We present a non-invasive, resonant cavity based approach to beam emittance measurement of a shot-to-shot non-circular beam pulse of multi-bunches. In a resonant cavity, desired field components can be enhanced up to Q_L_lambda/pi, where Q_L_lambda is the loaded Q of the resonance mode lambda, when the cavity resonant mode matches with the beam operating frequency. In particular, a Quad-cavity, with its quadrupole mode at beam operating frequency, extracts the beam quad-moment exclusively, utilizing the symmetry of the cavity and some simple networks to suppress common modes. Six successive beam quadrupole moment measurements, performed at different betatron phases in a linear transport system, allow us to determine the beam emittance, i.e., the beam size and shape in the beam's phase space. One measurement alone provides the rms-beam size if the beam position is given, for instance, by nearby beam-position-monitors. This paper describes the basic design and analysis of a Quad-cavity beam monitoring system.
Ultralow emittance ($\leq20$ nm, normalized) electron beams with $10^5$ electrons per bunch are obtained by tightly focusing an ultrafast ($\sim$ 100 fs) laser pulse on the cathode of a 1.6 cell radiofrequency 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 fo…
We present the design of a high repetition rate MeV energy ultrafast electron diffraction instrument based on a DC photoelectron gun and an SRF linac with multiple independently controlled accelerating and bunching cavities. The design is based on the existing Cornell photoinjector, which can readily be applied to the presented findings. Using particle tracking simulations in conjunction with multiobjective genetic algorithm optimization, we explore the smallest bunch lengths, emittance, and probe spot sizes achievable. We present results for both stroboscopic conditions (with single electrons…
One of the main concerns in the design of low emittance linear accelerators (linacs) is the preservation of beam emittance. Here we discuss one possible source of emittance dilution, the coupler kick, due to transverse electromagnetic fields in the accelerating cavities of the linac caused by the power coupler geometry. In addition to emittance growth, the coupler kick also produces orbit distortions. It is common wisdom that emittance growth from coupler kicks can be strongly reduced by using two couplers per cavity mounted opposite each other or by having the couplers of successive cavities …
Small, stray magnetic and RF fields in electron guns and injectors can perturb an electron beam and introduce correlations between the otherwise orthogonal transverse trajectories. These correlations couple the x and y dynamics which increases the transverse emittance. If the correlation becomes "diluted" or randomized in the beam transport then as the correlation disappears the 4D emittance increases. This paper discusses two important correlations common to most electron injectors. The first results from the coupling of a weak quadrupole field with beam rotation in a solenoid, and the second…
It is well-known that the electron beam quality required for applications such as FELs and ultra-fast electron diffraction can be degraded by the asymmetric fields introduced by the RF couplers of superconducting linacs. This effect is especially troublesome in the injector where the low energy beam from the gun is captured into the first high gradient accelerator section. Unfortunately modifying the established cavity design is expensive and time consuming, especially considering that only one or two sections are needed for an injector. Instead, it is important to analyze the coupler fields t…
With a low emittance and short-bunch electron beam at a high repetition rate realized by a superconducting linac, stimulated excitation of an optical cavity at the terahertz spectrum range has been shown. The electron beam passed through small holes in the cavity mirrors without being destroyed. A sharp resonance structure which indicated wide-band stimulated emission via coherent diffraction radiation was observed while scanning the round-trip length of the cavity.
We report the development of a multileaf collimator (MLC) for charged particle beams, based on independently actuated tungsten strips which can selectively scatter unwanted particles. The MLC is used in conjunction with an emittance exchange beamline to rapidly generate highly variable longitudinal bunch profiles. The developed MLC consists of 40 independent leaves that are 2 mm wide and can move up to 10 mm, and operates in an ultra high vacuum environment, enabled by novel features such as magnetically coupled actuation. An experiment at the Argonne Wakefield Accelerator, which previously us…
Radio frequency deflectors are widely used for time-resolved electron beam energy, emittance and radiation profile measurements in modern free electron laser facilities. In this paper, we present the beam dynamics aspects of the deflecting cavity of SXFEL user facility, which is located at the exit of the undulator. With a targeted time resolution around 10 fs, it is expected to be an important tool for time-resolved commissioning and machine studies for SXFEL user facility.
We studied the coupled beam motion in a storage ring between the transverse and longitudinal directions introduced by crab cavities. Analytic form of the linear decoupling transformation is derived. The equilibrium bunch distribution in an electron storage ring with a crab cavity is given, including contribution to the eigen-emittance induced by the crab cavity. Application to the short pulse generation scheme using crab cavities is considered.
