Ultra-low Emittance Research Articles

Performance of beam size monitor based on Kirkpatrick–Baez mirror at SSRF

High-energy photon source (HEPS) is a 6 GeV ultralow emittance storage ring light source to be built in Beijing, China. Both the horizontal and vertical beam sizes of the HEPS storage ring are below 10 µm. It is a challenge to measure such a small beam size in both directions. To this end, measurement by a Kirkpatrick–Baez (KB) mirror imaging system was evaluated. A test KB system for the Shanghai Synchrotron Radiation Facility storage ring was designed and tested. Two crossed cylindrical mirrors were used to image the dipole source point. Both mirrors can be moved in and out so that the monitor is interchangeable with the original X-ray pinhole monitor. The aberration and point spread function, which would cause image blur, were evaluated. A beam-based calibration scheme was used by varying the beam size with different quadrupole settings and fitting them with the corresponding theoretical values. We updated the original X-ray camera with a new camera having a 5-μm-thick LuAG/Ce scintillator, and the imaging result shows greatly decreased image blur.

DA optimization experiences in the HEPS lattice design

In the past decade, the so-called diffraction-limited storage ring (DLSR) light sources were proposed, promising much better radiation performance than available in the existing third generation light sources. Regarding the very strong focusing and chromatic sextupoles that required for reaching an ultralow emittance, to optimize the nonlinear dynamics and achieve an adequate dynamic aperture is an important topic in a DLSR design. In this paper we will present some tips distilled from the DA optimization experience of the High Energy Photon Source over the past ten years, hoping it could provide some aids to other ultralow-emittance designs.

Simulation study on luminosity feedback for horizontal beam stabilization at SuperKEKB

The SuperKEKB e+ e− collider uses highly focused ultra-low emittance bunches colliding every 4 ns to reach a very high luminosity of 8 × 1035cm−2s−1. It is essential to have an orbit feedback system at the Interaction Point (IP) to maintain the optimum overlap between the colliding beams in the presence of ground motion disturbances. For the horizontal motion, a luminosity monitoring system, based on measuring the rate of the Bhabha process at vanishing scattering angle, is developed as input signal to the feedback system. The relative precision needed for this monitor is studied in detail, for the different successive stages of luminosity operation, based on a full simulation of this system, including the detector, DAQ, lock-in amplifier, and feedback control.

Effects of energy spread on brightness and coherence of undulator sources.

The (spectral) brightness for partially transverse coherent sources such as synchrotron radiation and free-electron laser sources can be defined as the maximum of the Wigner distribution. Then, the brightness includes information on both coherence and wavefront characteristics of the radiation field. For undulator sources, it is customary to approximate the single-electron electric field at resonance with a Gaussian beam, leading to great simplifications. Attempts to account for the modified spatial and angular profile of the undulator radiation in the presence of detuning due to energy spread, currently build on the simplified brightness expression derived under the assumption of Gaussian beams. The influence of energy spread on undulator radiation properties is becoming important in view of diffraction-limited rings with ultralow emittance coming on-line. Here the effects of energy spread on the brightness of undulator radiation at resonance are discussed, as well as relevant relations with coherence properties.

Design Considerations for an Ultralow Emittance Storage Ring for the Canadian Light Source

Design Considerations for an Ultralow Emittance Storage Ring for the Canadian Light Source

Longitudinally Variable Field Dipole Design Using Permanent Magnets For CLIC Damping Rings

The latest CLIC damping ring lattice is based on magnets with longitudinally variable dipole fields in order to achieve ultralow beam emittance while keeping the ring circumference small. These magnets need to provide a focusing gradient of 11 T/m as well. The good field region radius is 5 mm. The field harmonics shall be in the order of 1E-4 of the main one. Since only a small variation of the field is requested, permanent magnets are the most cost-effective solution. Beam dynamics calculations have provided idealized field profiles, and magnetic calculations have been performed to check their feasibility. FEM electromagnetic computations are complicated because the cross-section of the magnet is not constant. Therefore, iron poles cannot be modeled by extrusion, and only 3-D computations are meaningful. Finally, this paper shows that small variations of field strength are possible by using movable parts.

