Equilibrium phase instability in the double RF system for Landau damping

A longitudinal coupled bunch instability in an electron storage ring was suppressed by the Landau damping in a double rf system composed of a second harmonic rf cavity. The damping became ineffective, however, above a beam current of 30 mA; the beam bunch slipped out of the optimum phase of the total rf voltage for the damping, which accompanied a simultaneous deformation of the total voltage. The unexpected phenomenon of the phase slip is explained by the concept of equilibrium phase instability of the beam bunch based on a rigid bunch model. The phase slip of the bunch was suppressed by introducing a phase feedback loop, resulting in an improvement of the maximum beam current for the damping. Discussions are made on various conditions of the equilibrium phase instability, including another possibility for avoiding the phase slip.     

Fast microwave detection system for coherent synchrotron radiation study at KEK: Accelerator test facility

A fast room temperature microwave detection system based on the Schottky Barrier-diode detector was created at the KEK ATF (Accelerator Test Facility). It was tested using Coherent Synchrotron Radiation (CSR) generated by the 1.28 GeV electron beam in the damping ring. The speed performance of the detection system was checked by observing the CSR from a multi-bunch (2.8 ns bunch separation time) beam. The theoretical estimations of CSR power yield from an edge of bending magnet as well as new injection tuning method are presented. A very high sensitivity of CSR power yield to the longitudinal electron distribution in a bunch is discussed.     

Experimental studies on the optimization of the accelerating field distribution in an IH lineac

A new method is being studied to control the accelerating field distribution in an IH lineac with a high energy gain. In spite of its high shunt impedance, a shortcoming of an IH structure is that a concentration of accelerating field occurs at the low-energy section of the cavity. One of the practical methods to control the accelerating-gap voltages is adjusting the electric-capacity distribution of the structure. This method, however, is not always suitable for an IH lineac when the particle-energy gain is larger than the incident particle energy.The basic idea of our experiments is to utilize an inductive method to “tune” the accelerating field distribution. In order to test the applicability of this method, accelerating field distributions and shunt impedances are measured in terms of these “tuner” configurations by using $\text{1}\text{4}$ scale models. The results are compared with the calculations based on a four-terminal network simulation.     

High-intensity multi-bunch beam generation by a photo-cathode RF gun

A multi-bunch photo-cathode RF gun system has been developed as an electron source for the production of intense quasi-monochromatic X-rays based on inverse Compton scattering. The desired multi-bunch beam is 100 bunches/pulse with a total charge of 500 nC and a bunch spacing of 2.8 ns. We modified the gun cavity of a ‘BNL-type IV’ RF gun to allow a CsTe cathode plug in the end plate. The system uses a four-dipole chicane beam line to allow the injection of laser light normal to the cathode surface. We compensate the gun cavity beam loading caused by the high-intensity multi-bunch electron beam by injecting the laser pulse before RF power has filled the cavity. We have achieved a total intensity of 220 nC in 100 bunches with a bunch-to-bunch energy spread under 1.3% (peak-to-peak). This paper concentrates on experiments to generate the high-intensity multi-bunch beam with compensation of the bunch-to-bunch energy spread due to heavy beam loading.     

Group velocity effect on resonant, long-range wake-fields in slow wave structures

Synchronous wake-fields in a dispersive waveguide are derived in a general explicit form on the basis of a rigorous electro-dynamical approach using Fourier transformations. The fundamental role of group velocity in wake-field propagation, calculation of attenuation, amplitudes, form-factors and loss-factors is analyzed for single bunch radiation. Adiabatic tapering of the waveguide and bunch density variation is taken into account analytically for the time-domain fields. Effects of field “compression/expansion” and group delays are demonstrated. The role of these effects is discussed for single bunch wake-fields, transient beam loading, BBU and HOMs. A novel waveguide structure with central rf coupling and both positive and negative velocities is proposed. It can be used effectively in both high-energy accelerators and single-section linacs.     

RF properties of a X-band hybrid photoinjector

An INFN-LNF/UCLA/SAPIENZA collaboration is developing a hybrid photoinjector in X-band. A hybrid photoinjector is a novel high brightness electron source that couples a standing wave cell cavity (acting as an RF gun) directly to a multi-cell travelling-wave structure. This configuration offers a number of advantages over the split standing wave/travelling-wave system. Most notably the reflected RF transient is almost completely suppressed, thus eliminating the need for a circulator and the bunch lengthening effect that occurs in the drift section of the split system. These properties allow scaling of the device to higher field and frequencies, which should dramatically improve beam brightness. The RF coupling between the standing and the traveling wave sections is accomplished in the fourth cell encountered by the beam, with the SW section electrically coupled to it on-axis. This mode of coupling is particularly advantageous, as it is accompanied by a 90° phase shift in the accelerating field, resulting in strong velocity bunching effects on the beam that reverse the usual bunch lengthening induced after the gun exit in standard 1.6 cell photoinjectors. In this scenario, from the beam dynamics point of view, it is seen that device may produce ten's of femtosecond beams at ∼3.5 MeV and the emittance compensation dynamics remains manageable even in the presence of strong compression. We present here a survey of the device characteristics. In particular we show the results of the electromagnetic simulations, a beam dynamics analysis related to the temperature tuning of the SW and TW section, and a RF characterization using bead pull and scattering coefficient measurements of a device prototype.     

