Publications (150)118.57 Total impact

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ABSTRACT: Wake function suppression is effected by ensuring that the mode frequencies of an Xband normal conducting (NC) accelerating structure of multiple cells are detuned and moderately damped by waveguide manifolds attached to the outer wall of the accelerator. We report on the dilution in the wake function suppression that occurs due to errors resulting from the fabrication process. After diffusion bonding 206 cells a nonuniform expansion in the cell geometry forces a substantial shift in the frequencies of select cells. We remap all circuit parameters to these shifted cell frequencies to predict the wake function. Experiments performed on the SLC at the SLAC National Accelerator Laboratory indicate that the wake function is well predicted by the circuit model.New Journal of Physics 11/2011; 11(3). DOI:10.1088/13672630/11/3/033013 · 3.67 Impact Factor 
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ABSTRACT: We present a resonant cavity approach for noninvasive, pulsetopulse, beam emittance measurements of noncircular multibunch beams. In a resonant cavity, desired field components can be enhanced up to Q{sub L}/, where Q{sub L} is the loaded quality factor of the resonant mode , when the cavity resonant mode matches the bunch frequency of a bunchtrain beam pulse. In particular, a quadcavity, with its quadrupole mode (TM for rectangular cavities) at beam operating frequency, rotated 45{sup o} with respect to the beamline, extracts the beam quadrupole moment exclusively, utilizing the symmetry of the cavity and some simple networks to suppress common modes. Six successive beam quadrupole moment measurements, performed at different betatron phases in a linear transport system determine the beam emittance, i.e. the beam size and shape in the beam's phase space, if the beam current and position at these points are known. In the presence of xy beam coupling, ten measurements are required. One measurement alone provides the rmsbeam size of a large aspect ratio beam. The resolution for such a measurement of rmsbeam size with the rectangular quadcavity monitor presented in this article is estimated to be on the order of ten microns. A prototype quadcavity was fabricated and preliminary beam tests were performed at the Next Linear Collider Test Accelerator (NLCTA) at the Stanford Linear Accelerator Center (SLAC). Results were mainly limited by beam jitter and uncertainty in the beam position measurement at the cavity location. This motivated the development of a positionemittance integrated monitor. 
Conference Paper: Design of a highcurrent injector and transport optics for the ilc electron source
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ABSTRACT: A train of 1.3ns micro bunches of longitudinally polarized electrons are generated in a 140kV DCgun based injector in the International Linear Collider electron source; a bunching system with extremely high bunching efficiency to compress the microbunch down to 20 ps FWHM is designed. Complete optics to transport the electron bunch to the entrance of the 5GeV damping ring injection line is developed. Starttoend multiparticle tracking through the beamline is performed including the bunching system, preacceleration, vertical chicane, 5 GeV superconducting booster linac, spin rotators and energy compressor. With optimizations of energy compression, 94% of the electrons from the DCgun are captured within the damping ring 6D acceptance  A<sub>x</sub> + A<sub>y</sub> les 0.09 m and DeltaEtimesDeltaz les (plusmn25 MeV) times (plusmn3.46 cm)  at the entrance of the damping ring injection line.Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007 
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ABSTRACT: Polarized electron beams generated by DC guns are routinely available at several accelerators including JLAB, Mainz and SLAC. These guns operate with a cathode bias on the order of 100 kV. To minimize space charge effects, relatively long bunches are generated at the gun and then compressed longitudinally external to the gun just before and during initial acceleration. For linear colliders, this compression is accomplished using a combination of rf bunchers. For the basic design of the International Linear Collider (ILC), a 120 kV DC photocathode gun is used to produce a series of nanosecond bunches that are each compressed by two subharmonic bunchers (SHBs) followed by an Lband buncher and capture section. The longitudinal bunching process results in a significantly higher emittance than produced by the gun alone. While highenergy experiments using polarized beams are not generally sensitive to the source emittance, there are several benefits to a lower source emittance including a simpler more efficient injector system and a lower radiation load during transport especially at bends as at the damping ring. For the ILC, the SHBs could be eliminated if the voltage of the gun is raised sufficiently. Simulations using the General Particle Tracer (GPT) package indicate that a cathode bias voltage of â¥200 kV should allow both SHBs to be operated at 433 or even 650 MHz, while â¥500 kV would be required to eliminate the SHBs altogether. Simulations can be used to determine the minimum emittance possible if the injector is designed for a given increased voltage. A possible alternative to the DC gun is an rf gun. Emittance compensation, routinely used with rf guns, is discussed for highervoltage DC guns. 
