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ILC Linac R&D at SLAC

Description: Since the ITRP recommendation in August 2004 to use superconducting rf technology for a next generation linear collider, the former NLC Group at SLAC has been actively pursuing a broad range of R&D for this collider (the ILC). In this paper, the programs concerning linac technology are reviewed. Current activities include the development of a Marx-style modulator and a 10 MW sheet-beam klystron, operation of an L-band (1.3 GHz) rf source using an SNS HVCM modulator and commercial klystrons, design of a more efficient and less costly rf distribution system, construction of a coupler component test stand, fabrication of a prototype positron capture cavity, beam tests of prototype S-band linac beam position monitors and preparations for magnetic center stability measurements of a prototype SC linac quad.
Date: August 9, 2006
Creator: Adolphsen, C.
Partner: UNT Libraries Government Documents Department

Advances in Normal Conducting Accelerator Technology from the X-Band Linear Collider Program

Description: In the mid-1990's, groups at SLAC and KEK began dedicated development of X-band (11.4 GHz) rf technology for a next generation, TeV-scale linear collider. The choice of a relatively high frequency, four times that of the SLAC 50 GeV Linac, was motivated by the cost benefits of having lower rf energy per pulse (hence fewer rf sources) and reasonable efficiencies at high gradients (hence shorter linacs). To realize such savings, however, requires operation at gradients and peak powers much higher than that hitherto achieved. During the past twelve years, these challenges were met through innovations on several fronts. This paper reviews these achievements, which include developments in the generation and transport of high power rf, and new insights into high gradient limitations.
Date: June 22, 2005
Creator: Adolphsen, C.
Partner: UNT Libraries Government Documents Department

Longitudinal Bunch Lengthening Compensation in High Charge RF Photoinjector

Description: In high charge RF photoinjectors for wakefield two beam acceleration studies, due to the strong longitudinal space charge, bunch lengthening between the photocathode and photoinjector exit is a critical issue. We present beam dynamics studies of bunch lengthening in an RF photoinjector for a high charge electron beam and describe methods to compensate the bunch lengthening to various degrees. In particular, the beam dynamics for bunch charge from 1nC to 30nC are studied for an S-band 2856 MHz photoinjector.
Date: October 3, 2008
Creator: Pei, S. & Adolphsen, C.
Partner: UNT Libraries Government Documents Department

Beam trajectory jitter in the SLC linac

Description: We present model-independent measurements of the vertical trajectory jitter of the positron beam in the Stanford Linear Collider (SLC) linac and discuss the results of studies aimed at isolating its source.
Date: May 1, 1995
Creator: Adolphsen, C. & Slaton, T.
Partner: UNT Libraries Government Documents Department

Parameters of the SLAC Next Linear Collider

Description: In this paper, the authors present the parameters and layout of the Next Linear Collider (NLC). The NLC is the SLAC design of a future linear collider using X-band RF technology in the main linacs. The collider would have an initial center-of-mass energy of 0.5 TeV which would be upgraded to 1 TeV and then 1.5 TeV in two stages. The design luminosity is > 5 {times} 10{sup 33} cm{sup {minus}2} sec {sup {minus}1} at 0.5 TeV and > 10{sup 34} cm{sup {minus}2} sec{sup {minus}1} at 1.0 and 1.5 TeV. They briefly describe the components of the collider and the proposed energy upgrade scenario.
Date: May 1, 1995
Creator: Raubenheimer, T.; Adolphsen, C. & Burke, D.
Partner: UNT Libraries Government Documents Department

RF Distribution Optimization in the Main Linacs of the ILC

Description: The nominal design gradient in the main linacs of the International Linear Collider (ILC) is 31.5 MV/m for a beam current of 9.0 mA. However, the superconducting cavities built to date have demonstrated a range in sustainable gradient extending well below this goal, being limited by Q drop-off and quenching. Thus, an economically feasible cavity acceptance rate will include a certain percentage of sub-performing cavities. An important question that needs to be addressed is, For a string of cavities rated to various levels of gradient and powered from a common source how can we optimize the overall gradient? Along with adjustable cavity coupling--or loaded Q factor--we assume adjustable RF power so that gradient can be leveled in nonnominal cavities, to avoid quench-inducing overshoots. In the ILC an RF unit comprises three cryomodules containing a total of 26 nine-cell cavities, which are fed by one klystron that nominally feeds equal power to all cavities. One simple way of running such a unit is to set RF power, beam arrival time, and all loaded Q's so that the power is matched and the gradient in all cavities equals the gradient limit in the poorest performing cavity. This conservative strategy, however, sacrifices gradient and can be improved upon. One improvement strategy is to adjust the cavity couplings individually (possible, since circulators are assumed in the baseline ILC design) or in pairs (when circulators are not needed) using the movable antennae of the fundamental mode couplers. Another strategy is to use variable power tap-offs (VTO's)[1] by which the RF power to succeeding pairs of cavities can be made to differ. These solutions will not be matched, resulting in power inefficiency and a gradient variation along the beam that needs to be limited. In this report we study the effect on overall gradient for various ...
Date: June 28, 2007
Creator: Bane, K.L.F.; Adolphsen, C.; Nantista, C. & /SLAC
Partner: UNT Libraries Government Documents Department