Two-color ionization injection using a plasma beatwave accelerator

Two-color laser ionization injection is a method to generate ultra-low emittance (sub-100 nm transverse normalized emittance) beams in a laser-driven plasma accelerator. A plasma beatwave accelerator is proposed to drive the plasma wave for ionization injection, where the beating of the lasers effectively produces a train of long-wavelength pulses. The plasma beatwave accelerator excites a large amplitude plasma wave with low peak laser electric fields, leaving atomically-bound electrons with low ionization potential. A short-wavelength, low-amplitude ionization injection laser pulse (with a small ponderomotive force and large peak electric field) is used to ionize the remaining bound electrons at a wake phase suitable for trapping, generating an ultra-low emittance electron beam that is accelerated in the plasma wave. Using a plasma beatwave accelerator for wakefield excitation, compared to short-pulse wakefield excitation, allows for a lower amplitude injection laser pulse and, hence, a lower emittance beam may be generated.

First optics and beam dynamics studies on the MAX IV 3 GeV storage ring

The MAX IV 3 GeV storage ring is the first light source to make use of a multibend achromat lattice to reach ultralow emittance. After extensive commissioning efforts, the storage ring is now ramping up its user program. We present results from beam commissioning of the MAX IV 3 GeV storage ring as well as a summary of the beam dynamics studies that have so for been carried out. We report on injection and accumulation using a single dipole kicker, top-up injection, slow orbit feedback, restoring the linear optics to design, effects of in-vacuum undulators with closed gaps, adjusting nonlinear optics to achieve design chromaticity correction and dynamic aperture sufficient for high injection efficiency and large Touschek lifetime.

Strongly Mismatched Regime of Nonlinear Laser–Plasma Acceleration: Optimization of Laser-to-Energetic Particle Efficiency

Laser electron accelerators utilize a bubble regime of nonlinear plasma waves driven as laser wakefields that, from theoretical considerations, require a matched laser spot size incident on plasma. A strongly mismatched regime of nonlinear laser–plasma acceleration in the bubble regime, favored by experiments, is introduced and modeled for optimization of laser-to-particle energy efficiency with application to the recently proposed laser positron accelerator. Strong mismatch, in contrast with the matched condition, arises from the incident laser spot size being much larger than that needed for equilibration of the laser ponderomotive and electron-ion charge-separation forces in the nonlinearly driven density structure of a plasma bubble. This is shown to be favorable for optimization of large self-injected electron charge and ultralow transverse emittance without precluding beam spectral shaping. It is shown that there are prominent signatures of the mismatched regime, strong optical-shock excitation, and bubble elongation, which are validated using multidimensional particle-in-cell simulations. This paper thus uncovers a generalized regime that apart from being used in many laser–plasma acceleration experiments also opens a novel pathway for a wide range of future applications.

The resonant multi-pulse ionization injection

The production of high-quality electron bunches in Laser Wake Field Acceleration relies on the possibility to inject ultra-low emittance bunches in the plasma wave. In this paper, we present a new bunch injection scheme in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses. In the Resonant Multi-Pulse Ionization injection scheme, the main portion of a single ultrashort (e.g., Ti:Sa) laser system pulse is temporally shaped as a sequence of resonant sub-pulses, while a minor portion acts as an ionizing pulse. Simulations show that high-quality electron bunches with normalized emittance as low as 0.08 mm × mrad and 0.65% energy spread can be obtained with a single present-day 100TW-class Ti:Sa laser system.

Single-stage plasma-based correlated energy spread compensation for ultrahigh 6D brightness electron beams

Plasma photocathode wakefield acceleration combines energy gains of tens of GeV m−1 with generation of ultralow emittance electron bunches, and opens a path towards 5D-brightness orders of magnitude larger than state-of-the-art. This holds great promise for compact accelerator building blocks and advanced light sources. However, an intrinsic by-product of the enormous electric field gradients inherent to plasma accelerators is substantial correlated energy spread—an obstacle for key applications such as free-electron-lasers. Here we show that by releasing an additional tailored escort electron beam at a later phase of the acceleration, when the witness bunch is relativistically stable, the plasma wave can be locally overloaded without compromising the witness bunch normalized emittance. This reverses the effective accelerating gradient, and counter-rotates the accumulated negative longitudinal phase space chirp of the witness bunch. Thereby, the energy spread is reduced by an order of magnitude, thus enabling the production of ultrahigh 6D-brightness beams.

