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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

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

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

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

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

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

Beam Dynamics Study of X-Band Linac Driven X-Ray FELS

Description: Several linac driven X-ray Free Electron Lasers (XFELs) are being developed to provide high brightness photon beams with very short, tunable wavelengths. In this paper, three XFEL configurations are proposed that achieve LCLS-like performance using X-band linac drivers. These linacs are more versatile, efficient and compact than ones using S-band or C-band rf technology. For each of the designs, the overall accelerator layout and the shaping of the bunch longitudinal phase space are described briefly. During the last 40 years, the photon wavelengths from linac driven FELs have been pushed shorter by increasing the electron beam energy and adopting shorter period undulators. Recently, the wavelengths have reached the X-ray range, with FLASH (Free-Electron Laser in Hamburg) and LCLS (Linac Coherent Light Source) successfully providing users with soft and hard X-rays, respectively. FLASH uses a 1.2 GeV L-band (1.3 GHz) superconducting linac driver and can deliver 10-70 fs FWHM long photon pulses in a wavelength range of 44 nm to 4.1 nm. LCLS uses the last third of the SLAC 3 km S-band (2.856 GHz) normal-conducting linac to produce 3.5 GeV to 15 GeV bunches to generate soft and hard X-rays with good spatial coherence at wavelengths from 2.2 nm to 0.12 nm. Newer XFELs (at Spring8 and PSI) use C-band (5.7 GHz) normal-conducting linac drivers, which can sustain higher acceleration gradients, and hence shorten the linac length, and are more efficient at converting rf energy to bunch energy. The X-band (11.4 GHz) rf technology developed for NLC/GLC offers even higher gradients and efficiencies, and the shorter rf wavelength allows more versatility in longitudinal bunch phase space compression and manipulation. In the following sections, three different configurations of X-band linac driven XFELs are described that operate from 6 to 14 GeV. The first (LOW CHARGE DESIGN) has an electron bunch charge ...
Date: December 13, 2011
Creator: Adolphsen, C.; Limborg-Deprey, C.; Raubenheimer, T.O.; Wu, J.; /SLAC; Sun, Y. et al.
Partner: UNT Libraries Government Documents Department

Dark Currents and Their Effect on the Primary Beam in an X-band Linac

Description: We numerically study properties of primary dark currents in an X-band accelerating structure. For the H60VG3 structure considered for the Next Linear Collider (NLC) we first perform a fairly complete (with some approximations) calculation of dark current trajectories. These results are used to study properties of the dark current leaving the structure. For example, at accelerating gradient of 65 MV/m, considering two very different assumptions about dark current emission around the irises, we find that the fraction of emitted current leaving the structure to be a consistent {approx} 1%. Considering that {approx} 1 mA outgoing dark current is seen in measurement, this implies that {approx} 100 mA (or 10 pC per period) is emitted within the structure itself. Using the formalism of the Lienard-Wiechert potentials, we then perform a systematic calculation of the transverse kick of dark currents on a primary linac bunch. The result is {approx} 1 V kick per mA (or per 0.1 pC per period) dark current emitted from an iris. For an entire structure we estimate the total kick on a primary bunch to be {approx} 15 V. For the NLC linac this translates to a ratio of (final) vertical beam offset to beam size of about 0.2. However, with the assumptions that needed to be made--particularly the number of emitters and their distribution within a structure--the accuracy of this result may be limited to the order of magnitude.
Date: May 27, 2005
Creator: Bane, K.L.F.; Dolgashev, V.A.; Raubenheimer, T.; Stupakov, G.V.; Wu, J.H. & /SLAC
Partner: UNT Libraries Government Documents Department

FACET Emittance Growth

Description: FACET, the Facility for Advanced Accelerator and Experimental Tests, is a new facility being constructed in sector 20 of the SLAC linac primarily to study beam driven plasma wakefield acceleration. The FACET beamline consists of a chicane and final focus system to compress the 23 GeV, 3.2 nC electron bunches to {approx}20 {micro}m long and {approx}10 {micro}m wide. Simulations of the FACET beamline indicate the short-duration and large, 1.5% rms energy spread beams may suffer a factor of four emittance growth from a combination of chromaticity, incoherent synchrotron radiation (ISR), and coherent synchrotron radiation (CSR). Emittance growth is directly correlated to head erosion in plasma wakefield acceleration and is a limiting factor in single stage performance. Studies of the geometric, CSR, and ISR components are presented. Numerical calculation of the rms emittance can be overwhelmed by long tails in the simulated phase space distributions; more useful definitions of emittance are given. A complete simulation of the beamline is presented as well, which agrees with design specifications.
Date: April 5, 2011
Creator: Frederico, J; Hogan, M.J.; Nosochkov, Y.; Litos, M.D.; Raubenheimer, T. & /SLAC
Partner: UNT Libraries Government Documents Department