TTF HOM Data Analysis with Curve Fitting Method

Description: To investigate the possibility of using HOM signals induced in SC cavities as beam and cavity diagnostics, narrow band (20 MHz) data was recorded around the strong TE111-6(6{pi}/9-like) dipole modes (1.7 GHz) in the 40 L-band (1.3 GHz) cavities at the DESY TTF facility. The analyses of these data have so far focused on using a Singular Value Decomposition (SVD) technique to correlate the signals with each other and data from conventional BPMs to show the dipole signals provide an alternate means of measuring the beam trajectory. However, these analyses do not extract the modal information (i.e., frequencies and Q's of the nearly degenerate horizontal and vertical modes). In this paper, we described a method to fit the signal frequency spectrum to obtain this information, and then use the resulting mode amplitudes and phases together with conventional BPM data to determine the mode polarizations and relative centers and tilts. Compared with the SVD analysis, this method is more physical, and can also be used to obtain the beam position and trajectory angle.
Date: July 14, 2009
Creator: Pei, S.; Adolphsen, C.; Li, Z.; Bane, K.; Smith, J. & /SLAC
Partner: UNT Libraries Government Documents Department

Wakefield Damping in a Pair of X-Band Accelerators for Linear Colliders

Description: We consider means to damp the wake-field left behind ultra-relativistic charges. In particular, we focus on a pair of travelling wave accelerators operating at an X-band frequency of 11.424 GHz. In order to maximize the efficiency of acceleration, in the context of a linear collider, multiple bunches of charged particles are accelerated within a given pulse of the electromagnetic field. The wake-field left behind successive bunches, if left unchecked, can seriously disturb the progress of trailing bunches and can lead to an appreciable dilution in the emittance of the beam. We report on a method to minimize the influence of the wake-field on trailing bunches. This method entails detuning the characteristic mode frequencies which make-up the electromagnetic field, damping the wake-field, and interleaving the frequencies of adjacent accelerating structures. Theoretical predictions of the wake-field and modes, based on a circuit model, are compared with experimental measurements of the wake-field conducted within the ASSET facility at SLAC. Very good agreement is obtained between theory and experiment and this allows us to have some confidence in designing the damping of wake-fields in a future linear collider consisting of several thousand of these accelerating structures.
Date: December 18, 2006
Creator: Jones, R.M.; Adolphsen, C.E.; Wang, J.W.; Li, Z. & /SLAC
Partner: UNT Libraries Government Documents Department

Higher Order Mode Heating Analysis for the ILC Superconducting Linacs

Description: The superconducting cavities and interconnects in the 11 km long linacs of the International Linear Collider (ILC) are designed to operate at 2K, where cooling costs are very expensive. It is thus important to minimize cryogenic heat loads. In addition to an unavoidable static load and the dynamic load of the fundamental 1.3 GHz accelerating rf, a further heat source is presented by the higher order mode (HOM) power deposited by the beam. Such modes will be damped by specially designed HOM couplers attached to the cavities (for trapped modes), and by ceramic dampers at 70K that are located between the eight or nine cavity cryomodules (for propagating modes). Brute force calculation of the higher frequency modes excited in a string of cryomodules is limited by computing capacity (see, e.g. [1]). M. Liepe has calculated {approx} 400 longitudinal TM modes in 3 superconducting cavities plus absorbers, up to 8 GHz [2]. Joestingmeier, et al., have used a ray tracing calculation to find the effect at higher frequencies, specifically in the range of tens of GHz and above [3]. In this report we present a scattering matrix approach, which we apply to an rf unit comprising 26 cavities and 3 absorbers. We perform calculations at sample frequencies (up to 20 GHz) to predict the effectiveness of the ceramic dampers in limiting HOM heat deposition at 2K.
Date: October 27, 2010
Creator: Bane, K.L.F.; Nantista, C.; Adolphsen, C.; /SLAC & ,
Partner: UNT Libraries Government Documents Department

