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ABSTRACT: The shape of an RF pulse is distorted upon propagating through an X-band 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 mode-matching 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;
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[show abstract]
[hide abstract]
ABSTRACT: The shape of an RF pulse is distorted upon propagating through an X-band 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 mode-matching 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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[show abstract]
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ABSTRACT: In order to prevent electrical breakdown occurring in the JLC/NLC (Japanese Linear Collider/Next Linear Collider) X-band 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 long-range 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 long-range 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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V.A. Dolgashev,
C. Adolphsen,
D.L. Burke,
G. Bowden,
R.M. Jones,
J. Lewandowski,
Z. Li,
R. Loewen, R.H. Miller,
C. Ng,
C. Pearson,
R.D. Ruth,
S.G. Tantawi,
J.W. Wang,
P. Wilson
[show abstract]
[hide abstract]
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 two-mile linear accelerator uses 3 meter long S-band 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 S-band 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, S-band 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
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[show abstract]
[hide abstract]
ABSTRACT: We present a non-invasive, resonant cavity based approach to beam emittance measurement of a shot-to-shot non-circular beam pulse of multi-bunches. In a resonant cavity, desired field components can be enhanced up to Q_L_lambda/pi, where Q_L_lambda is the loaded Q of the resonance mode lambda, when the cavity resonant mode matches with the beam operating frequency. In particular, a Quad-cavity, with its quadrupole mode at beam operating frequency, extracts the beam quad-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, allow us to determine the beam emittance, i.e., the beam size and shape in the beam's phase space. One measurement alone provides the rms-beam size if the beam position is given, for instance, by nearby beam-position-monitors. This paper describes the basic design and analysis of a Quad-cavity beam monitoring system.
10/2002;
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ABSTRACT: In order to mitigate the effects of electrical breakdown (which have been found to occur in SLAC X-band 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.
07/2002;
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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.
07/2002;
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R. H. Miller,
R. M. Jones,
C. Adolphsen,
G. Bowden,
V. Dolgashev,
N. Kroll Z. Li,
R. Loewen,
C. Ng,
C. Pearson,
T. Raubenheimer R. Ruth,
S. Tantawi,
J W Wang
[show abstract]
[hide abstract]
ABSTRACT: Early tests of short low group velocity and standing wave structures indicated the viability of operating X-band 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 S-band linac to its fourth harmonic at X-band. 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 disk-loaded 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
09/2001;
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[show abstract]
[hide abstract]
ABSTRACT: Recent experiments at SLAC [1,2] and CERN [3] have revealed evidence of significant deformation in the form of "pitting" of the cells of the 1.8m series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned Structure). This pitting occurs in the high group velocity (vg /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 90cm structure consisting of 83 cells with an initial vg/c = 0.0506 (known as H90VG5) and secondly, an even shorter structure of length 60cm consisting of 55 cells with an initial vg /c = 0.03 (known as H60VG3). The feasibility of using these structures to accelerate a charged beam over 10km 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 beam-induced dipole wakefields and on the beam dynamics encountered on tracking the progress of the beam through several thousand accelerating structures. [1] C. Adolphsen, ROAA003, this conf. [2] R.H. Miller et al, FPAH062, this conf. [3] L. Groening et al, MPPH039, this conf
08/2001;
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[show abstract]
[hide abstract]
ABSTRACT: Operating the SLAC/KEK DDS (Damped Detuned Structure) X-band 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 beam-induced 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.
08/2001;
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[show abstract]
[hide abstract]
ABSTRACT: Operating the SLAC/KEK DDS (damped detuned structure) X-band
linacs at high gradients (in excess of 70 MV/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. However, in designing new
accelerating structures care must be taken to ensure that the
beam-induced transverse wakefields do not cause the beam to become
unstable. Here, we report on damping transverse wakefields in two
different short structures: a 90 cm 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
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
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R.H. Miller,
R.M. Jones,
C. Adolphsen,
G. Bowden,
V. Dolgashev,
N. Kroll,
Z. Li,
R. Loewen,
C. Ng,
C. Pearson,
T. Raubenheimer,
R. Ruth,
S. Tantawi,
J.W. Wang
[show abstract]
[hide abstract]
ABSTRACT: Early tests of short low group velocity and standing wave
structures indicated the viability of operating X-band 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 S-band linac
to its fourth harmonic at the X-band. 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 disk-loaded 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
structures
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
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[show abstract]
[hide abstract]
ABSTRACT: We present a non-invasive, resonant cavity based approach to beam
emittance measurement of a shot-to-shot non-circular beam pulse of
multi-bunches. In a resonant cavity, desired field components can be
enhanced up to Q<sub>Lλ</sub>/π where Q<sub>Lλ</sub> is
the loaded Q of the resonance mode λ, when the cavity resonant
mode matches with the beam operating frequency. In particular, a
Quad-cavity, with its quadrupole mode at beam operating frequency,
extracts the beam quad-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, allow us to determine the beam
emittance, i.e., the beam size and shape in the beam's phase space. One
measurement alone provides the rms-beam size if the beam position is
given, for instance, by nearby beam-position-monitors. This paper
describes the basic design and analysis of a Quad-cavity beam monitoring
system
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
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[show abstract]
[hide abstract]
ABSTRACT: The JLC/NLC linac must accelerate multi-bunch beams in order to