Beam collisions with a crossing angle at the interaction point have been applied in high intensity colliders to reduce the effects of parasitic collisions which induce emittance growth and beam lifetime deterioration. The crossing angle causes the geometrical reduction of the luminosity. Crab cavity can be one of the most promising ways to compensate the crossing angle and to realize effective head-on collisions. Moreover, the crab crossing mitigates the synchro-betatron resonances due to the crossing angle. Crab cavity experiment in SPS is proposed for deciding on a full crab-cavity implement…
We present a theoretical description of resonant radiofrequency (RF) deflecting cavities in TM$_{110}$ mode as dynamic optical elements for ultrafast electron microscopy. We first derive the optical transfer matrix of an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D phase space propagation of a Gaussian electron distribution through the cavity. We derive closed, analytic expressions for the increase in transverse emittance and energy spread of the electron distribution. We demonstrate that for the special case of a beam focused in the center of the cavity, the low…
A high-current emittance-compensated RF photoinjector is a key enabling technology for a high-power CW FEL. A preliminary design of a normal-conducting, 2.5-cell pi-mode, 700-MHz CW RF photoinjector that will be built for demonstration purposes, is completed. This photoinjector will be capable of accelerating a 100-mA electron beam (3 nC per bunch at 35 MHz bunch repetition rate) to 2.7 MeV while providing an emittance below 7 mm-mrad at the wiggler. More than 1 MW of RF power will be fed into the photoinjector cavity through two ridge-loaded tapered waveguides. The waveguides are coupled to t…
The beam parameters of complete prototype of the bERLinPro injector that consisted of a lead cathode in an 1.6 cell superconducting cavity were measured. The measurements are discussed in chapter 2 and indicate that the beam emittance is governed by a multitude of contributions from the cathode and the injector. Cathodes for practical use must have a high enough QE to deliver the required current at moderate illumination intensities and be able to sustain the operation for several days. Alkali antimonides are good candidates for this purpose, as discussed in chapter 3.2. The preparation of suc…
A six-dimensional muon ionization cooling in a helical magnet channel has been studied. The cooling performance which is analytically evaluated by solving the exact Hamiltonian is reproduced in numerical simulation. One of the key beam elements for the helical channel is a dense-hydrogen gas-filled RF cavity which realizes a compact cooling channel. Besides, a beam-induced gas plasma in the cavity can generate a plasma-focusing effect. This will generate extremely small betatron function, which realizes extremely low emittance beam.
Accelerator-based terahertz (THz) radiation has been expected to realize a high-power broadband source. Employing a low-emittance and short-bunch electron beam at a high repetition rate, a scheme of coherent diffraction-radiation in an optical cavity layout is proposed. The scheme's stimulated radiation process between bunches can greatly enhance the efficiency of the radiation emission. We performed an experiment with a superconducting linac constructed as an energy recovery linac (ERL) test facility. The electron beam passes through small holes in the cavity mirrors without being destroyed. …
The resonant laser cooling of circular accelerator beams of relativistic charged particle is studied. It is shown that in the approximation of the given external electromagnetic wave amplitude (small gain free electron laser) the emittance of a beam of charged particles decreases. In the field of particle energy about 100 in the mass energy units the beam energy losses are negligible. The discovered effect can be used for cooling of charged particle beams in various accelerators. The significant cooling rates are possible to achieve by placing of the cooling device in the appropriate points of…
The external-Q factor of a resonant mode of a cavity represents the strength of the electromagnetic coupling between this resonant mode and the transmission mode of a waveguide coupled to the cavity. In this paper, we derive formulas to explicitly give the external Q of the cavities coupled with the waveguide through capacitive and inductive coupling. The derivation is based on the classical method that uses the reflection coefficient to estimate the coupling strength. Although the external Q is evaluated via three-dimensional computer simulations currently, these formulas may be useful for ma…
We present the results of numerical simulations and experimental studies about the effects of resonant and random excitations on proton losses, emittances, and beam distributions in the Large Hadron Collider (LHC). In addition to shedding light on complex nonlinear effects, these studies are applied to the design of hollow electron lenses (HEL) for active beam halo control. In the High-Luminosity Large Hadron Collider (HL-LHC), a considerable amount of energy will be stored in the beam tails. To control and clean the beam halo, the installation of two hollow electron lenses, one per beam, is b…
We propose an innovative beam cooling scheme based on laser driven plasma wakefields to address the challenge of high luminosity generation for a future linear collider. For linear colliders, beam cooling is realised by means of damping rings equipped with wiggler magnets and accelerating cavities. This scheme ensures systematic reduction of phase space volume through synchrotron radiation emission whilst compensating for longitudinal momentum loss via an accelerating cavity. In this paper, the concept of a plasma wiggler and its effective model analogous to a magnetic wiggler are introduced; …
In work is considered average transverse dynamics of an electron beam in the autoresonant laser. It is shown, that in approach of the given external electromagnetic wave (small gain free electron laser) transverse emittance of a beam of the charged particles decreases. This for the first time found out effect can be used for cooling beams of various accelerators of the charged particles. In the field of particle energy about 100 in the mass energy units, beam energy losses are negligible. This method of charge beams cooling may be applied to electron, muon and various heavy particle beams.
