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Ions in the linacs of future linear colliders

Description: Ions have been identified as a potential limitation in high current storage rings, but they are not usually thought to be a problem in electron linear accelerators. In this paper, we consider the effects of ions in the linacs of future linear colliders. Future linear collider designs call for long trains of closely spaced bunches and/or very dense bunches. Thus, significant ion densities can be generated through the collisional ionization process and trapping'' in a long train of bunches or through the tunneling ionization with very dense bunches. We consider two principal effects of these ions: first, they provide skew fields which cause transverse betatron coupling and increase the vertical emittance of the flat beams, and, second, the ion fields increase the rate of filamentation, making correction of the emittance dilutions more difficult; this could lead to tighter alignment tolerances. Both of these effects are verified with simulations. We will not consider other potential limitations that the ions impose. We will consider these effects in four illustrative linear collider designs: the NLC, the old NLC design, the DESY S-band design, and the Russian design, VLEPP.
Date: July 1, 1992
Creator: Raubenheimer, T. O. & Chen, P.
Partner: UNT Libraries Government Documents Department

Chicane and wiggler based bunch compressors for future linear colliders

Description: In this paper, we discuss bunch compressors for future linear colliders. In the past, the bunch compression optics has been based upon achromatic cells using strong sextupoles to correct the dispersive and betatron chromaticity. To preserve the very small emittances required in most future collider designs, these schemes tend to have very tight alignment tolerances. Here, we describe bunch compressors based upon magnetic chicanes or wigglers which do need sextupoles to correct the chromatic emittance dilution. The dispersive chromaticity cancels naturally and the betatron chromaticity is not a significant source of emittance dilution. Thus, these schemes allow for substantially reduced alignment tolerances. Finally, we present a detailed design for the NLC linear collider.
Date: May 1, 1993
Creator: Raubenheimer, T. O.; Emma, P. & Kheifets, S.
Partner: UNT Libraries Government Documents Department

Computer studies of a combined-function bend magnet for a proposed redesign of the SLAC SLC damping rings

Description: A proposed redesign of the SLAC SLC Damping Ring requires a combined-function bending magnet. The magnet will operate with a main field of 1.8338 T, and quadrupole and sextupole gradients dB{sub y}/dx, d{sup 2}B{sub y}/dx{sup 2} of {minus}14.1 T/m and {minus}477 T/m{sup 2}, respectively. Because the orbit sagitta in the magnet is in excess of 2 cm, the pole will be curved with a 2 m radius of curvature. Furthermore, since the current must be variable over a range of {plus_minus}2 percent, we have considered using vanadium permendur poles to avoid a adverse saturation effects. Studies were done using POISSON in 2-D and TOSCA for 3-D end effects.
Date: April 1, 1993
Creator: Early, R. A. & Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Electron transport of a Linac Coherent Light Source (LCLS) using the SLAC linac

Description: A linac configuration providing a low emittance high peak current electron beam is under study for a potential Linac Coherent Light Source (LCLS) based on the SLAC accelerator. The parameters of the final electron bunch are nearing the technological limits of present accelerators in both transverse and longitudinal phase space. In this note we describe a layout of the RF gun, linac, and bunch compressors to deliver the required bunch properties. We consider a bunch that is generated by an rf gun and accelerated to 7 GeV in 900 m of SLAC linac structure before it enters the wiggler. We assume that the rf gun generates a gaussian beam with an energy of 10 MeV, a population N = 6 {times} 10{sup 9}e{sup {minus}}, an rms length {sigma}{sub z} = 0.5 mm, an rms energy spread {sigma}{sub {delta}} = 0.2%, and normalized rms emittances {gamma}{epsilon}{sub x,y} = 3 mm-mrad. At the end of the linac, we require that the peak current {cflx I} {approx_gt} 2.5 kA and the peak-to-peak energy spread {Delta}{delta} {approx_lt} 0.2%. To obtain the required high peak current, we need to compress the bunch length by a factor greater than 10. In deciding at what position in the linac to compress we need to consider three issues: the longitudinal wakefield in the linac, this increases the beam`s energy spread and is harder to compensate with short bunches, the transverse wakefield and rf deflections in the linac, these increase the transverse emittance of the beam and are more severe for long bunches, and the effects of phase and current jitter which will change the bunch length and therefore the peak current of the beam. In this paper, we will describe how we compress the bunch to meet these three criteria. Then, we will briefly describe the bunch compressor ...
Date: May 1, 1993
Creator: Bane, K. L.; Raubenheimer, T. O. & Seeman, J. T.
Partner: UNT Libraries Government Documents Department