obtain a luminosity >3×10<sup>34</sup> cm<sup>-2</sup>
sec<sup>-1</sup> at a center of mass energy of 1 TeV. It is essential
for the structure design to minimize the long and short-range dipole
wakefields to prevent emittance degradation and the beam breakup
instability (BBU). In addition, the structures must operate at a high RF
gradient to minimize the linac cost. High-power testing of prototype
structures at SLAC has shown noticeable damage. The damage is largest in
the front of the structure, where the group velocity is high, and there
is minimal or no damage at the back end, where the group velocity is
low. Theoretical analysis using a simple circuit model suggests using
structures with a lower group velocity, on the order of a few percent,
would be a way of avoiding damage. For the standard 2π/3 accelerating
mode, it is difficult to lower the group velocity without losing
efficiency or increasing the wakefields. With this in mind, we have
taken the phase advance as an additional parameter in structure
optimization. We found that a low group velocity structure at higher
phase advance can maintain high RF efficiency and low wakefields. In
this paper, we study the impact of phase advance on structure
performance. We then optimize the NLC S-band and X-band structures to
meet design requirements
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
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[show abstract]
[hide abstract]
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 beam-induced dipole wakefields and on the beam
dynamics encountered on tracking the progress of the beam through
several thousand accelerating structures
Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001
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Z Li,
N. T. Folwell,
K Ko,
R. J. Loewen,
E. W. Lundahl,
B. McCandless, R. H. Miller,
R.D. Ruth,
M. D. Starkey,
Y Sun,
J W Wang,
T. Higo
[show abstract]
[hide abstract]
ABSTRACT: The complexity of the Round Damped Detuned Structue (RDDS) for the JLC/NLC main linac is driven by the considerations of rf efficiency and dipole wakefield suppression. As a time and cost saving measure for the JLC/NLC, the dimensions of the 3D RDDS cell are being determined through computer modeling to within fabrication precision so that no tuning may be needed once the structures are assembled. The tolerances on the frequency errors for the RDDS structure are about one MHz for the fundamental mode and a few MHz for the dipole modes. At the X-band frequency, these correspond to errors of a micron level on the major cell dimensions. Such a level of resolution requires highly accurate field solvers and vast amount of computer resources. A parallel finite-element eigensolver Omega3P was developed at SLAC that runs on massively parallel computers such as the Cray T3E at NERSC. The code was applied in the design of the RDDS cell dimensions that are accurate to within fabrication precision. We will present the numerical approach of using these codes to determine the RDDS dimensions and compare the numerical predictions with the cold test measurements on RDDS prototypes that are diamond-turned using these dimensions.
10/2000;
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J W Wang,
C. Adolphsen,
G. B. Bowden,
D. L. Burke,
J. Cornuelle,
V. A. Dolgashev,
W. B. Fowkes,
R.K. Jobe,
R. M. Jones,
K Ko, [......],
G. Stupakov,
T. Higo,
Y. Funahashi,
Y Higashi,
N. Hitomi,
T Suzuki,
K Takata,
T. Takatomi,
N. Toge,
Y Watanabe
[show abstract]
[hide abstract]
ABSTRACT: As a joint effort in the JLC/NLC research program, we have developed a new type of damped detuned accelerator structure with optimized round-shaped cavities (RDDS). This paper discusses some important R&D aspects of the first structure in this series (RDDS1). The design aspects covered are the cell design with sub-MHz precision, HOM detuning, coupling and damping technique and wakefield simulation. The fabrication issues covered are ultra-precision cell machining with micron accuracy, assembly and diffusion bonding technologies to satisfactorily meet bookshelf, straightness and cell rotational alignment requirements. The measurements described are the RF properties of single cavities and complete accelerator section, as well as wakefields from the ASSET tests at SLAC. Finally, future improvements are also discussed.
10/2000;
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[show abstract]
[hide abstract]
ABSTRACT: In damping the wakefield generated by an electron beam traversing several thousand X-band linacs in the NLC we utilise a Gaussian frequency distribution of dipole modes to force the modes to deconstructively interfere, supplemented with moderate damping achieved by coupling these modes to four attached manifolds. Most of these modes are adequately damped by the manifolds. However, the modes towards the high frequency end of the lower dipole band are not adequately damped because the last few cells are, due to mechanical fabrication requirements, not coupled to the manifolds. To mitigate this problem in the present RDDS1 design, the output coupler for the accelerating mode has been designed so as to also couple out those dipole modes which reach the output coupler cell. In order to couple out both dipole mode polarizations, the output coupler has four ports. We also report on the results of a study of the benefits which can be achieved by supplementing manifold damping with local damping for a limited number of cells at the downstream end of the structure.
09/2000;
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[show abstract]
[hide abstract]
ABSTRACT: In fabricating the first X-Band RDDS (Rounded Damped Detuned Structure) accelerator structure, microwave measurements are made on short groups of discs prior to bonding the discs of the entire structure. The design dispersion curves are compared with the frequency measurements. The theory utilised is based on a circuit model adapted to a short stack of slowly varying non-uniform discs. The model reveals the nature of the modes in the structure and may also be used to refit the experimental data to the parameters in a model of the wakefield given earlier [1]. This method allows a more faithful determination of the wakefield that a beam will experience as it traverses the structure. Results obtained on the frequencies are compared to the original design. [1] R.M.Jones, et al, EPAC96 (also SLAC-PUB-7187)
09/2000;
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[show abstract]
[hide abstract]
ABSTRACT: The main linacs of the Next Linear Collider (NLC) will contain several thousand X-band RDDS (Rounded Damped Detuned Structures). The transverse wakefield in the structures is reduced by detuning the modal frequencies such that they destructively interfere and by four damping manifolds per structure which provide weak damping. Errors in the fabrication of the individual cells and in the alignment of the cells will reduce the cancellation of the modes. Here, we calculate the tolerances on random errors in the synchronous frequencies of the cells and the cell-to-cell alignment.
09/2000;
