We present an algorithm for calculating the impedance of infinitely long beam pipes with arbitrary cross section. The method is not restricted to ultrarelativistic beams or perturbative approximations with respect to the wall surface impedance or skin penetration depth. We exemplify our algorithm with a calculation of the impedance for rectangular metallic beam pipes. Unlike the situation in the perturbative regime, where the beam pipe geometry modifies the metallic resistive wall impedances by only a multiplicative factor, the beam pipe geometry has a more complex influence on the impedance when nonultrarelativistic effects are significant and in the ultrarelativistic regime at both small and large frequencies. Since our algorithm requires the boundary conditions at the beam pipe wall to be provided as linear relations between the transverse components of the electromagnetic field, we discuss a general algorithm to calculate these boundary conditions for multilayer beam pipes with arbitrary cross section.

In radiation pressure dominated laser ion acceleration schemes, transverse target deformation and Rayleigh-Taylor (RT)-like instability always develop quickly, break the acceleration structure, limit the final accelerated ion energy, and lower the beam quality. To overcome these issues, we propose a target design named dual parabola targets consisting of a lateral thick part and a middle thin part, each with a parabolic front surface of different focus positions. By using such a target, through interactive laser and target shaping processes, the central part of the thin target will detach from the whole target and a microtarget is formed. This enables the stable acceleration of the central part of the target to high energy with high quality since usual target deformation and RT-like instabilities with planar targets are suppressed. Furthermore, this target design reduces the laser intensity required to optimize radiation pressure acceleration by more than 1 order of magnitude compared to normal flat targets with similar thickness and density. Two-dimensional particle-in-cell simulations indicate that a quasimonoenergetic proton beam with peak energy over 200 MeV and energy spread around 2% can be generated when such a solid target (with density $400n_c$ and target thickness $0.5{\lambda }_0$) is irradiated by a 100 fs long circularly polarized laser pulse at focused intensity $I_L\sim 9.2\times {10}^{21}\mathrm{W}/{\mathrm{cm}}^2$.

Recently cold cathodes based on field emission have drawn attention and been considered to drive accelerators and free electron lasers, due to the progress in field emitter arrays and planar emitters like diamond films. In this paper, we reviewed the characteristics of field emission in rf fields. Simulations of S-band rf guns consisting of a cathode cell and a full cell were done. We showed that a shorter cathode cell with a length of 0.25–0.3 of $\lambda /2$ is in favor of obtaining both low emittance and low energy spread bunches when the amplitude of electric field on the cathode surface ranges from 60 to $80\mathrm{MV}/\mathrm{m}$. A single cell test cavity has been installed to study field emission of diamond films and the measured beam current showed a good agreement with theoretical calculations.

Fully coherent x rays can be generated by self-seeding x-ray free electron lasers (XFELs). Self-seeding by a forward Bragg diffraction (FBD) monochromator has been recently proposed [G. Geloni, V. Kocharyan, and E. Saldin, J. Mod. Opt. 58, 1391 (2011)] and demonstrated [J. Amann et al., Nat. Photonics 6, 693 (2012)]. Characteristic time ${\mathcal{T}}_0$ of FBD determines the power, spectral, and time characteristics of the FBD seed [Yu. Shvyd’ko and R. Lindberg, Phys. Rev. ST Accel. Beams 15, 100702 (2012)]. Here we show that for a given electron bunch with duration ${\sigma }_{\mathrm{e}}$ the spectral flux of the self-seeding XFEL can be maximized, and the spectral bandwidth can be respectively minimized by choosing ${\mathcal{T}}_0\sim {\sigma }_{\mathrm{e}}/\pi$ and by optimizing the electron bunch delay ${\tau }_e$. The choices of ${\mathcal{T}}_0$ and ${\tau }_e$ are not unique. In all cases, the maximum value of the spectral flux and the minimum bandwidth are primarily determined by ${\sigma }_{\mathrm{e}}$. Two-color seeding takes place if ${\mathcal{T}}_0\ll {\sigma }_{\mathrm{e}}/\pi$. The studies are performed, for a Gaussian electron bunch distribution with the parameters, close to those used in the short-bunch (${\sigma }_{\mathrm{e}}\simeq 5\mathrm{fs}$) and long-bunch (${\sigma }_{\mathrm{e}}\simeq 20\mathrm{fs}$) operation modes of the Linac Coherent Light Source XFEL.