A beam-based alignment technique for correction of accelerator structure misalignments

Description: This paper describes a method of reducing the transverse emittance dilution in linear colliders due to transverse wakefields arising-from misaligned accelerator structures. The technique is a generalization of the Wake-Free correction algorithm. The structure alignment errors are measured locally by varying the bunch charge and/or bunch length and measuring the change in the beam trajectory. The misalignments can then be corrected by varying the beam trajectory or moving structures. The results of simulations are presented demonstrating the viability of the technique.
Date: August 1, 1994
Creator: Kubo, K. & Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Method to evaluate steering and alignment algorithms for controlling emittance growth

Description: Future linear colliders will likely use sophisticated beam-based alignment and/or steering algorithms to control the growth of the beam emittance in the linac. In this paper, a mathematical framework is presented which simplifies the evaluation of the effectiveness of these algorithms. As an application, a quad alignment that uses beam data taken with the nominal linac optics, and with a scaled optics, is evaluated in terms of the dispersive emittance growth remaining after alignment.
Date: April 1, 1993
Creator: Adolphsen, C. & Raubenheimer, T.
Partner: UNT Libraries Government Documents Department

Emittance dilution by ions in the SLC arcs

Description: Since the start of flat-beam operation in 1993 typical vertical IP spot sizes in the SLC are well below 1 micron, and the luminosity is highly sensitive to dilutions of the vertical emittance. In the arcs, the ionization of the residual gas by the bunch head gives rise to an ion cloud, whose electric field deflects the tail of the bunch and may cause an unrecoverable filamentation in phase space. The effect of the ions is particularly important in regions with large dispersion, due to the strong correlation of energy and longitudinal position within a bunch. Computer simulations of a bunch passage in the presence of ions have been performed to estimate the magnitude of the emittance dilution.
Date: June 1, 1994
Creator: Emma, P.; Raubenheimer, T. & Zimmermann, F.
Partner: UNT Libraries Government Documents Department

A possible redesign of the SLAC SLC damping rings

Description: We describe a possible replacement for the SLC damping rings that would generate beams with normalized horizontal emittances of {gamma}{epsilon}{sub x} = 9 {times} 10{sup {minus}6} m-rad when uncoupled; this is more than three times smaller than that generated by the current rings. The primary difference between the new design and the current ring is the arc cell structure; the insertion regions, the kickers, and the RF are essentially unchanged. The new cell uses a single combined function bending magnet, roughly 70 cm in length, to replace the two bends, defocusing quadrupole, and defocusing sextupoles in the current FODO cell; the focusing quadrupole and sextupoles, used in the current cell, are also used in the new structure. The length of the new cell is identical to that of the current cell and thus nine of these new cells would simply replace the nine FODO cells in each arc of the rings.
Date: April 1, 1993
Creator: Raubenheimer, T. O.; Early, R.; Limberg, T.; Moshammer, H. & Spencer, J.
Partner: UNT Libraries Government Documents Department

Proposed emittance upgrade for the SLC damping rings

Description: One way to improve luminosity is to reduce transverse emittance by changing damping partitions. We consider the options in relation to the constraints. Besides modifications of the basic DR configuration the options include closed-orbit offsets in the quadrupoles, addition of strong multipoles and replacement of existing rectangular nosepieces on the dipoles (shim angles {phi}{equivalent_to}{theta}/2) with rotatable inserts. Measurements indicate the possibility of dynamically tuning {phi}>{ge}45{degrees} with decreases in {tau}{sub x} and {epsilon}{sub x} of {ge}50%. We discuss damping mechanisms to motivate the desirable field characteristics as well as nonlinear contours to cancel dipole harmonic errors (B>2T here) or to provide chromatic corrections. Such inserts could also be used to make cheaper, more compact rings with better impedance by reducing the number of conventional multipoles without impairing the stability. Estimated hardware costs are 250$/dipole end or multipole equivalent.
Date: June 1, 1994
Creator: Early, R.; Limberg, T.; Moshammer, H.; Raubenheimer, T.; Skarpaas, K. & Spencer, J.
Partner: UNT Libraries Government Documents Department