Luminosity Loss due to Beam Distortion and the Beam-Beam Instability

Description: In a linear collider, sources of emittance dilution such as transverse wakefields or dispersive errors will couple the vertical phase space to the longitudinal position within the beam (the so-called ''banana effect''). When the Intersection Point (IP) disruption parameter is large, these beam distortions will be amplified by a single bunch kink instability which will lead to luminosity loss. We study this phenomena both analytically using linear theory and via numerical simulation. In particular, we examine the dependence of the luminosity loss on the wavelength of the beam distortions and the disruption parameter. This analysis may prove useful when optimizing the vertical disruption parameter for luminosity operation with given beam distortions.
Date: June 30, 2005
Creator: Wu, Juhao; Raubenheimer, T.O.; Chao, A.W.; Seryi, A.; /SLAC; Sramek, C.K. et al.
Partner: UNT Libraries Government Documents Department

Suppressing Electron Cloud in Future Linear Colliders

Description: Any accelerator circulating positively charged beams can suffer from a build-up of an electron cloud (EC) in the beam pipe. The cloud develops through ionization of residual gases, synchrotron radiation and secondary electron emission and, when severe, can cause instability, emittance blow-up or loss of the circulating beam. The electron cloud is potentially a luminosity limiting effect for both the Large Hadron Collider (LHC) and the International Linear Collider (ILC). For the ILC positron damping ring, the development of the electron cloud must be suppressed. This paper discusses the state-of-the-art of the ongoing SLAC and international R&D program to study potential remedies.
Date: May 27, 2005
Creator: Pivi, M; Kirby, R.E.; Raubenheimer, T.O.; /SLAC; Le Pimpec, F. & /PSI, Villigen
Partner: UNT Libraries Government Documents Department

Emittance and trajectory control in the main linacs of the NLC

Description: The main linacs of the next generation of linear colliders need to accelerate the particle beams to energies of up to 750 GeV while maintaining very small emittances. This paper describes the main mechanisms of static emittance growth in the main linacs of the Next Linear Collider (NLC). The authors present detailed simulations of the trajectory and emittance control algorithms that are foreseen for the NLC. They show that the emittance growth in the main linacs can be corrected down to about 110%. That number is significantly better than required for the NLC design luminosity.
Date: September 1, 1996
Creator: Assmann, R.; Adolphsen, C.; Bane, K.; Raubenheimer, T.O. & Thompson, K.
Partner: UNT Libraries Government Documents Department

LIAR -- A new program for the modeling and simulation of linear accelerators with high gradients and small emittances

Description: Linear accelerators are the central components of the proposed next generation of linear colliders. They need to provide acceleration of up to 750 GeV per beam while maintaining very small normalized emittances. Standard simulation programs, mainly developed for storage rings, do not meet the specific requirements for high energy linear accelerators. The authors present a new program LIAR (LInear Accelerator Research code) that includes wakefield effects, a 4D coupled beam description, specific optimization algorithms and other advanced features. Its modular structure allows to use and to extend it easily for different purposes. They present examples of simulations for SLC and NLC.
Date: September 1, 1996
Creator: Assmann, R.; Adolphsen, C.; Bane, K.; Raubenheimer, T.O.; Siemann, R. & Thompson, K.
Partner: UNT Libraries Government Documents Department

Ion effects in the SLC electron damping ring

Description: The authors report on the ion-related beam behavior in the electron damping ring during unusually poor vacuum conditions in the weeks that followed a catastrophic kicker chamber failure that contaminated the ring vacuum system. The vacuum gradually improved over several months of beam operation, during which time the vertical emittance remained blown up by a factor of 2. The emittance blowup was accompanied by a transverse instability that produced jitter in the extracted beam size. Both the characteristic spectrum of self-excited betatron sidebands and the emittance blowup exhibited a threshold behavior with beam current and vacuum pressure. This behavior depended strongly on the betatron tune and it was found that the ion effects could be minimized by operating just below the 1/2 integer resonance.
Date: May 1, 1997
Creator: Krejcik, P.; Pritzkau, D.; Raubenheimer, T.; Ross, M. & Zimmermann, F.
Partner: UNT Libraries Government Documents Department

Observation and analysis of static deflections from transverse long-range wakefields in the SLC