Static beam-based alignment for the Ring-To-Main-Linac of the Compact Linear Collider

The Compact Linear Collider (CLIC) is a future multi-TeV collider for the post-Large Hadron Collider era. It features high-gradient acceleration and ultra-low emittance to achieve its ambitious goals of high collision energy and peak luminosity. Beam-based alignment (BBA) techniques are mandatory for CLIC to preserve the ultra-low emittances from the damping rings to the interaction point. In this paper, a detailed study of BBA techniques has been carried out for the entire 27 km long ``Ring-To-Main-Linac'' (RTML) section of the CLIC, to correct realistic static errors such as element position offsets, angle, magnetic strength and dynamic magnetic centre shifts. The correction strategy is proved to be very effective and leads to a relaxation of the pre-alignment tolerances for the component installation in the tunnel. This is the first time such a large scale and complex lattice has been corrected to match the design budgets. The techniques proposed could be applied to similarly sized facilities, such as the International Linear Collider, where a similar RTML section is used, or free-electron lasers, which, being equipped with linacs and bunch compressors, present challenges similar to those of the CLIC RTML. Moreover, a new technique is investigated for the emittance tuning procedure: the direct measurement of the interactions between the beams and a set of a few consecutive laser wires. The speed of this technique can be faster comparing to the traditional techniques based on emittance reconstructed from beam size measurements at several positions.

Design of a Superconducting Longitudinal Gradient Bend Magnet for the SLS Upgrade

The Paul Scherrer Institute is planning a major upgrade of the Swiss Light Source (SLS-2) with the development of a new storage ring lattice providing ultralow emittance while maintaining the locations of the undulator-based beam lines, in order to meet the growing demand of the users for higher photon flux and brilliance. The preliminary design of the lattice features a multibend achromat with seven bending magnets in each of the 12 arcs. This compact low-emittance lattice will include the use of high field longitudinal gradient bends (LGB), whose design and fabrication are particularly challenging. The superconducting dipole will have a hyperbolic longitudinal field variation with a narrow peak (FWHM of about 40-70 mm) to limit energy spread and the radiated power. The total magnet length should not exceed 400 mm with a field integral of around 0.6 Tm. The peak field value at the beam position has to be around 6 T. The LGB has to host a room temperature vacuum chamber and an antichamber to release the synchrotron radiation. This requirement determines the magnet aperture and with it the peak field at the conductor location that drives the choice of the superconducting material for the winding. In this paper, the design of the LGB magnet is presented.

Lattice design for an ultra-low emittance synchrotron light source

In this paper, we investigate a lattice design for an ultra-low emittance storage ring with intermediate energy of 3 GeV and circumference of 576 m. We present the design results for a seven-band achromat lattice with the natural emittance of 90 pm-rad. The base line is based on 20 straight sections with the length of 6 m. The minimum distance between different magnets is 10 cm and it has been tried to keep the lattice structure as simple as possible. The brilliance of radiated synchrotron radiation from a 2.24 m In-vacuum undulator installed in the lattice is 4×1021photons∕smm2mm2mrad20.1%BW . We will address the challenging points of designed ultra-low emittance storage ring lattice such as strong nonlinearity and collective effects. The dynamic aperture of the lattice is comparatively sufficient for off-axis injection and the lifetime is relatively large without using any harmonic cavity. Since the proposed lattice structure in some aspects is similar to MAX IV storage ring, we will make some comparison between two lattices in different sections.

Direct Measurement of Sub-10fs Relativistic Electron Beams with Ultralow Emittance.

Ultralow emittance (≤20 nm, normalized) electron beams with 10^{5} electrons per bunch are obtained by tightly focusing an ultrafast (∼100 fs) laser pulse on the cathode of a 1.6 cell radio frequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to <10 fs rms bunch length. The measurement is performed using a thick high-voltage deflecting cavity which is shown to be well suited to measure ultrashort durations of bunching beams, provided that the beam reaches a ballistic longitudinal focus at the cavity center.