The preservation of low emittance flat beams

Description: Many future linear collider designs require beams with very small transverse emittances and large emittance ratios {epsilon}{sub x} {much_gt} {epsilon}{sub y}. In this paper, we will discuss issues associated with the preservation of these small emittances during the acceleration of the beams. The primary sources of transverse emittance dilution in a high energy linear accelerator are the transverse wakefields, the dispersive errors, RF deflections, and betatron coupling. We will discuss the estimation of these effects and the calculation of tolerances that will limit the emittance dilution with a high degree of confidence. Since the six-dimensional emittance is conserved and only the projected emittances are increased, these dilutions can be corrected if the beam has not filamented (phase mixed). We discuss methods of correcting the dilutions and easing the tolerances with beam-based alignment and steering techniques, and non-local trajectory bumps. Finally, we discuss another important source of luminosity degradation, namely, pulse-to-pulse jitter.
Date: April 1, 1993
Creator: Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Ions in the linacs of future linear colliders

Description: Ions have been identified as a potential limitation in high current storage rings, but they are not usually thought to be a problem in electron linear accelerators. In this paper, we consider the effects of ions in the linacs of future linear colliders. Future linear collider designs call for long trains of closely spaced bunches and/or very dense bunches. Thus, significant ion densities can be generated through the collisional ionization process and ``trapping`` in a long train of bunches or through the tunneling ionization with very dense bunches. We consider two principal effects of these ions: first, they provide skew fields which cause transverse betatron coupling and increase the vertical emittance of the flat beams, and, second, the ion fields increase the rate of filamentation, making correction of the emittance dilutions more difficult; this could lead to tighter alignment tolerances. Both of these effects are verified with simulations. We will not consider other potential limitations that the ions impose. We will consider these effects in four illustrative linear collider designs: the NLC, the old NLC design, the DESY S-band design, and the Russian design, VLEPP.
Date: July 1, 1992
Creator: Raubenheimer, T. O. & Chen, P.
Partner: UNT Libraries Government Documents Department

Centrifugal space-charge force for bunched beams

Description: In 1990, E. Lee showed for a DC beam that the effect of the centrifugal space-charge force (CSCF) on an electron`s transverse motion in a end is cancelled to lowest order by the effect of the potential depression of the beam. However, this cancellation does not in general occur when the beam is bunched. We solve for the harmonic scalar and vector potentials and explicitly find the energy-independent transverse and longitudinal space-charge forces. We make estimates of the emittance growth as a function of bend angle and path length resulting from these forces.
Date: August 1, 1994
Creator: Carlsten, B. E. & Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Calculations of emittance and damping time effects in the SLC damping rings

Description: In a recent NDR machine experiment the transverse emittance was studied as a function of store time and tune. To explain the observed transverse emittance damping time constants, the magnetic measurement data of the longitudinal field of the bending magnets had to be taken into account. The variation of the transverse emittances with tune due to misalignments and the associated anomalous dispersion is studied as well as the effect of synchrobetatron coupling due to dispersion in the RF cavities.
Date: March 1, 1992
Creator: Limberg, T.; Moshammer, H.; Raubenheimer, T.; Spencer, J. & Siemann, R.
Partner: UNT Libraries Government Documents Department

Measured emittance versus store time in the SLC damping ring

Description: Emittance studies at the SLC North Damping Ring led to precise measurements of the damping time using three independent methods. These measurements were done at three different locations: (1) in the ring using a fast gated video camera which allows the acquisition of the image of the synchrotron light from a single turn, (2) using the extracted beam and a single wire scanner in the ring-to-linac transport line, and (3) in the linac using four wire scanners. In addition the extracted beam emittance was studied as a function of various parameters. A significant dependence on the tune was observed.
Date: March 1, 1992
Creator: Decker, F. J.; Emma, P.; Krejcik, P.; Limberg, T.; Minty, M.; Moshammer, H. et al.
Partner: UNT Libraries Government Documents Department

Over-compression, a method to shape the longitudinal bunch distribution for a reduced energy spread

Description: In the Stanford Linear Collider the energy spread of the bunches at the end of the linac is dominated by longitudinal wakefields. A short, high current bunch with a Gaussian shape will produce a double-horned energy distribution. It can be shown that certain charge distributions with a sharp rise time (about rectangular or half-Gaussian) will give no additional energy spread due to the linac, since the generated wakefield and the rf-curvature cancel each other exactly. In this paper different methods are presented on how to achieve such distributions by using non-linear dependences in the RTL (Ring-To-Linac) compression region. A simple and effective method to achieve such a distribution is by over-compression. When not fully compressing the bunch, there are two settings of the compressor voltage, under and over-compression, which give the same core bunch length in the linac. By switching from the under to the over-compressed setting, the tails are reduced from more than Gaussian to less than Gaussian beam tails. This results in a roughly rectangular shape which will give the wakefield-rf cancellation. Simulations, measurements and their implications are discussed.
Date: July 1, 1994
Creator: Decker, F. J.; Holtzapple, R. & Raubenheimer, T.
Partner: UNT Libraries Government Documents Department