Transverse wakefield effects in the two-beam accelerator

Transverse wakefield effects in the high-gradient accelerating structure of the two-beam accelerator (TBA) [1–3] are analyzed theoretically using three different models. The first is a very simple two-particle model due to Wilson [4]; the second, due to Chao et al. [5], is for a beam with uniform charge distribution, constant betatron wavelength, and a linear wake approximation. Both of these models give analytic scaling laws. The third model has a Gaussian beam (represented by 11 superparticles), energy variation across the bunch, acceleration, variation of betatron focusing with energy, and variation of the wakefield from linearity. The three models are compared, and the third model is used to explore the wakefield effects when accelerator parameters such as energy, energy spread, injection energy, accelerating gradient, and betatron wavelength are varied. Also explored are the sensitivity of the beam to the wakefield profile to the longitudinal charge distribution. Finally, in consideration of wakefield effects, possible parameters of a TBA are presented.     

Practical Design for Improving the Sensitivity to Search for Dark Matter Axions with Rydberg Atoms

A microwave single-photon detector was developed with highly-excited alkaline Rydberg-atoms in a cooled resonant cavity to search for dark matter axions. This detector belongs to a microwave single-photon counter, thus being free from the standard quantum limit (SQL). High sensitivity of the present detector system was demonstrated by measuring the thermal blackbody radiations in the cavity at temperatures as low as 70 mK where the sensitivity is below the SQL. The detection sensitivity of the present system is mainly limited by stray electric fields present in the detection region. Practical design of a new experimental scheme with a guiding electric field through the atomic-beam trajectory is here presented and discussed to avoid the effect of stray electric field and thus to improve the detection sensitivity.      

Bunch length measurement of picosecond electron beams from a photoinjector using coherent transition radiation

The bunch length of an electron beam derived from the UCLA Saturnus photoinjector has been measured using a 45° CTR foil. The sudden change of electrons boundary conditions cause them to radiate (transition radiation) with the spectral power entirely dependent upon the degree of coherency, which strongly relates to the beam size. A polarizing Michelson interferometer allowed measurement of the auto-correlation of the coherent transition radiation signal. An analysis method was developed to compensate for undetected low-frequency radiation and systematically extract the bunch length information for a specific beam model. This analysis allowed observation of pulse lengthening due to the space charge, as well as compression with the variation of the RF injection phase. The hypothesis of a satellite beam has been also tested using this analysis.     

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.     

A 25.5 MHz double-coaxial λ/4-resonator as a rebuncher in heavy ion linac system

A 25.5 MHz double-coaxial λ/4-resonator has been constructed to be used as a rebuncher between a 25.5 MHz RFQ linac and a 51 MHz interdigital-H linac for the acceleration of short-lived nuclei. By employing a double coaxial structure, the resonator length is only 130 cm; the length of the natural λ/4-resonator is 294 cm. The resonator, 69 cm in inner diameter, has six acceleration gaps. The bare shunt impedance is 40.6 MΩ/m. The ions with a charge-to-mass ratio 1/10 can be rebunched by feeding an rf power of 1.4 kW.     

Analytic beam spread function for ocean optics applications

A discrete ordinates code is developed with which to compute the beam spread function (BSF) without invoking the small-angle scattering approximation or performing Monte Carlo calculations. The computed BSF is used to predict the response of a detector versus its distance to the origin of a highly collimated beam, its angle with respect to the beam, and the two local angles that specify the detector orientation. Numerical results have been obtained for water models that simulate a clear ocean, a coastal ocean, and a turbid harbor. Six orders of magnitude or more change in the detector response caused by scattered photons can be predicted for different detector locations while simultaneously obtaining small changes for different detector orientations. This capability is useful for assessment of the sensitivity of the detector response to the interpretation of time-independent underwater imaging systems or visibility models.     

Trace gas detection by laser intracavity photothermal spectroscopy

A novel laser intracavity photothermal detector is described. In this scheme, sample absorption of the pump laser power takes place within the cavity of a probe He-Ne laser causing modulation in the gain and in turn the output power. Comparison of this intracavity detector with two other photothermal techniques, namely, phase fluctuation optical heterodyne spectroscopy and thermal beam deflection, is made in terms of practicality and sensitivity. For in situ measurements, sensitivity of 0.5 x 10(-7) cm(-1) for a probe length of 3 cm has been achieved.     

Birefringent beam splitter for intracavity mode selection in high-power multimirror lasers

The design and application of an uncoated sapphire plate that acts as both the beam splitter and the output coupler of an interferometric laser resonator are described. Output coupling is provided at one of the surfaces by p-polarized (TM) reflection near the Brewster angle, and axial-mode selection is enforced at the other surface by s-polarized (TE) reflection at the same angle of incidence. The design is discussed in the context of the phase-locked, rf linac free-electron laser, in which the coupling of adjacent optical pulses at the beam splitter induces temporal phase coherence among all the pulses in the output beam; this coherence is manifested in the frequency domain as a reduction in the number of axial modes per rf frequency interval. The Michelson and Fox-Smith resonator designs are compared, and applications to cavity dumping are discussed.     