Baseline design of a typical x-ray free electron laser (FEL) undulator assumes a planar configuration which results in a linear polarization of the FEL radiation. However, many experiments at x-ray FEL user facilities would profit from using a circularly polarized radiation. As a cheap upgrade, one can consider an installation of a short helical (or cross-planar) afterburner, but then one should have an efficient method to suppress the powerful linearly polarized background from the main undulator. In this paper we propose a new method for such a suppression: an application of the reverse taper in the main undulator. We discover that in a certain range of the taper strength, the density modulation (bunching) at saturation is practically the same as in the case of a nontapered undulator while the power of linearly polarized radiation is suppressed by orders of magnitude. Then strongly modulated electron beam radiates at full power in the afterburner. Considering the SASE3 undulator of the European XFEL as a practical example, we demonstrate that soft x-ray radiation pulses with peak power in excess of 100 GW and an ultimately high degree of circular polarization can be produced. The proposed method is rather universal, i.e., it can be used at SASE FELs and seeded (self-seeded) FELs, with any wavelength of interest, in a wide range of electron beam parameters, and with any repetition rate. It can be used at different x-ray FEL facilities, in particular at Linac Coherent Light Source after installation of the helical afterburner in the near future.

An rf Wien filter (WF) can be used in a storage ring to measure a particle’s electric dipole moment (EDM). If the WF frequency equals the spin precession frequency without WF, and the oscillating WF fields are chosen so that the corresponding transverse Lorentz force equals zero, then a large source of systematic errors is canceled but the EDM signal is not. This effect, discovered by simulation, can be called the “partially frozen spin” effect.

A novel optics concept, the achromatic telescopic squeezing (ATS) scheme has been invented in the context of the Large Hadron Collider (LHC) upgrade studies, and chosen as the baseline scheme for the optics and layout of the recently approved high luminosity LHC project (HL-LHC). This scheme offers an extremely powerful and flexible machinery in order to strongly reduce ${\beta }^{*}$ in a symmetric or asymmetric way (i.e. without necessarily imposing the same ${\beta }^{*}$ in both planes), while perfectly controlling the chromatic aberrations induced, namely the linear and nonlinear chromaticities, the off-momentum $\beta$ beating, and the spurious dispersion from the large crossing angle which is required at small ${\beta }^{*}$ in the particular case of the (HL)-LHC. The initial motivations of the scheme will be reviewed, followed by a detailed description of its fundamental theoretical foundations. An effective construction of ATS optics will be given and its main features illustrated in the case of the LHC and HL-LHC.

The future high intensity upgrade project of the J-PARC (Japan Proton Accelerator Research Complex) MR (Main Ring) includes developments of high gradient rf cavities and magnet power supplies for high repetition rate. The scenario describing the cavity replacements is reported. By the replacement plan, the total acceleration voltage will be almost doubled, while the number of rf stations remains the same. The key issue is the development of a high gradient rf system using high impedance magnetic alloy, FT3L. The FT3L is produced by the transverse magnetic field annealing although the present cavity for the J-PARC adopts the magnetic alloy, FT3M, which is annealed without magnetic field. After the test production using a large spectrometer magnet in 2011, a dedicated production system for the FT3L cores was assembled in 2012. This setup demonstrated that we can produce material with 2 times higher ${\mu }_p^{\prime }\mathrm{Qf}$ product compared to the cores used for present cavities. In this summer, the production system was moved to the company from J-PARC and is used for mass production of 280 FT3L cores for the J-PARC MR. The cores produced in the first test production are already used for standard machine operation. The operation experience shows that the power loss in the cores was reduced significantly as expected.

In this paper we describe two methods that demonstrate an energy evolution at the ends of a long bunch. The first method is a nondestructive diagnostic that extends the response matrix technique, to provide insight into the time-sliced transverse behavior of a coasting bunch in a ring. This method uncovers intense beam physics from the resulting longitudinal energy variations at the ends of a rectangular bunch. Measurements of the tune profile, using this method, agree with analytical predictions resulting in a percent error of 0.32% and 2.45% (head and tail respectively). Measurements and calculations presented in this paper are presented for a beam with an intensity of 0.901 (beam current of 21 mA). The head and the tail tunes, averaged over four ring chambers, also illustrate the intensity variation axially along the bunch. This is exhibited through tune-shift measurements at the ends of the bunch. As confirmed through calculations, the measured tune shift at the head is smaller than the tail. This results from the sum of both the transverse coherent tune shift from image forces with the component from longitudinal space charge. The sum of the two components produces a smaller tune shift at the head of the beam and a larger tune shift at the tail. The second method presented in this paper optically illustrates the centroid “cork-screwing” motion within the beam ends, at a single location on the ring. The motion is resolvable using this diagnostic since it is capable of axially time-slicing regions of the beam with different energies. The difference between the two methods presented is that the first is a nondestructive diagnostic that can be extended to multiple turns whereas the second method is a destructive first turn imager.