Optimizing the lattice design of a diffraction-limited storage ring with a rational combination of particle swarm and genetic algorithms**Supported by NSFC (11475202, 11405187) and Youth Innovation Promotion Association CAS (2015009)

In the lattice design of a diffraction-limited storage ring (DLSR) consisting of compact multi-bend achromats (MBAs), it is challenging to simultaneously achieve an ultralow emittance and a satisfactory nonlinear performance, due to extremely large nonlinearities and limited tuning ranges of the element parameters. Nevertheless, in this paper we show that the potential of a DLSR design can be explored with a successive and iterative implementation of the multi-objective particle swarm optimization (MOPSO) and multi-objective genetic algorithm (MOGA). For the High Energy Photon Source, a planned kilometer-scale DLSR, optimizations indicate that it is feasible to attain a natural emittance of about 50 pm·rad, and simultaneously realize a sufficient ring acceptance for on-axis longitudinal injection, by using a hybrid MBA lattice. In particular, this study demonstrates that a rational combination of the MOPSO and MOGA is more effective than either of them alone, in approaching the true global optima of an explorative multi-objective problem with many optimizing variables and local optima.

On-line dispersion estimation and correction scheme for the Compact Linear Collider

The Compact Linear Collider (CLIC) has stringent component alignment tolerances in order to preserve the ultralow emittance of the utilized particle beams. Beam-based alignment techniques have been designed to relax these tolerances to realizable values. In this paper, a scheme is presented that is capable of mitigating besides the effects of static misalignments also dynamic misalignments caused by ground motion. It is based on the well-known dispersion-free steering (DFS) algorithm, with the peculiarity that it can perform its correction during the usual operation (on-line). This is enabled by performing the necessary dispersion measurements by introducing only negligibly small beam energy changes (per mille level). It has been found that this on-line correction becomes sensitive to the imperfections of transverse wakefields and structure tilts. These sensitivities have been studied via analytical models and in simulations and appropriate countermeasures to improve the robustness of the method have been proposed. The correction performance and robustness properties of the improved algorithm have been studied in detail with respect to all relevant static and dynamic imperfections in a realistic scenario. The presented scheme is not only a potentially important operational tool for CLIC, but the findings with respect to robustness properties for different imperfections are of general interest for the application of the dispersion-free steering algorithm.

Beam optics and dynamics in electron storage rings with ultralow emittance

Due to recent advances in the physics and technology of relativistic circular colliders, it is now possible to design storage rings whose emittance is close to the diffraction threshold of synchrotron radiation (e ~ 10 pm for the radiation critical wavelength of λ c ~ 1A). Methods for minimizing emittance by optimizing the magnetic structure without reducing the dynamic aperture and the energy acceptance, as well as possible implementations of these methods, are discussed in this paper.

Flat electron beam sources for DLA accelerators

In this paper we discuss the application of the flat beam transform to generate beams suitable for injection into slab-symmetric dielectric laser-driven accelerators (DLAs). A study of the focusing requirements to keep the particles within the tight apertures characterizing these accelerators shows the benefits of employing ultralow beam emittances. The slab geometry of the many dielectric accelerating structures strongly favors the use of flat beams with large ratio between vertical and horizontal emittances. We employ particle tracking simulations to study the application of the flat beam transform for two injector designs, a DC non relativistic photogun and a 1.6 cell S-band RF photoinjector, obtaining in both cases emittance ratios between the horizontal and vertical plane in excess of 100 in agreement with simple analytical estimates. The 4MeV RF photoinjector study-case can be directly applied to the UCLA Pegasus beamline and shows normalized emittances down to <3nm in the vertical dimension for beam charges up to 20fC, enabling a two-stage DLA experiment.

Statistical analysis of the limitation of half integer resonances on the available momentum acceptance of the High Energy Photon Source

In a diffraction-limited storage ring, half integer resonances can have strong effects on the beam dynamics, associated with the large detuning terms from the strong focusing and strong sextupoles as required for an ultralow emittance. In this study, the limitation of half integer resonances on the available momentum acceptance (MA) was statistically analyzed based on one design of the High Energy Photon Source (HEPS). It was found that the probability of MA reduction due to crossing of half integer resonances is closely correlated with the level of beta beats at the nominal tunes, but independent of the error sources. The analysis indicated that for the presented HEPS lattice design, the rms amplitude of beta beats should be kept below 1.5% horizontally and 2.5% vertically to reach a small MA reduction probability of about 1%.