Chromatic correction in the SLC bunch length compressors

Description: The SLC Ring to Linac (RTL) transport lines employ intense bending and strong transverse focusing to produce the momentum compaction needed for bunch length compression prior to S-band acceleration. In the presence of the large rf induced energy spread needed for compression the consequent chromatic effects -- viz. the variation with energy of residual output dispersion and of the RTL transfer matrix, threaten to destroy the small emittances produced by the damping rings. We report on the tuning methods that have been developed and used to implement the sextupole based chromatic correction scheme. 6 refs., 4 figs.
Date: June 1, 1991
Creator: Adolphsen, C.E.; Emma, P.J.; Fieguth, T.H. & Spence, W.L.
Partner: UNT Libraries Government Documents Department

Effect of wakefields on first order transport in the SLC linac

Description: The limitation in increasing the beam current in the SLC linac comes from the emittance growth caused by wakefields. Simulations of the beam transport that model the wakefield dynamics are being done to study methods to control this growth. To verify the theoretical estimates of the wakefield strengths assumed in these simulations, data were taken which are sensitive to their effect on the first order linac transport. Specifically, the dependence of single beam loading and betatron motion on beam current was measured in the range of 0--5.10{sup 10} to 3.5--10{sup 10} electrons per bunch. This paper presents these data together with comparisons to results from simulations. 5 refs., 7 figs.
Date: June 1, 1991
Creator: Adolphsen, C.; Bane, K.L.F. & Seeman, J.T.
Partner: UNT Libraries Government Documents Department

The trajectory control in the SLC linac

Description: Due to wake field effects, the trajectories of accelerated beams in the Linac should be well maintained to avoid severe beam breakup. In order to maintain a small emittance at the end of the Linac, the tolerance on the trajectory deviations become tighter when the beam intensities increase. The existing two beam trajectory correction method works well when the theoretical model agrees with the real machine lattice. Unknown energy deviations along the linac as well as wake field effects can cause the real lattice to deviate from the model. This makes the trajectory correction difficult. Several automated procedures have been developed to solve these problems. They are: an automated procedure to frequently steer the whole Linac by dividing the Linac into several small regions; an automated procedure to empirically correct the model to fit the real lattice and eight trajectory correcting feedback loops along the linac and steering through the collimator region with restricted corrector strengths and a restricted number of correctors. 6 refs., 2 figs.
Date: May 1, 1991
Creator: Hsu, I.C.; Adolphsen, C.E.; Himel, T.M. & Seeman, J.T.
Partner: UNT Libraries Government Documents Department

Emittance dilution due to slow alignment drifts in the main linacs of the NLC

Description: The tight tolerances in the main linacs of the Nest Linear Collider (NLC) result in a large sensitivity of the beam emittance to slow alignment drifts. Once the accelerator is tuned, the optimized emittances must be maintained. Slow alignments drifts will make resteering and reoptimization necessary. The frequency of these linac reoptimizations is an important parameter that determines how well the linear collider can be operated. We present simulation results that address this question for the main linacs of the NLC. We will show that the effects of alignment drifts can indeed be handled.
Date: September 1, 1996
Creator: Assmann, R.; Adolphsen, C. & Bane, K.
Partner: UNT Libraries Government Documents Department

Recent results & plans for the future on SLAC Damped Detuned Structures (DDS)

Description: The cells in the SLAC DDS are designed in such a way that the transverse modes excited by the beam are detuned in a Gaussian fashion so that destructive interference causes the wake function to decrease rapidly and smoothly. Moderate damping provided by four waveguide manifolds running along the outer wall of the accelerator is utilized to suppress the reappearance of the wake function at long ranges where the interference becomes constructive again. The newly developed spectral function method, involving a continuum of frequencies, is applied to analyze the wake function of the DDS I design and to study the dependence of the wake function on manifold termination. The wake function obtained with the actually realized manifold terminations is presented and compared to wake function measurements recently carried out at the ASSET facility installed in the SLAC LINAC.
Date: October 1, 1996
Creator: Kroll, N.M.; Jones, R.M. & Adolphsen, C.
Partner: UNT Libraries Government Documents Department