The influence of heat treatments at 122, 400, and $800°\mathrm{C}$ on the field emission of large-grain and single-crystal high-purity niobium samples has been investigated. Buffered chemical polishing of $40\mu \mathrm{m}$ and high pressure ultrapure water rinsing under clean-room conditions resulted in smooth surfaces with a linear surface roughness of 46 to 337 nm. By means of field emission scanning microscopy, an increasing number of emitters up to $40/{\mathrm{cm}}^2$ with temperature were found at surface fields up to $160\mathrm{MV}/\mathrm{m}$. Two different mechanisms of emitter activation were found, i.e. activation by the applied electric field and activation by temperature. Some emitters with an onset surface field of 50 to $100\mathrm{MV}/\mathrm{m}$ appeared already after the low-temperature bakeout. Correlated scanning-electron-microscopy/energy-dispersive-x-ray measurements revealed particles and surface defects as emitters. Their activation will be discussed with respect to the thickness of the insulating oxide layer.

We report on experimental studies on the harmonic interaction between an optical laser and a relativistic electron beam in an undulator up to the 15th order. In this experiment, a significant energy modulation is imprinted on the beam longitudinal phase space through the electron-laser interaction when the laser frequency is the 3rd, 5th, 7th, 9th or 15th harmonic of the fundamental resonant frequency of the undulator. The experimental results are in good agreement with theory, and indicate that high harmonic interactions in undulators with large $K$ values and small phase errors can be quite efficient. The results confirm the basic physics of harmonic interaction with a goal toward ushering forward the development of many high harmonic based applications in free-electron lasers.

Correction of beta-beat is of great importance for performance improvement of high energy accelerators, like the Relativistic Hadron Ion Collider (RHIC). At RHIC, using the independent component analysis method, linear optical functions are extracted from the turn by turn beam position data of the ac dipole driven betatron oscillation. Despite the constraint of a limited number of available quadrupole correctors at RHIC, a global beta-beat correction scheme using a beta-beat response matrix method was developed and experimentally demonstrated. In both rings, a factor of 2 or better reduction of beta-beat was achieved within available beam time. At the same time, a new scheme of using horizontal closed orbit bump at sextupoles to correct beta-beat in the arcs was demonstrated in the Yellow ring of RHIC at beam energy of 255 GeV, and a peak beta-beat of approximately 7% was achieved.

It has been shown that the direct seeding can enhance the performance of a free-electron laser (FEL) in terms of its spectral, temporal, and coherence properties and reduces fluctuations in FEL output energy and arrival-time jitter. The properties of the used seed photon pulse are of high importance. In this paper, we describe the influence of the $M^2$ onto the achievable power contrast between the direct seeded and the unseeded FEL radiation. The results of these studies are compared with the data from the high harmonic generation direct seeding experiment “sFLASH” in Hamburg, Germany. A method to measure $M^2$ from a single transverse intensity distribution of the high harmonics beam at waist is discussed.

We describe theoretically and numerically the interaction of a laser- or LINAC-accelerated beam of relativistic electrons with a high intensity optical lattice, resulting from the superposition of two transverse laser pulses. The bunch is trapped and guided within the potential channels of the optical lattice, leading to betatron oscillations. We describe the emission from individual particles and from the bunch, and analyze its spectrum, considering the dominant incoherent radiation as well as possible effects of partial coherency. Analysis of the emitted radiation should provide useful information on the characteristics of the electron beam, and its interaction with the optical lattice.

This is a complementary paper to the recently published paper on the two-point multipactor (MP2) in a superconducting cavity near its equator [V. Shemelin, Phys. Rev. ST Accel. Beams 16, 012002 (2013)]. Now, the analytical description of the one-point multipactor (MP1) is suggested following the approach used for MP2. Such a description gives a clear physical understanding of many features of the phenomenon. Influence of the cavity shape on the existence of this or another kind of the discharge is analyzed. A concept of a geometrical parameter of multipactor is introduced. This parameter is different for different kinds of MP and defines limits of its existence.

We discuss the possibility in principle of stripping extraction in combination with reverse bends in isochronous separate-sector cyclotrons (and/or fixed field alternating gradient accelerators). If one uses reverse bends between the sectors (instead of or in combination with drifts) and places stripper foils at the sector exit edges, the stripped beam has a reduced bending radius and it should be able to leave the cyclotron within the range of the valley—even if the beam is stripped at less than full energy. We are especially interested in stripping of ${\mathrm{H}}_2^{+}$, as it doubles the charge to mass ratio of the ions. However the method could be applied to other ions or ionized molecules as well. For the production of proton beams by stripping extraction of an ${\mathrm{H}}_2^{+}$ beam, we discuss possible designs for three types of machines: First, a low-energy cyclotron for the simultaneous production of several beams at multiple energies—for instance 15, 30, and 70 MeV—thus allowing beam delivery on several isotope production targets. In this case it can be an advantage to have a strong energy dependence of the direction of the extracted beam. Second, we consider a fast variable-energy proton machine for cancer therapy that should allow extraction (of the complete beam) at all energies in the range of about 70 MeV to about 250 MeV into the same beam line. Third, we consider a high-intensity high-energy machine, where the main design goals are extraction with low losses, low activation of components, and high reliability. Especially if such a machine is considered for an accelerator driven system (ADS), this extraction mechanism has advantages: Beam trips by the failure of electrostatic elements could be avoided and the turn separation would be less critical, which allows operation at lower main cavity voltages. This would in turn reduce the number of rf trips. The price that has to be paid for these advantages is an increase in size and/or field strength compared to proton machines with standard extraction at the final energy.