Some applications of particle-in-cell codes to problems of high intensity beams

The technique of using the “particle-in-cell” method was developed in the field of plasma physics [1]. Borrowed for problems of intense beams, it becomes an especially powerful tool because such problems frequently use single species “plasmas” and so pose a less severe requirement on the computer. We will look at several problems in which the PIC code method has been useful. The first is the classical Pierce gun in a transient or short pulse mode. Here the transverse beam optics is strongly affected by the time dependence of the current. The second is a study of high power klystrons searching for the source of an instability. The third is the high power rf source called the “lasertron” which is under development at SLAC. The interesting new development for the lasertron simulation is the introduction of a double gap output cavity for improved efficiency. The lasertron and klystron simulations are steady state solutions to rf problems with high-Q cavities. In order to limit the computation to a realistic time, these simulations use an external equivalent circuit which can communicate with the beam tunnel through ports placed at the locations of the rf cavities. Application for electron beams generally require using a fully relativistic electromagnetic code such as MASK. In some applications, the computation can be speeded up by limiting the solution of the fields to the electrostatic conditions. This can be especially helpful if the degree of precision required demands very large numbers of macroparticles. As a last example, we will show the use of this technique for a problem involving emittance growth for a high intensity beam for heavy ion fusion.     

A High-Sensitivity Microwave-Single-Photon Detector with Rydberg Atoms at Low Temperature

A high-sensitivity microwave-single-photon detector was developed in Kyoto, in which microwave photons in a resonant cavity cooled at very low temperatures are absorbed by highly excited Rydberg atoms and the Rydberg atoms thereby promoted to a higher excited state are then selectively field-ionized and detected. This scheme allows us to count microwave photons one by one, thus provide a single-photon counting without the limit of standard quantum limit (SQL). The apparatus “CARRACK” for the single-photon detector was constructed based on this scheme, where the cavity was cooled down to 10 mK range to reduce the background of thermal blackbody photons from the cavity wall. The apparatus has served for years to search for dark matter axions in the 10 μeV (∼2.4 GHz) mass region. Thermal blackbody photons in a microwave resonant cavity at temperatures as low as 70 mK have been measured, the sensitivity being below the SQL limit. A number of improvements in the detection efficiency and sensitivity have been planned and will be reported. Applications of the detector to fundamental physics are also discussed shortly.      

Spectral characterization of integrated acousto-optic tunable filters by means of laser frequency modulation spectroscopy

The spectral characteristics of an integrated acousto-optic tunable filter (AOTF) as well as its responsivity to the rf driving signal and sensitivity to temperature changes are experimentally investigated and quantified using a diode-laser-based interrogation system. A spectroscopic technique, exploiting the rf frequency modulation of the laser beam and the phase-sensitive detection of the AOTF transmission, has been used for this purpose. That allows for the generation of a dispersivelike signal, which serves as a reference for tracking any wavelength change of the filter's peak with high resolution. The possibility of using the integrated AOTF as a spectrum analyzer with this interrogation scheme for fiber Bragg grating (FBG) strain sensing is also discussed.     

Disk-and-washer cavities for an accelerator

Studied is the behavior of disk-and-washer accelerator cavities, in which the single radial stem support is adopted. The support leads about 70% larger and 20% smaller rf power loss than that of the calculated value in the normal cell and in the end cell, respectively. The distortion of the field is limited in the region of the so-called coupling cells. The properties of the coupled cavities, as the dispersion curve or the effect of tuners on the relative axial field is measured and well explained by the biperiodic coupled resonator model. The manufacturing of the actual cavity is also described, in which electroplating and welding methods are developed. The transverse coupling impedance calculated for a multiple-cell cavity are comparable with the instability experiments made in the TRISTAN accumulation ring.     

Low radiation level detection with room temperature InAs detector

Recently, room temperature or near room temperature InAs detectors are widely used in laser warning receivers, process control monitors, temperature sensors, etc. requiring linear operation over many decades of the sensitivity range. The linearity of zero biased Si, InGaAs and Ge detectors is thoroughly discussed in the literature, contrary to InAs detectors. In an earlier work of the authors it has been demonstrated that applying a bootstrap circuit to a Ge detector – depending on the frequency of the operation – will virtually increase the shunt resistance of the detector by 3–6 decades compared to the detector alone. In the present work, a similar circuitry was applied to a room temperature InAs detector, the differences between the bootstrapped Ge and bootstrapped InAs detector are underlined. It is shown, how the bootstrap circuit channels the photogenerated current to the feedback impedance decreasing with many decades the detectable low level limit of the detector – I/V converter unit. The linearity improvement results are discussed as a function of the chopping frequency, calculated and measured values are compared, the noise sources are analyzed and noise measurement results are presented.