Search Results (18 total)

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and γ-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO₂ laser beam.

At a warm linear collider the short time interval at which bunches will pass near each other in the interaction region may lead to significant alteration of the bunches positions. In this paper we quantify the intensity of this effect and show that it can be addressed by a fast intrapulse feedback system.

We develop a new method to simulate coherent synchrotron radiation numerically. It is based on the mesh calculation of the electromagnetic field in the frequency domain. We make an approximation in the Maxwell equation which allows a mesh size much larger than the relevant wavelength so that the computing time is tolerable. Using the equation, we can perform a mesh calculation of coherent synchrotron radiation in transient states with shielding effects by the vacuum chamber. The simulation results obtained by this method are compared with analytic solutions. Though, for the comparison with theories, we adopt simplifications such as longitudinal Gaussian distribution, zero-width transverse distribution, horizontal uniform bend, and a vacuum chamber with rectangular cross section, the method is applicable to general cases.

Based on the requirements from a conceptual design of a polarized positron beam for future linear colliders, we constructed a special collision system with a short focal length of 150 mm of the laser beams so as to produce γ rays through inverse Compton scattering. In order to achieve efficient laser-electron collisions, we created a special optics to produce very small e^--beam sizes of σ_ex0=7.6   μm and σ_ey0=5.4   μm in the horizontal and vertical directions at the collision point. Using laser light with a wavelength of 532 nm and an e^- beam of 1.28 GeV, provided from the ATF-damping ring at KEK, we generated 2×10⁵ γ rays with a time duration of 26 ps in rms, leading to a peak brightness of 1.7×10¹⁸/(mrad² mm² 0.1%bandwidth s) near to the maximum energy of 56 MeV.

A vertical coupled-bunch instability was observed for a positron beam at the Beijing Electron Positron Collider (BEPC). The experimental results show that the instability has similar characteristics as that observed in the Photon Factory of KEK several years ago. The instability at BEPC can be explained by the effect of an electron cloud which is produced in the beam chamber by synchrotron light hitting the wall.

Since the resistive wall impedance for a beam pipe of a nonround cross section depends on the coordinates of a witness particle, the witness particle receives an incoherent tune shift. When the expression for the impedance of an infinitely thick chamber is applied to the calculation of this tune shift, it becomes infinite. We have derived the resistive wall impedance for a chamber with a finite thickness and calculated the tune shift. There is no ambiguity in this expression for the tune shift, because it is automatically finite.

A theory concerning the relation between the heating rate and temperature of hadron beams is formulated from a quantum point of view. This theory predicts that the heating rate can be reduced by increasing the lattice periodicity of the accelerator with its fixed tunes and circumference. This prediction is quite consistent with simulation results.

The soft-Gaussian approximation is often employed in computer simulations of strong-strong beam-beam interactions in storage rings. Its defect on the coherent oscillation frequency is pointed out and a possible remedy using Hermitian polynominals is discussed.

7.6×10⁶ x-ray photons per 3.5 ps pulse are detected within a 1.8–2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW CO₂ laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a σ  =  32 μm spot. Our next plan is to demonstrate 10¹⁰ x-ray photons per pulse using a CO₂ laser of ∼1 TW peak power.

The dynamics of particles in laser pulse-driven wakefields over multistages in a collider is studied. A map of phase space dynamics over a stage of wakefield acceleration induced by a laser pulse (or electron beam) is derived. The entire system of a collider is generated with a product of multiple maps of wakefields, drifts, magnets, etc. This systems map may include offsets of various elements of the accelerator, representing noise and errors arising from the operation of such a complex device. We find that an unmitigated strong focusing of the wakefield coupled with the alignment errors of the position (or laser beam aiming) of each wakefield stage and the unavoidable dispersion in individual particle betatron frequencies leads to a phase space mixing and causes a transverse emittance degradation. The rate of the emittance increase is proportional to the number of stages, the energy of the particles, the betatron frequency, the square of the misalignment amplitude, and the square of the betatron phase shift over a single stage. The accelerator with a weakened focus in a channel can, therefore, largely suppress the emittance degradation due to errors.