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ABSTRACT: Measurement of the beam quadrupole moment at several locations can be used to reconstruct the beam envelope and emittance parameters. The measurements can be performed in a nonintercepting way using a set of quadrupolemode cavities. We present a cavity design with an optimized quadrupole moment shunt impedance. The cavity properties can be characterized using a wire test method to insure symmetry about the central axis, and alignment to nearby position sensing cavities. The design and characterization of the prototype structure is discussed.11/2006; DOI:10.1063/1.2409188 
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ABSTRACT: We present a resonantcavity approach for noninvasive, pulsetopulse, beam emittance measurements of noncircular multibunch beams. In a resonant cavity, desired field components can be enhanced up to QLlambda/pi, where QLlambda is the loaded quality factor of the resonant mode lambda, when the cavity resonant mode matches the bunch frequency of a bunchtrain beam pulse. In particular, a quad cavity, with its quadrupole mode (TM220 for rectangular cavities) at beam operating frequency, rotated 45° with respect to the beamline, extracts the beam quadrupole moment exclusively, utilizing the symmetry of the cavity and some simple networks to suppress common modes. Six successive beam quadrupolemoment measurements, performed at different betatron phases in a linear transport system, determine the beam emittance, i.e., the beam size and shape in the beam's phase space, if the beam current and position at these points are known. In the presence of xy beam coupling, ten measurements are required. One measurement alone provides the rms beam size of a large aspect ratio beam. The resolution for such a measurement of rms beam size with the rectangular quadcavity monitor presented in this article is estimated to be on the order of 10 mum. A prototype quad cavity was fabricated and preliminary beam tests were performed at the Next Linear Collider Test Accelerator at the Stanford Linear Accelerator Center. The results were mainly limited by beam jitter and uncertainty in the beam position measurement at the cavity location. This motivated the development of a positionemittance integrated monitor [J. S. Kim et al., Rev. Sci. Instrum. 76, 073302 (2005)].Review of Scientific Instruments 12/2005; 76(12):5109125109. DOI:10.1063/1.2149191 · 1.58 Impact Factor 
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ABSTRACT: Many particle accelerator applications can benefit from online pulsebypulse nonintercepting emittance measurement system. Recently, there has been much interest in performing such a measurement with a set of resonant quadrupolemode cavities. This article explores a geometry to achieve an enhanced shunt impedance in such a cavity by adding a set of posts forming capacitive gaps near the beam pipe outer radius. For typical diagnostic cavity applications, a fivefold increase in shunt impedance can be obtained with this method. The effect of errors in cavity fabrication on the required mode structure are explored. 
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ABSTRACT: Linear colliders and FEL facilities need fast, nondestructive beam position and profile monitors to facilitate machine tuneup, and for use with feedback control. FARTECH, Inc., in collaboration with SLAC, is developing a resonant cavity diagnostic to simultaneously measure the dipole, quadrupole and sextupole moments of the beam distribution. Measurements of dipole and quadrupole moments at multiple locations yield information about beam orbit and emittance. The sextupole moment can reveal information about beam asymmetry which is useful in diagnosing beam tail deflections caused by shortrange dipole wakefields. In addition to the resonance enhancement of a singlecell cavity, use of a multicell standingwave structure further enhances signal strength and improves the resolution of the device. An estimated resolution is better than 1 m in rms beam size and better than 1 nm in beam position.Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005 
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ABSTRACT: The progress of multiple bunches of charged particles down the main Lband linacs of the ILC (International Linear Collider) can be disrupted by wakefields. These wakefields correspond to the electromagnetic fields excited in the accelerating cavities and have both longrange and shortrange components. The horizontal and vertical modal components of the wakefield will be excited at slightly different frequencies (the dipole mode frequency degeneracy’s are split) due to inevitable manufacturing errors. We simulate the progress of the ILC beam down the collider under the influence of these wakefields. In particular, we investigate the consequences on the final emittance dilution of the beam of coupling of the horizontal to the vertical motion of the beam. 