Description: In the SLC main linac a train of three bunches is accelerated. The leading positron bunch is followed by two bunches of electrons. When the positron bunch passes off-axis through the Rf structures, it excites dipole modes in the structures, for example long-range transverse wakefields which deflect the subsequent electron bunches. Although the magnitude of the deflections is small one can infer the deflections by measuring the trajectory differences while changing the spacing between the positron and electron bunches. Knowing the positron trajectory the misalignments of the accelerating RF structures with respect to the BPM`s can be calculated. The authors present measurements from the SLC linac and discuss the data analysis and errors.
Date: July 1, 1997
Creator: Assmann, R.W.; Decker, F.J.; Raimondi, P. & Raubenheimer, T.O.
Partner: UNT Libraries Government Documents Department

Alignment tolerance of accelerating structures and corrections for future linear colliders

Description: The alignment tolerance of accelerating structures is estimated by tracking simulations. Both single-bunch and multi-bunch effects are taken into account. Correction schemes for controlling the single and multi-bunch emittance growth in the case of large misalignment are also tested by simulations.
Date: June 1, 1995
Creator: Kubo, K.; Adolphsen, C.; Bane, K.L.F.; Raubenheimer, T.O. & Thompson, K.A.
Partner: UNT Libraries Government Documents Department

Diagnostic for dynamic aperture

Description: In large accelerators and low beta colliding beam storage rings, the strong sextupoles, which are required to correct the chromatic effects, produce strong nonlinear forces which act on particles in the beam. In addition in large hadron storage rings the superconducting magnets have significant nonlinear fields. To understand the effects of these nonlinearities on the particle motion there is currently a large theoretical effort using both analytic techniques and computer tracking. This effort is focused on the determination of the 'dynamic aperture' (the stable acceptance) of both present and future accelerators and storage rings. A great deal of progress has been made in understanding nonlinear particle motion, but very little experimental verification of the theoretical results is available. In this paper we describe 'dynamic tracking', a method being studied at the SPEAR storage ring, which can be used to obtain experimental results which are in a convenient form to be compared with the theoretical predictions.
Date: April 1, 1985
Creator: Morton, P.L.; Pellegrin, J.L.; Raubenheimer, T.; Rivkin, L.; Ross, M.; Ruth, R.D. et al.
Partner: UNT Libraries Government Documents Department

A damping ring design for future linear colliders

Description: In this paper we present a preliminary design of a damping ring for the TeV Linear Collider (TLC), a future linear collider with an energy of 1/2 to 1 TeV in the center of mass. Because of limits on the emittance, repetition rate and longitudinal impedance, we use combined function FODO cells with wigglers in insertion regions; there are approximately 22 meters of wigglers in the 155 meter ring. The ring has a normalized horizontal emittance, including the effect of intrabeam scattering, which is less than 3 /times/ 10/sup /minus/6/ and an emittance ratio of epsilon/sub x/ approx. 100epsilon/sub y/. It is designed to damp bunches for 7 vertical damping times while operating at a repetition rate of 360 Hz. Because of these requirements on the emittance and the damping per bunch, the ring operates at 1.8 GeV and is relatively large, allowing more bunches to be damped at once. 10 refs., 5 figs., 2 tabs.
Date: March 1, 1989
Creator: Raubenheimer, T.O.; Gabella, W.E.; Morton, P.L.; Lee, M.J.; Rivkin, L.Z. & Ruth, R.D.
Partner: UNT Libraries Government Documents Department

Suppression of Secondary Electron Emission using Triangular Grooved Surface in the ILC Dipole and Wiggler Magnets

Description: The development of an electron cloud in the vacuum chambers of high intensity positron and proton storage rings may limit machine performance. The suppression of electrons in a magnet is a challenge for the positron damping ring of the International Linear Collider (ILC) as well as the Large Hadron Collider. Simulation show that grooved surfaces can significantly reduce the electron yield in a magnet. Some of the secondary electrons emitted from the grooved surface return to the surface within a few gyrations, resulting in a low effective secondary electron yield (SEY) of below 1.0 A triangular surface is an effective, technologically attractive mitigation with a low SEY and a weak dependence on the scale of the corrugations and the external magnetic field. A chamber with triangular grooved surface is proposed for the dipole and wiggler sections of the ILC and will be tested in KEKB in 2007. The strategy of electron cloud control in ILC and the optimization of the grooved chamber such as the SEY, impedance as well as the manufacturing of the chamber, are also discussed.
Date: July 6, 2007
Creator: Wang, L.; Bane, K.; Chen, C.; Himel, T.; Munro, M.; Pivi, M. et al.
Partner: UNT Libraries Government Documents Department