We describe a method to measure a nanometer beam size during collisions at future e⁺e^- linear colliders by using e⁺e^- pairs. A huge number of pairs are deflected in a strong Coulomb potential made by an oncoming beam. Since the potential is a function of the beam size (σ_x,σ_y), the pairs are expected to carry this information, especially in their angular distributions. We investigated this process in detail by simulation, using a computer program abel under realistic experimental conditions as well as by analytic studies. Besides the beam size, a vertical displacement between two beams and rotations in the transverse beam profile can be precisely measured. Because of the high statistics of pairs, the results from these measurements can be used for feedback during real-time operation of linear colliders.

The equilibrium polarization of the electron spin has been measured at the TRISTAN electron-positron storage ring. The measurement was carried out by using a laser polarimeter detecting an asymmetry in the Compton scattering of a circularly polarized laser beam on polarized electrons. The laser polarimeter allows for simultaneous measurements of the transverse and longitudinal polarization with sufficient accuracy. The polarization level was relatively high, 40% at an electron-beam energy of 29 GeV, and the polarization vector was inclined at an angle of about 30°.

It is shown that a scheme of beam-beam collisions which makes a head-on collision in a transversely boosted frame is applicable to storage-ring colliders. This scheme allows a large crossing angle at the collision point without an excitation of synchrotron-betatron resonances, and will give merits in designing high-luminosity colliders.

The correction terms to the Sokolov-Ternov radiation formula due to variation of the magnetic field strength along the electron trajectory are calculated up to the second order in the power expansion of B_τ / B, where τ is the formation time of radiation. It is found that the field-gradient effect reduces radiation intensity in the classical regime, and enhances it in the quantum regime. This is then applied to quantum beamstrahlung with Gaussian variation in e⁺e^- bunch currents. The correction is shown to be substantial for beam parameters suggested by Himel and Siegrest.

We report on our recent simulation results on the disruption effects during the interaction of round e⁺e^- beams in linear colliders. It is found that in addition to the well-known disruption parameter D, the disruption effects also depend on another parameter A≡σ_z/β^*, where σ_z is the bunch length and β^* the β function at the interaction point. It turns out that while the luminosity enhancement factor H_D is insensitive to A only in the small-D (D≲1) regime, the disruption-angle enhancement factor H_θ behaves oppositely. Specifically, we found that for large D, H_θ is suppressed as 1/ √D , and H_D increases monotonically without saturation. Moreover, for fixed D, H_D varies as a function of ln(1/A). Computer analysis further suggests that in the large-D and small-A regime a confinement mechanism is developed near the beginning of the collision: particles once pinched tend to be trapped in a much smaller radius throughout the process. A theoretical model is provided to qualitatively explain the above findings.

A nonlinear perturbation theory is developed for longitudinal instabilities of a coasting beam in particle accelerators. In contrast to the linear theory, the present perturbation approach to the Vlasov equation demonstrates that the part of the particle distribution function averaged over the azimuthal angle around the ring has the time dependence of second order in perturbed quantities. A set of differential equations in time is derived for the average distribution (or the energy spread) and the perturbation amplitude. A mapping of the time derivative of the energy spread onto the complex impedance plane is obtained and it shows that the region where the energy spread increases exists even in the stable area. The energy spread for an initially unstable beam is shown to increase beyond the threshold of the stability and eventually to converge to a final value determined by the initial energy spread and the threshold value. Quantitative agreements are obtained between the experimental results at the CERN ISR and the theoretical overshoot formula in the case where the initial energy spread is close to the threshold.

An ambiguity exists in the prescription for deriving n-body potentials (n ≥ 4) from the S matrix in quantum field theory. The prescription can be modified in such a way that the potentials coincide with the classical potentials.