A damped detuned structure for the next linear collider

Description: An X-band Damped Detuned Structure (DDS) for NLC has been fabricated as part of a collaboration between KEK and SLAC. The individual cells were diamond point machined and microwave tested at KEK. The cells were diffusion bonded at SLAC. The structure has been cold tested. The time dependence of the beam induced dipole wakefields have been measured with the SLC beam in the test station ASSET. The structure is designed so that the dipole modes have an approximately gaussian density distribution in the frequency domain. This gives an approximately gaussian decrease of the wakefields for short times (about 10 ns), which is produced by the interference among the 206 modes in the lowest dipole mode band of the 206 cell structure. Without damping, however, the wakefields then rise back to a level which is approximately equal to the expected incoherent level from the 206 modes. The damping is accomplished by means of 4 rectangular slots or manifolds (approximately 5 mm by 10 mm) equally spaced in azimuth around the structure and running the full length of the structure. These manifolds act as single mode rectangular waveguides for the lowest band dipole modes, but are cut off for the accelerating mode. The manifolds are coupled to every cell in the structure, except for 3 at each end, by means of radial slots. Each of the four manifolds will have the dipole mode frequencies traveling in both directions and so are terminated on both ends. The structure will be installed in the NLC Test Accelerator this fall.
Date: September 1, 1996
Creator: Miller, R.H.; Adolphsen, C. & Bane, K.L.
Partner: UNT Libraries Government Documents Department

Long-range wakefields and split-tune lattice at the SLC

Description: At the SLC, a train consisting of one positron bunch followed by two electron bunches is accelerated in the linac, each separated by about 60 ns. Long-range transverse wakefields from the leading bunch were found to cause up to a factor of three increase in beam jitter for the trailing bunches. Incoming jitter is efficiently damped by BNS damping, but excitations in the middle of the linac from sources such as long-range wakefields can grow in amplitude. To measure the wake function, the time difference between the positron and electron bunches was changed, determining the frequency and strength of the dominant mode contributing to the dipole Wakefield. By splitting the horizontal and vertical phase advance, or {open_quote}tune{close_quote}, of the magnetic lattice, it was possible to decrease the resonant excitation from these wakefields and thereby reduce the jitter of the electron beam by a factor of two.
Date: August 1, 1996
Creator: Decker, F.J.; Adolphsen, C.E. & Assmann, R.
Partner: UNT Libraries Government Documents Department

Performance issues, downtime recovery and tuning in the Next Linear Collider (NLC)

Description: The Next Linear Collider (NLC) consists of several large subsystems, each of which must be operational and tuned in order to deliver luminosity. Considering specific examples, we study how the different subsystems respond to various perturbations such as ground motion, temperature changes, drifts of beam-position monitors etc., and we estimate the overall time requirements for tuning and downtime recovery of each subsystem. The succession of subsystem failures and recoveries as well as other performance degradations can be modeled as a Markov process, where each subsystem is characterized, e.g., by its failure rate and recovery time. Such a model allows the prediction of the overall NLC availability. Our mathematical description of a linear collider is benchmarked against the known performance of the Stanford Linear Collider (SLC).
Date: May 1, 1997
Creator: Zimmermann, F.; Adolphsen, C. & Assmann, R.
Partner: UNT Libraries Government Documents Department

Microwave analysis of the damped detuned accelerator structure

Description: Damped and detuned accelerating structures (DDS), designed to minimize the effects of long range wakefields excited by bunchtrains, are presently under investigations at SLAC. The authors report the first studies of beam-induced microwave signals in a prototype DDS. The DDS is a 206 cell, nearly constant gradient structure, employing Gaussian detuning, and four symmetrically placed waveguide manifolds to damp the first-band dipole modes. They describe the manifold and output coupler design, bench measurements, and measurements with beam during the ASSET experiment. Dipole mode signals have been used to steer the beam to the structure center and minimize the wakefield kick.
Date: September 1, 1996
Creator: Seidel, M.; Adolphsen, C. & Fowkes, W.R.
Partner: UNT Libraries Government Documents Department

Beam current monitors in the NLCTA

Description: The current profile along the 126 ns, multi-bunch beam pulse in the Next Linear Collider Test Accelerator (NLCTA) is monitored with fast toroids (rise time {approximately} 1 ns). Inserted at several positions along the beam line, they allow one to track current transmission as a function of position along the bunch train. Various measurements, such as rise time, current, width, and slope, are made on the digitized signals, which can be corrected in software by means of stored frequency response files. The design and implementation of these devices is described.
Date: May 1, 1997
Creator: Nantista, C. & Adolphsen, C.
Partner: UNT Libraries Government Documents Department