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ABSTRACT: The shape of an RF pulse is distorted upon propagating through an Xband accelerator structure due to dispersive effects. This distortion together with beam loading introduce energy spread between 192 bunches. In order to minimize this energy spread we modify the input RF pulse shape. The pulse propagation, energy gain, and beam loading are modelled with a modematching computer code and a circuit model. A 2D model and a circuit model of a complete 60 cm structure, consisting of 55 cells and input and output couplers is analyzed. This structure operates with a 5pi/6 phase advance per cell. Dispersive effects for this structure are more significant than for previously studied 2pi/3 phase advance accelerating structures. Experimental results are compared with the theoretical model and excellent agreement is obtained for the propagation of an RF pulse through the structure.08/2003; DOI:10.2172/813185 
Conference Paper: Status of Xband standing wave structure studies at SLAC
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ABSTRACT: Accelerating gradient is one of the major parameters of a linear accelerator. It determines the length of the accelerator and its power consumption. The SLAC twomile linear accelerator uses 3 meter long Sband traveling wave (TW) accelerating structures. The average gradient in the linac is about 20 MV/m. This gradient corresponds to a maximum surface electric field of about 40 MV/m. An operational gradient of 40 MV/m was reported for a 1.5 m constant impedance TW structure for the SLC positron injector. This corresponds to a maximum surface field of 80 MV/m. A typical operational gradient for standing wave (SW) structures of a medical linear accelerator is 30 MV/m, with surface electric fields of 130 MV/m at a pulse width of several microseconds (longer than the working pulse width for SLAC TW structures). SW structures for Sband rf guns routinely operate at maximum surface fields of 130 MV/m (∼2 μs pulse width). We emphasize an operational gradient with a very low fault rate in comparison to much higher gradients obtained in dedicated high gradient test structures. The operational surface fields in the above mentioned SW structures are obviously higher than in TW, Sband structures. Design considerations, results of high power tests and future plans are discussed in this paper.Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003 
Conference Paper: Optimized wakefield suppression & emittance dilutionimposed alignment tolerances in Xband accelerating structures for the JLC/NLC
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ABSTRACT: In order to prevent electrical breakdown occurring in the JLC/NLC (Japanese Linear Collider/Next Linear Collider) Xband structures several new structures are under investigation. These accelerating structures represent an evolutionary design of the DDS series of structures. The phase advance per cell has been varied and the detailed elliptical shape of the cell has been varied in order to simultaneously minimize the group velocity, the surface electromagnetic fields and the pulse temperature rise on the copper surface. It is also important to ensure that the wakefield induced by multiple bunches traversing the accelerating structures does not disrupt trailing bunches. The longrange wakefield must be decreased adequately in order to prevent a BBU (beam break up) instability occurring and to ensure that emittance dilution due to the higher order modes is kept to acceptable levels. The longrange wakefield is forced to decohere by detuning all of the frequencies such that the mode density of frequencies is approximately Gaussian. In order to minimize the impact of the wakefield on the beam dynamics we change the bandwidth and the standard deviation of the Gaussian distribution of frequencies such that a "cost function" is minimized. Interleaving of cell frequencies of adjacent structures is required to adequately damp the wakefield of each particular structure under consideration. The resulting alignment tolerances imposed on the cells and structures is significantly looser alignment tolerances with the use of the code.Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003 
