Search Results (14 total)

We report on a recent set of measurements of the transverse wakefields from longitudinally tapered collimators. The measurements were performed with a low-emittance 1.19 GeV beam in the SLAC linac by inserting a collimator aperture into the beam path and reconstructing the vertical deflection of the beam as a function of the vertical position of the aperture. Each collimator in the experiment was designed to present a relatively large transverse impedance and to minimize the impedance from other contributions such as resistivity. In addition, the collimator parameters were chosen to provide some insight into the scaling of the transverse geometric wakefield as a function of the collimator’s geometry. A description of the experimental apparatus and the aperture design, the method of data collection and analysis, and a comparison to theoretical and numerical predictions are presented.

Coherent synchrotron radiation (CSR) can play an important role by not only increasing the energy spread and emittance of a beam, but also leading to a potential instability. Previous studies of the CSR induced longitudinal instability were carried out for the CSR impedance due to dipole magnets. However, many storage rings include long wigglers where a large fraction of the synchrotron radiation is emitted. This includes high-luminosity factories such as DAPHNE, PEP-II, KEK-B, and CESR-C as well as the damping rings of future linear colliders. In this paper, the instability due to the CSR impedance from a wiggler is studied assuming a large wiggler parameter K. The primary consideration is a low-frequency microwavelike instability, which arises near the pipe cutoff frequency. Detailed results are presented on the growth rate and threshold for the damping rings of several linear collider designs. The different scaling between the wiggler CSR impedance and the dipole CSR impedance suggests an optimization for the damping ring design.

Future linear colliders may require a nonzero crossing angle between the two beams at the interaction point (IP). This requirement in turn implies that the beams will pass through the strong interaction region solenoid with an angle, and thus that the component of the solenoidal field perpendicular to the beam trajectory is nonzero. The interaction of the beam and the solenoidal field in the presence of a crossing angle will cause optical effects not observed for beams passing through the solenoid on axis; these effects include dispersion, deflection of the beam, and synchrotron radiation effects. For a purely solenoidal field, the optical effects which are relevant to luminosity exactly cancel at the IP when the influence of the solenoid’s fringe field is taken into account. Beam size growth due to synchrotron radiation in the solenoid is proportional to the fifth power of the product of the solenoidal field, the length of the solenoid, and the crossing angle. Examples based on proposed linear collider detector solenoid configurations are presented.

Most studies of coherent synchrotron radiation (CSR) have considered only the radiation from independent dipole magnets. However, in the damping rings of future linear colliders, a large fraction of the radiation power will be emitted in damping wigglers. In this paper, the longitudinal wakefield and impedance due to CSR in a wiggler are derived in the limit of a large wiggler parameter K. After an appropriate scaling, the results can be expressed in terms of universal functions, which are independent of K. Analytical asymptotic results are obtained for the wakefield in the limit of large and small distances, and for the impedance in the limit of small and high frequencies.

In this paper, we present a series of analytic expressions to predict the beam dynamics in a long linear accelerator. These expressions can be used to model the linac optics, calculate the magnitude of the wakefields, estimate the emittance dilution due to misaligned accelerator components, and estimate the stability and jitter limitations. The analytic expressions are based on the results of simple physics models and are useful to understand the parameter sensitivities. They are also useful when using simple codes or spreadsheets to optimize a linac system.

Electron bunches of high charge (up to 10 nC) are compressed in length in the Compact Linear Collider Test Facility magnetic chicane to less than 0.4 mm rms. The short bunches radiate coherently in the chicane magnetic field, and the horizontal and longitudinal phase space density distributions are affected. This paper reports the results of beam emittance and momentum measurements. Horizontal and vertical emittances and momentum spectra were measured for different bunch compression factors and bunch charges. In particular, for 10 nC bunches, the mean beam momentum decreased by about 5% while the rms momentum spread increased from 2% to 8%. The experimental results are compared with simulations made with the code TraFiC4.