Beam parameters of a possible emittance-dynamics test area for NLC studies at the SLC

Description: A group at SLAC has studied the possibility of using the Stanford Linear Collider (SLC) to generate short-bunch small-emittance beams similar to those required for the Next Linear Collider (NLC). The conclusion is that such beams are feasible and that an experimental area for testing many concepts related to NLC beams can be provided with a reasonable addition of hardware to the existing SLC Linac. Some of the concepts that can be tested are: (1) effect tolerances of double bunch length compression, (2) wakefields of ultra-short bunches in accelerating structures, (3) the acceleration of short intense multiple bunches, (4) the generation and preservation of bunches with 100 to 1 emittances ratios, (5) beam deflections by collimators, (6) energy and energy spread control of multiple short bunches, and (7) vibration effects and trajectory stability for low emittance beams.
Date: August 1, 1992
Creator: Seeman, J. T.; Fieguth, T.; Kheifets, S.; Raubenheimer, T. & Yeremian, A. D.
Partner: UNT Libraries Government Documents Department

The generation and acceleration of low emittance flat beams for future linear colliders

Description: Many future linear collider designs call for electron and positron beams with normalized rms horizontal and vertical emittances of {gamma}{epsilon}{sub x} = 3{times}10{sup {minus}6} m-rad and {gamma}{epsilon}{sub y} = 3{times}10{sup {minus}8} m-rad; these are a factor of 10 to 100 below those observed in the Stanford Linear Collider. In this dissertation, we examine the feasibility of achieving beams with these very small vertical emittances. We examine the limitations encountered during both the generation and the subsequent acceleration of such low emittance beams. We consider collective limitations, such as wakefields, space charge effects, scattering processes, and ion trapping; and also how intensity limitations, such as anomalous dispersion, betatron coupling, and pulse-to-pulse beam jitter. In general, the minimum emittance in both the generation and the acceleration stages is limited by the transverse misalignments of the accelerator components. We describe a few techniques of correcting the effect of these errors, thereby easing the alignment tolerances by over an order of magnitude. Finally, we also calculate ``fundamental`` limitations on the minimum vertical emittance; these do not constrain the current designs but may prove important in the future.
Date: November 1, 1991
Creator: Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department

Linac Coherent Light Source (LCLS) at 2--4 nm using the SLAC linac

Description: The authors describe the possible use of the SLAC linac to drive a unique, powerful, short wavelength Linac Coherent Light Source (LCLS). Using the FEL principle, lasing is achieved in a single pass of a high peak current electron beam through a long undulator by self-amplified-spontaneous-emission (SASE). The main components are a high-brightness electron RF gun with a photocathode, two electron bunch length compressors, the existing SLAC linac, beam diagnostics, and a long undulator combined with a FODO quadrupole focusing system. The RF gun, to be installed about 1 km from the end of the SLAC linac, would produce a single bunch of 6 x 10{sup 9} electrons with an invariant emittance of about 3 mm-mrad and a bunch length of about 500 {mu}m. That bunch is then accelerated to 100 MeV and compressed to a length of about 200 {mu}m. The main SLAC linac accelerates the bunch to 2 GeV were a second bunch compressor reduces the length to 30--40 {mu}m and produces a peak current of 2--3 kA. The bunch is then accelerated to 7--8 GeV and transported to a 50--70 m long undulator. Using electrons below 8 GeV, the undulator could operate at wavelengths down to 2 nm, producing about 10 GW peak power in sub-ps light pulses. At a linac repetition rate of 120 Hz, the average power is about 1 W. Linac operation at lower beam energies provides longer wavelength radiation. After the undulator, the beam is deposited in a dump. The LCLS light pulses are then distributed to multiple user stations using grazing incident mirrors. Length compression, emittance control, phase stability, FEL design criteria, and parameter tolerances are discussed. A demonstration experiment is also described which uses the SLAC linac and (possibly) the PALADIN undulator to study SASE to power saturation at wavelengths of ...
Date: August 1, 1993
Creator: Seeman, J. T.; Bane, K.; Boyce, R.; Loew, G.; Morton, P.; Nuhn, H. D. et al.
Partner: UNT Libraries Government Documents Department