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ABSTRACT: The shape of an RF pulse is distorted upon propagating through an Xband accelerator structure due to dispersive effects. This distortion together with beam loading introduce energy spread between 192 bunches. In order to minimize this energy spread we modify the input RF pulse shape. The pulse propagation, energy gain, and beam loading are modelled with a modematching computer code and a circuit model. A 2D model and a circuit model of a complete 60 cm structure, consisting of 55 cells and input and output couplers is analyzed. This structure operates with a 5π/6 phase advance per cell. Dispersive effects for this structure are more significant than for previously studied 2π/3 phase advance accelerating structures. Experimental results are compared with the theoretical model and excellent agreement is obtained for the propagation of an RF pulse through the structure.Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003 
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ABSTRACT: The progress of a multiple bunches of electrons through several thousand accelerator structures results in a wakefield which if left unchecked will kick successive bunches off the axis of the accelerator and can at the very least dilute the final luminosity of the final colliding beams, or at worst can lead to a BBU (Beam Break Up) instability. In order to damp the wakefields to acceptable levels for travelling wave structures we detune the frequencies of the cells and we couple out the field to four adjacent manifolds. Optimizing the manifoldcell coupling for several hundred cells and changing the bandwidth parameters of the distribution has in previous structures been achieved by a process of trial and error. Here, we report on an optimized Fortran code that has been specifically written with the aim minimizing the sum of the squares of the RMS and standard deviation of the sum wakefield. Sparse matrix techniques are employed to reduce the computational time required for each frequency step. The wakefield is minimized whilst ensuring that no significant local surface heating occurs due to slots cuts into the accelerator cells to couple out the wakefield. 
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ABSTRACT: In order to mitigate the effects of electrical breakdown (which have been found to occur in SLAC Xband traveling wave structures) standing wave structures are being considered for the NLC linac. At SLAC, structures consisting of 15 cells operating in the p accelerating mode are being tested for their electrical breakdown characteristics. In this paper the tuning requirement on the cavities is elucidated by utilizing a circuit model of the structure. The sensitivity of the field to both random and systematic errors is also discussed. 
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ABSTRACT: Earlier RDDS (Rounded Damped Detuned Structures) [1,2], designed, fabricated and tested at SLAC, in collaboration with KEK, have been shown to damp wakefields successfully. However, electrical breakdown has been found to occur in these structures and this makes them inoperable at the desired gradient. Recent results [3] indicate that lowering the group velocity of the accelerating mode reduces electrical breakdown events. In order to preserve the filling time of each structure a high synchronous phase advance (150 degrees as opposed to 120 used in previous NLC designs) has been chosen. Here, damping of the wakefield is analyzed. Manifold damping and interleaving of structure cell frequencies is discussed. These wakefields impose alignment tolerances on the cells and on the structure as a whole. Tolerance calculations are performed and these are compared with analytic estimations. 
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ABSTRACT: Early tests of short low group velocity and standing wave structures indicated the viability of operating Xband linacs with accelerating gradients in excess of 100 MeV/m. Conventional scaling of traveling wave traveling wave linacs with frequency scales the cell dimensions with l. Because Q scales as l1/2, the length of the structures scale not linearly but as l3/2 in order to preserve the attenuation through each structure. For NLC we chose not to follow this scaling from the SLAC Sband linac to its fourth harmonic at Xband. We wanted to increase the length of the structures to reduce the number of couplers and waveguide drives which can be a significant part of the cost of a microwave linac. Furthermore, scaling the iris size of the diskloaded structures gave unacceptably high short range dipole wakefields. Consequently, we chose to go up a factor of about 5 in average group velocity and length of the structures, which increases the power fed to each structure by the same factor and decreases the short range dipole wakes by a similar factor. Unfortunately, these longer (1.8 m) structures have not performed nearly as well in high gradient tests as the short structures. We believe we have at least a partial understanding of the reason and will discuss it below. We are now studying two types of short structures with large apertures with moderately good efficiency including: 1) traveling wave structures with the group velocity lowered by going to large phase advance per period with bulges on the iris, 2) pi mode standing wave structures 