Beam-based alignment of quadrupoles by variation of quadrupole strength is a widely used technique in accelerators today. We describe the dominant systematic limitation of such algorithms, which arises from the change in the center position of the quadrupole as the strength is varied, and derive expressions for the resulting error. In addition, we derive an expression for the statistical resolution of such techniques in a periodic transport line, given knowledge of the line's transport matrices, the resolution of the beam position monitor system, and the details of the strength variation procedure. These results are applied to the Next Linear Collider main linear accelerator, an 11 km accelerator containing 750 quadrupoles and 5 000 accelerator structures. We find that, in principle, a statistical resolution of 1μm is easily achievable, but the systematic error due to variation of the magnetic centers could be several times larger.

Measurements of the beam emittance during bunch compression in the CLIC Test Facility (CTF-II) are described. The measurements were made with different beam charges and different energy correlations versus the bunch compressor settings which were varied from no compression through the point of full compression and to overcompression. Significant increases in the beam emittance were observed with the maximum emittance occurring near the point of full (maximal) compression. Finally, evaluation of possible emittance dilution mechanisms indicates that coherent synchrotron radiation was the most likely cause.

In colliding-beam facilities, the “final-focus system” must demagnify the beams to attain the very small spot sizes required at the interaction points. The first final-focus system with local chromatic correction was developed for the Stanford Linear Collider, where very large demagnifications were desired. This same conceptual design has been adopted by all of the future linear collider designs as well as the Superconducting Super Collider, the Stanford and KEK B Factories, and the proposed Muon Collider. In this paper, the overall layout, physics constraints, and optimization techniques relevant to the design of final-focus systems for high-energy electron-positron linear colliders are reviewed. Finally, advanced concepts to avoid some of the limitations of these systems are discussed.

We report the results of observations of a new regime of ion instabilities at the Advanced Light Source (ALS). With artificially increased pressure and gaps in the bunch train large enough to avoid multiturn ion trapping, we observed a factor of 2–3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. The observations are qualitatively consistent with the “fast beam-ion instability” [T. O. Raubenheimer and F. Zimmermann, Phys. Rev. E 52, 5487 (1995); G. V. Stupakov et al., Phys. Rev. E 52, 5499 (1995)], which can arise even when the ions are not trapped over multiple beam passages. This effect may be important for many future accelerators.

The ionization of residual gas by an electron beam in an accelerator generates ions that can resonantly couple to the beam through a wave propagating in the beam-ion system. A beam-ion instability is studied for a multibunch train, taking into account the decoherence of ion oscillations due to the ion frequency spread. It is shown that while the decoherence does not completely suppress the instability, it makes the growth rate smaller. A comparison of analytical and numerical results indicates a good agreement with direct macroparticle simulation of the instability.

The interaction of an electron beam with residual gas ions results in mutually driven transverse oscillations. This effect arises during the passage of a single train of bunches. An equivalent instability mechanism is encountered in positron beams where ionization electrons oscillate within a single bunch. In either case, the oscillations grow exponentially with an exponent proportional to t^1/2. In this report, the rise time of the instability is calculated analytically by a perturbation series approach and is compared with computer simulations. Growth rates are evaluated for several existing or proposed storage rings and linear accelerators; the effect considered could be a significant limitation in many of the future designs.

Talman [Phys. Rev. Lett. 56, 1429 (1986)] has proposed a novel relativistic effect that occurs when a charged particle beam is bent in the magnetic field from an external dipole. The consequence of this effect is that the space-charge forces from the particles do not exhibit the usual inverse-square energy dependence and some part of them are, in fact, independent of energy. This led to speculation that this effect could introduce significant emittance growth for a bending electron beam. Subsequently, it was shown that this effect’s influence on the beam’s transverse motion is canceled for a dc beam by a potential depression within the beam (to first order in the beam radius divided by the bend radius). In this paper, we extend the analysis to include short bunch lengths (as compared to the beam pipe dimensions) and find that there is no longer the cancellation for forces both transverse to and in the direction of motion. We provide an estimate for the emittance growth as a function of bend angle, beam radius, and current, and for magnetic compression of an electron bunch.