Betatron phase advance measurement at SPEAR

Description: There are many reasons to determine the betatron phase advance between two azimuthal positions in a circular accelerator or storage ring. We have measured the betatron phase advance between various pairs of azimuthal points in the SPEAR Storage Ring by two different methods. The first method is to excite a steady state coherent betatron oscillation with a network analyzer. The second method is to excite a free coherent betatron oscillation with an impulse kick, and to digitally sample the transverse position of the beam at the pickup stations. The results of these digital samples are Fourier analyzed with a computer to obtain the phase advance. The second method is discussed, and the experimental results compared to theory.
Date: February 1, 1987
Creator: Morton, P.L.; Pellegrin, J.L.; Raubenheimer, T. & Ross, M.
Partner: UNT Libraries Government Documents Department

Beam parameters of a possible emittance-dynamics test area for NLC studies at the SLC

Description: A group at SLAC has studied the possibility of using the Stanford Linear Collider (SLC) to generate short-bunch small-emittance beams similar to those required for the Next Linear Collider (NLC). The conclusion is that such beams are feasible and that an experimental area for testing many concepts related to NLC beams can be provided with a reasonable addition of hardware to the existing SLC Linac. Some of the concepts that can be tested are: (1) effect tolerances of double bunch length compression, (2) wakefields of ultra-short bunches in accelerating structures, (3) the acceleration of short intense multiple bunches, (4) the generation and preservation of bunches with 100 to 1 emittances ratios, (5) beam deflections by collimators, (6) energy and energy spread control of multiple short bunches, and (7) vibration effects and trajectory stability for low emittance beams.
Date: August 1, 1992
Creator: Seeman, J.T.; Fieguth, T.; Kheifets, S.; Raubenheimer, T. & Yeremian, A.D.
Partner: UNT Libraries Government Documents Department

A new trajectory correction technique for linacs

Description: In this paper, we describe a new trajectory correction technique for high energy linear accelerators. Current correction techniques force the beam trajectory to follow misalignments of the Beam Position Monitors. Since the particle bunch has a finite energy spread and particles with different energies are deflected differently, this causes chromatic'' dilution of the transverse beam emittance. The algorithm, which we describe in this paper, reduces the chromatic error by minimizing the energy dependence of the trajectory. To test the method we compare the effectiveness of our algorithm with a standard correction technique in simulations on a design linac for a Next Linear Collider. The simulations indicate that chromatic dilution would be debilitating in a future linear collider because of the very small beam sizes required to achieve the necessary luminosity. Thus, we feel that this technique will prove essential for future linear colliders. 3 refs., 6 figs., 2 tabs.
Date: June 1, 1990
Creator: Raubenheimer, T.O. & Ruth, R.D.
Partner: UNT Libraries Government Documents Department

Damping ring designs for a TeV Linear Collider

Description: In this paper we present a damping ring design for the TLC (TeV Linear Collider). The ring operates at 1.8 GeV. It has normalized emittances of elepsilon/sub x/ = 2.8 mrad and elepsilon/sub y/ = 25.4 nmrad. The damping times are /tau//sub x/ = 2.5 ms and /tau//sub y/ = 4.0 ms. To achieve these extremely low emittances and fast damping times, the ring contains 22 m of wigglers. 30 refs., 7 figs., 7 tabs.
Date: December 1, 1988
Creator: Raubenheimer, T.O.; Rivkin, L.Z. & Ruth, R.D.
Partner: UNT Libraries Government Documents Department

Analytic estimates of coupling in damping rings

Description: In this paper we present analytic formulas to estimate the vertical emittance in weakly coupled electron/positron storage rings. We consider contributions from both the vertical dispersion and linear coupling of the betatron motions. In addition to simple expressions for random misalignments and rotations of the magnets, formulas are presented to calculate the vertical emittance blowup due to orbit distortions. The orbit distortions are assumed to be caused by random misalignments, but because the closed orbit is correlated from point to point, the effects must be treated differently. We consider only corrected orbits. Finally, the analytic expressions are compared with computer simulations of storage rings with random misalignments. 6 refs., 3 figs.
Date: March 1, 1989
Creator: Raubenheimer, T.O. & Ruth, R.D.
Partner: UNT Libraries Government Documents Department