of two pinhole arrays, two silicon-diode detectors, and two thin-film nickel bolometers. Each of the two pinhole arrays is paired with a single silicon diode. Each array consists of a $38\times 38$ square array of 10-$\mu \mathrm{m}$-diameter pinholes in a 50-$\mu \mathrm{m}$-thick tantalum plate. The arrays achromatically attenuate the x-ray flux by a factor of $\sim 1800$. The use of such arrays for the attenuation of soft x rays was first proposed by Turner and co-workers [Rev. Sci. Instrum. 70, 656 (1999)]. The attenuated flux from each array illuminates its associated diode; the diode’s output current is recorded by a data-acquisition system with 0.6-ns time resolution. The arrays and diodes are located 19 and 24 m from the source, respectively. Because the diodes are designed to have an approximately flat spectral sensitivity, the output current from each diode is proportional to the x-ray power. The nickel bolometers are fielded at a slightly different angle from the array-diode combinations, and view (without pinhole attenuation) the same x-ray source. The bolometers measure the total x-ray energy radiated by the source and—on every shot—provide an in situ calibration of the array-diode combinations. Two array-diode pairs and two bolometers are fielded to reduce random uncertainties. An analytic model (which accounts for pinhole-diffraction effects) of the sensitivity of an array-diode combination is presented.

We investigate the effect of shear in the flow of charged particle equilibria that are unstable to the coherent synchrotron radiation (CSR) instability. Shear may act to quench this instability because it acts to limit the size of the region with a fixed phase relation between emitters. The results are important for the understanding of astrophysical sources of coherent radiation where shear in the flow is likely.

A technique is described for the tomographic mapping of transverse phase space in beams with space charge. Most prior studies where performed at high energy where space charge was negligible and therefore not considered in the analysis. The tomographic reconstruction process is compared with results of simulations using the particle-in-cell code WARP. The new tomographic technique is tested for beams with different intensities (both emittance and space-charge dominated), and with different initial distributions. Effects of various errors in the data collection process on the reconstructed phase space are discussed. It is shown that the crucial factor is not necessarily the number of projections but the range of angles over which the projections are taken. This study also includes a number of experimental results on tomographic phase space mapping performed on the University of Maryland Electron Ring.

A novel design of a movable mask (collimator) for high-current accelerators is proposed. The mask head is supported by a ceramic rod to reduce interference with the beam. One side of the support is coated with a thin ($\sim 1\mu \mathrm{m}$) conductive material to avoid extra charge up of the head. The head is also made of ceramics to avoid direct damage from an intense beam. To investigate the availability of the new movable mask, the rf properties, such as the impedances of trapped modes and the loss factors, were evaluated by simulation codes. The frequencies and the Q values of the trapped modes were also measured using a test model, and compared with calculated ones. Two trial modes for KEKB are being manufactured, and will be tested with beams next year.

The fixed field alternating gradient (FFAG) accelerator is proposed as a muon accelerator because of its large aperture and no need of magnet ramping. In particular, the nonscaling type of FFAG has been studied because of its simple magnets and its unique acceleration method using the path out of the rf bucket. A recent 6D tracking study reveals, however, that the time of flight difference due to transverse amplitude causes the longitudinal emittance blowup which limits the transverse acceptance. This is a serious problem for an accelerator that is expected to accelerate a muon beam with huge transverse emittance. Two methods of curing the problem are examined by particle tracking. One is higher rf voltage and the other is higher harmonic rf in addition to the fundamental one. A 50% increase of rf voltage suppresses the emittance blowup. Second and third harmonic rf also improve the final momentum spread. Both methods work fine in a single FFAG system. However, a few percent of particle loss is inevitable when two FFAG are cascaded.

A Vlasov equilibrium of the Kapchinskij-Vladimirskij form is obtained for a periodically twisted ellipse-shaped charged-particle beam in a nonaxisymmetric periodic magnetic focusing field. The single-particle Hamiltonian dynamics is analyzed self-consistently. A constant of motion analogous to the Courant-Snyder invariant is found. The equilibrium distribution function is constructed. The statistical properties of the beam equilibrium are studied. In the zero-temperature limit, the generalized envelope equations derived from the kinetic equilibrium theory recover the generalized envelope equations obtained in the cold-fluid equilibrium theory. Examples of periodically twisted elliptic beam equilibria are presented, and potential applications are discussed. For ribbon-beam amplifier and ribbon-beam klystron applications, the kinetic equilibrium theory predicts that the effect of beam temperature on the beam envelopes is negligibly small.

The ability to generate small transverse emittance is perhaps the main limiting factor for the performance of high-gain x-ray free-electron lasers (FELs). Noting that beams from an rf photocathode gun can have energy spread much smaller than required for efficient FEL interaction, we present a method to produce normalized transverse emittance at or below about $0.1\mu \mathrm{m}$, which will lead to a significantly shorter length undulator as well as a lower electron beam energy for an x-ray FEL project. The beam manipulation consists of producing an unequal partition of the initially equal emittances into two dissimilar emittances by a flat-beam technique and exchanging the larger transverse emittance with a smaller longitudinal emittance. We study various issues involved in the manipulation. In particular, a new emittance exchange optics we found enables an exact emittance exchange necessary for this scheme.

We have recently considered acceleration of protons and heavy ions in a fixed-field alternating-gradient accelerator with nonscaling lattice and linear field profile. To avoid the problem of frequency modulation for acceleration of low-energy beams over a too short period of time, and to boost the acceleration rate, the method of harmonic number jump (HNJ) was proposed. This method allows the use of constant frequency acceleration, for instance using superconducting cavities, despite the fact that the beam velocity may vary considerably. Of course in this case the accelerating voltage and rf phase will need to be programmed accordingly as we shall show. We shall study first the motion of synchronous particles, and then of those with deviating initial conditions. We estimate the area and height of the rf buckets that are to contain the beam bunches with the added condition of the HNJ. We finally investigate methods to allow the program of energy gain as required by the HNJ method, including the effect of the cavity transit time factor.

Plasma production via field ionization occurs when an incoming particle beam is sufficiently dense that the electric field associated with the beam ionizes a neutral vapor or gas. Experiments conducted at the Stanford Linear Accelerator Center explore the threshold conditions necessary to induce field ionization by an electron beam in a neutral lithium vapor. By independently varying the transverse beam size, number of electrons per bunch, or bunch length, the radial component of the electric field is controlled to be above or below the threshold for field ionization. Additional experiments ionized neutral xenon and neutral nitric oxide by varying the incoming beam’s bunch length. A self-ionized plasma is an essential step for the viability of plasma-based accelerators for future high-energy experiments.