RF systems for the NLCTA

Description: This paper describes an X-Band and RF system for the Next Linear Collider Test Accelerator. The RF system consists of a 90 MeV injector and a 540 MeV linac. The main components of the injector are two low-Q single-cavity prebunchers and two 0.9-m-long detuned accelerator sections. The linac system consists of six 1.8-m-long detuned and damped detuned accelerator sections powered in pairs. The rf power generation, compression, delivery, distribution and measurement systems consist of klystrons, SLED-II energy compression systems, rectangular waveguides, magic-T`s, and directional couplers. The phase and amplitude for each prebuncher is adjusted via a magic-T type phase shifter/attenuator. Correct phasing between the two 0.9 m accelerator sections is obtained by properly aligning the sections and adjusting two squeeze type phase shifters. Bunch phase and bunch length can be monitored with special microwave cavities and measurement systems. The design, fabrication, microwave measurement, calibration, and operation of the sub-systems and their components are briefly presented.
Date: May 1, 1997
Creator: Wang, J.W.; Adolphsen, C. & Atkinson, R.
Partner: UNT Libraries Government Documents Department

Low-level RF signal processing for the Next Linear Collider Test Accelerator

Description: In the X-band accelerator system for the Next Linear Collider Test Accelerator (NLCTA), the Low Level RF (LLRF) drive system must be very phase stable, but concurrently, be very phase agile. Phase agility is needed to make the Stanford Linear Doubler (SLED) power multiplier systems Energy work and to shape the RF waveforms to compensate beam loading in the accelerator sections. Similarly, precision fast phase and amplitude monitors are required to view, track, and feed back on RF signals at various locations throughout the system. The LLRF is composed of several subsystems: the RF Reference System generates and distributes a reference 11.424 GHz signal to all of the RF stations, the Signal Processing Chassis creates the RF waveforms with the appropriate phase modulation, and the Phase Detector Assembly measures the amplitude and phase of monitor3ed RF signals. The LLRF is run via VXI instrumentation. These instruments are controlled using HP VEE graphical programming software. Programs have been developed to shape the RF waveform, calibrate the phase modulators and demodulators, and display the measured waveforms. This paper describes these and other components of the LLRF system.
Date: May 12, 1997
Creator: Holmes, S.; Ziomek, C. & Adolphsen, C.
Partner: UNT Libraries Government Documents Department

Status and results from the next linear collider test accelerator

Description: The design for the Next Linear Collider (NLC) at SLAC is based on two 11.4 GHz linacs operating at an unloaded acceleration gradient of 50 MV/m increasing to 85 MV/m as the energy is increased from {1/2} TeV to 1 TeV in the center of mass. During the past several years there has been tremendous progress on the development of 11.4 GHz (X-band) RF systems. These developments include klystrons which operate at the required power and pulse length, pulse compression systems that achieve a factor of four power multiplication and structures that are specially designed to reduce long-range wakefields. Together with these developments, we have constructed a {1/2} GeV test accelerator, the NLC Test Accelerator (NLCTA). The NLCTA will serve as a test bed as the design of the NLC is refined. In addition to testing the RF system, the NLCTA is designed to address many questions related to the dynamics of the beam during acceleration, in particular the study of multibunch beam loading compensation and transverse beam break-up. In this paper we present the status of the NLCTA and the results of initial commissioning.
Date: August 1, 1996
Creator: Ruth, R.D.; Adolphsen, C. & Allison, S.
Partner: UNT Libraries Government Documents Department

Results from the SLAC NLC test accelerator

Description: The design for the Next Linear Collider (NLC) at SLAC is based on two 11.4 GHz linacs operating at an unloaded acceleration gradient of 50 MV/m increasing to 85 MV/m as the energy is increased from 1/2 TeV to 1 TeV in the center of mass. During the past several years there has been tremendous progress on the development of 11.4 GHz (X-band) RF systems. These developments include klystrons which operate at the required powder and pulse length, pulse compression systems that achieve a factor of four power multiplication and structures that are specially designed to reduce long-range wakefields. Together with these developments, we have constructed a 1/2 GeV test accelerator, the NLC Test Accelerator (NLCTA). The NLCTA will serve as a test bed as the design of the NLC is refined. In addition to testing the RF system, the NLCTA is designed to address many questions related to the dynamics of the beam during acceleration, in particular, multibunch beam-loading compensation and transverse beam break-up. In this paper we describe the NLCTA and present results from initial experiments.
Date: June 1, 1997
Creator: Ruth, R.D.; Adolphsen, C. & Allison, S.
Partner: UNT Libraries Government Documents Department