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ABSTRACT: Operating the SLAC/KEK DDS (Damped Detuned Structure) Xband linacs at high gradients (in excess of 70MV/m) has recently been found to be limited by the accelerator structures breaking down and as a consequence severe damage occurs to the cells which makes the structures inoperable. A series of recent experiments at SLAC indicates that arcing in the structures is significantly reduced if the group velocity of the accelerating mode is reduced and additionally it has been discovered that reducing the length of the accelerating structure also limits the number and intensity of breakdown events [1]. However, in designing new accelerating structures care must be taken to ensure that the beaminduced transverse wakefields do not cause the beam to become unstable. Here, we report on damping transverse wakefields in two different short structures: a 90cm traveling wave structure in which the wakefield is coupled out to four attached manifolds and secondly, in a standing wave structure in which a limited number of cells heavily damp down the wakefield. [1] C. Adolphsen, ROAA003, this conf. 
Conference Paper: Dipole wakefield suppression in high phase advance detuned linear accelerators for the JLC/NLC designed to minimise electrical breakdown and cumulative BBU
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ABSTRACT: Recent experiments at SLAC and CERN have revealed evidence of significant deformation in the form of "pitting" of the cells of the 1.8 m series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned Structure). This pitting occurs in the high group velocity (v <sub>g</sub>/c = 0.012) end of the accelerating structure and little evidence of breakdown has been found in the lower group velocity end of the structure. Additional, albeit preliminary experimental evidence , suggests that shorter and lower group velocity structures have reduced breakdown events with increasing accelerating field strengths. Two designs are presented here, firstly a 90 cm structure consisting of 83 cells with an initial v<sub>g</sub>/c = 0.0506 (known as H90VG5) and secondly, an even shorter structure of length 60 cm consisting of 55 cells with an initial v<sub>g</sub>/c = 0.03 (known as H60VG3). The feasibility of using these structures to accelerate a charged beam over 10 km is investigated. The particular issue focussed upon is suppression of the dipole wakefields via detuning of the cell frequencies and by locally damping individual cells in order to avoid BBU (Beam Break Up). Results are presented on beaminduced dipole wakefields and on the beam dynamics encountered on tracking the progress of the beam through several thousand accelerating structuresParticle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001 
Conference Paper: Room temperature accelerator structures for linear colliders
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ABSTRACT: Early tests of short low group velocity and standing wave structures indicated the viability of operating Xband linacs with accelerating gradients in excess of 100 MeV/m. Conventional scaling of traveling wave traveling wave linacs with frequency scales the cell dimensions with λ. Because Q scales as λ<sup>1/2</sup>, the length of the structures scale not linearly but as λ<sup>3/2 </sup> in order to preserve the attenuation through each structure. For the NLC we chose not to follow this scaling from the SLAC Sband linac to its fourth harmonic at the Xband. We wanted to increase the length of the structures to reduce the number of couplers and waveguide drives which can be a significant part of the cost of a microwave linac. Furthermore, scaling the iris size of the diskloaded structures gave unacceptably high short range dipole wakefields. Consequently, we chose to go up a factor of about 5 in average group velocity and length of the structures, which increases the power fed to each structure by the same factor and decreases the short range dipole wakes by a similar factor. Unfortunately, these longer (1.8 m) structures have not performed nearly as well in high gradient tests as the short structures. We believe we have at least a partial understanding of the reason and will discuss it below. We are now studying two types of short structures with large apertures with moderately good efficiency including: 1) traveling wave structures with the group velocity lowered by going to large phase advance per period with bulges on the iris, 2) π mode standing wave structuresParticle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
Publication Stats
2k  Citations  
118.57  Total Impact Points  
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Institutions

1965–2001

Stanford University
 • Stanford Linear Accelerator Center
 • High Energy Physics Laboratory
Stanford, CA, United States


1985

Mountain View College
Mountain View, California, United States


1978

Yale University
 Department of Physics
New Haven, Connecticut, United States


1976

Bielefeld University
Bielefeld, North RhineWestphalia, Germany