Beam-beam effects in eRHIC, the proposed energy recovery linac (ERL)-based electron-ion collider (EIC) at BNL, have several unique features distinguishing them from those in hadron and lepton colliders. Taking the advantage of the fact that the electron beam is used only once, we expect the luminosity to be 10 times that of a ring-ring collision scheme with similar parameters. However, without instituting proper treatments, the quality of electron and hadron beams can undergo degradation or even beam loss, driven by the beam-beam interactions. Kink instability is one of the most important collective effects of the ion ring due to the special beam-beam interaction in an ERL-based EIC. In this article, we present novel treatments for kink instability in ERL-based EICs including a scheme dedicated to suppressing instability.

We present a new model-independent dynamic feedback technique, rotation rate tuning, for automatically and simultaneously tuning coupled components of uncertain, complex systems. The main advantages of the method are: (1) it has the ability to handle unknown, time-varying systems, (2) it gives known bounds on parameter update rates, (3) we give an analytic proof of its convergence and its stability, and (4) it has a simple digital implementation through a control system such as the experimental physics and industrial control system (EPICS). Because this technique is model independent it may be useful as a real-time, in-hardware, feedback-based optimization scheme for uncertain and time-varying systems. In particular, it is robust enough to handle uncertainty due to coupling, thermal cycling, misalignments, and manufacturing imperfections. As a result, it may be used as a fine-tuning supplement for existing accelerator tuning/control schemes. We present multiparticle simulation results demonstrating the scheme’s ability to simultaneously adaptively adjust the set points of 22 quadrupole magnets and two rf buncher cavities in the Los Alamos Neutron Science Center (LANSCE) Linear Accelerator’s transport region, while the beam properties and rf phase shift are continuously varying. The tuning is based only on beam current readings, without knowledge of particle dynamics. We also present an outline of how to implement this general scheme in software for optimization, and in hardware for feedback-based control/tuning, for a wide range of systems.

Laser ion acceleration provides for compact, high-intensity ion sources in the multi-MeV range. Using a pulsed high-field solenoid, for the first time high-intensity laser-accelerated proton bunches could be selected from the continuous exponential spectrum and delivered to large distances, containing more than ${10}^9$ particles in a narrow energy interval around a central energy of 9.4 MeV and showing $\le 30\mathrm{mrad}$ envelope divergence. The bunches of only a few nanoseconds bunch duration were characterized 2.2 m behind the laser-plasma source with respect to arrival time, energy width, and intensity as well as spatial and temporal bunch profile.

Strategies for mitigating ionization-induced beam head erosion in an electron-beam-driven plasma wakefield accelerator (PWFA) are explored when the plasma and the wake are both formed by the transverse electric field of the beam itself. Beam head erosion can occur in a preformed plasma because of a lack of focusing force from the wake at the rising edge (head) of the beam due to the finite inertia of the electrons. When the plasma is produced by field ionization from the space charge field of the beam, the head erosion is significantly exacerbated due to the gradual recession (in the beam frame) of the 100% ionization contour. Beam particles in front of the ionization front cannot be focused (guided) causing them to expand as in vacuum. When they expand, the location of the ionization front recedes such that even more beam particles are completely unguided. Eventually this process terminates the wake formation prematurely, i.e., well before the beam is depleted of its energy. Ionization-induced head erosion can be mitigated by controlling the beam parameters (emittance, charge, and energy) and/or the plasma conditions. In this paper we explore how the latter can be optimized so as to extend the beam propagation distance and thereby increase the energy gain. In particular we show that, by using a combination of the alkali atoms of the lowest practical ionization potential (Cs) for plasma formation and a precursor laser pulse to generate a narrow plasma filament in front of the beam, the head erosion rate can be dramatically reduced. Simulation results show that in the upcoming “two-bunch PWFA experiments” on the FACET facility at SLAC national accelerator laboratory the energy gain of the trailing beam can be up to 10 times larger for the given parameters when employing these techniques. Comparison of the effect of beam head erosion in preformed and ionization produced plasmas is also presented.