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Proceedings ArticleDOI

A proposed injector for the LCLS linac

12 May 1997-Vol. 3, pp 2855-2857
TL;DR: In this paper, the injector beamline has been designed using the SUPERFISH, POISSON, PARMELA, and TRANSPORT codes in a consistent way to simulate the beam from the S-band RF gun up to the entrance of the main accelerator linac where the beam energy is 150 MeV.
Abstract: The Linac Coherent Light Source (LCLS) will use the last portion of the SLAC accelerator as a driver for a short wavelength FEL. The injector must produce 1-nC, 3-ps rms electron bunches at a repetition rate of up to 120 Hz with a normalized rms emittance of about 1 mm-mrad. The injector design takes advantage of the photocathode RF gun technology developed since its conception in the mid 1980's, in particular the S-band RF gun developed by the SLAC/BNL/UCLA collaboration, and emittance compensation techniques developed in the last decade. The injector beamline has been designed using the SUPERFISH, POISSON, PARMELA, and TRANSPORT codes in a consistent way to simulate the beam from the gun up to the entrance of the main accelerator linac where the beam energy is 150 MeV. PARMELA simulations indicate that at 150 MeV, space charge effects are negligible.

Summary (2 min read)

Introduction

  • The Linac Coherent Light Source (LCLS) will use the last portion of the SLAC accelerator as a driver for a short wavelength FEL.
  • The injector design takes advantage of the photocathode rf gun technology developed since its conception in the mid 1980's [1], in particular the S-band rf gun developed by the SLAC/BNL/UCLA collaboration [2], and emittance compensation techniques developed in the last decade [3,4].
  • The injector beamline has been designed using the SUPERFISH, POISSON, PARMELA, and TRANSPORT codes in a consistent way to simulate the beam from the gun up to the entrance of the main accelerator linac where the beam energy is 150 MeV.
  • PARMELA simulations indicate that at 150 MeV, space charge effects are negligible.

1 BEAMLINE

  • Many diagnostics have been designed into the injector to characterize the beam and to aid in tuning the injector.
  • In the drift section from the gun to the first accelerator there will be a beam position monitor (BPM) a current monitor, and a profile screen/Faraday cup pop-in device.
  • A full transverse emittance diagnostic section is included prior to the dog-leg which consists of four wirescanners [6] separated by 45˚ of x and y betatron phase advance.
  • Emittance and energy scrapers are also planned in this region.

2 SIMULATION

  • Emittance growth and compensation in the LCLS injector from the gun to the end of the fourth accelerator section has been thoroughly studied using PARMELA simulations.
  • Much care has been taken to symmetrize the dominant dipole rf fields in the gun [2] thus assuming cylindrical symmetry in the rf fields is not unreasonable.
  • The magnetic field at the gun is generated by two solenoids placed symmetrically up and downstream of the cathode such that the axial magnetic field at the cathode is zero.
  • Thus the beam radius is drastically reduced at the entrance of the third accelerator section to reduce the emittance growth due to radial electric field effects.
  • The core emittance (excluding a 7.7% halo) at 150 MeV is 1.08 mm-mrad for the truncated Gaussian temporal bunch shape.

3 DOG-LEG INFLECTOR

  • The function of the dog-leg inflector is to transport the 150 MeV electron beam from the new injector linac into the existing SLAC linac.
  • It is possible to design the dogleg as a first bunch compression stage, but this necessitates a large incoming correlated energy spread of 1- 2% and the chromaticity of the quadrupole magnets within the dog-leg—required for linear achromaticity—generate large second order dispersion which requires sextupole compensation.
  • The beamline is 4 meters long with two 14˚, 20-cm long bends providing the ~1 meter inflection.
  • Note, the incoming rms energy spread from the injector is 0.13%.
  • The system is therefore, for all practical purposes, isochronous with no significant chromatic emittance dilution.

4 SUMMARY

  • The injector design for the LCLS takes advantage of the present development of photocathode rf gun technology to produce 150 MeV, 1 nC, 3 ps rms electron bunches with a normalized rms emittance of 1.1 mmmrad.
  • The necessary beamline and rf systems have been defined to produce such beams and the design includes sufficient diagnostic components to aide in the production and characterization of the electron bunches.

6 REFERENCES

  • J. S. Fraser, et. al., "Photocathodes in Accelerator Applications", Proceedings of the IEEE Particle Accelerator Conference, Wash.
  • M. C. Ross, et. al., "Wire Scanners for Beam Size and Emittance Measurements at the SLC", Proceedings of the IEEE Particle Accelerator Conference, San Francisco, 1991, p 1201. [7].

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SLAC-PUB-7362
A Proposed Injector for the LCLS Linac
*Work supported by Department of Energy contract DE–AC03–76SF00515.
Stanford Linear Accelerator Center
Stanford University, Stanford, CA 94309
A.D. Yeremian, V. K. Bharadwaj, P. Emma, R.H. Miller,
D.T. Palmer, M.D. Woodley
November 1996
Presented at 17th IEEE Particle Accelerator Conference (PAC 97): Accelerator Science,
Technology, and Applications, May12–May16, 1997, Vancouver, B. C., Canada

A PROPOSED INJECTOR FOR THE LCLS LINAC
*
A. D. Yeremian, V. K. Bharadwaj, P. Emma, R. H. Miller,
D. T. Palmer, M. D. Woodley
Stanford Linear Accelerator Center
Stanford, California 94309
Abstract
The Linac Coherent Light Source (LCLS) will use the
last portion of the SLAC accelerator as a driver for a short
wavelength FEL. The injector must produce 1-nC, 3-ps
rms electron bunches at a repetition rate of up to 120 Hz
with a normalized rms emittance of about 1 mm-mrad.
The injector design takes advantage of the photocathode rf
gun technology developed since its conception in the mid
1980's [1], in particular the S-band rf gun developed by
the SLAC/BNL/UCLA collaboration [2], and emittance
compensation techniques developed in the last decade
[3,4]. The injector beamline has been designed using the
SUPERFISH, POISSON, PARMELA, and
TRANSPORT codes in a consistent way to simulate the
beam from the gun up to the entrance of the main
accelerator linac where the beam energy is 150 MeV.
PARMELA simulations indicate that at 150 MeV, space
charge effects are negligible.
1 BEAMLINE
The injector beamline for LCLS consists of a 1.6 cell
S-band photocathode rf gun, 4 SLAC type 3 m S-band
constant gradient traveling wave accelerator sections,
emittance compensation solenoids at the gun (after
accelerator section 2 and after accelerator section 3), a 14˚
achromatic, isochronous translation bend system (dog-leg)
into the main linac, and instrumentation for beam
diagnostics as shown in Fig. 1.
The rf distribution system is also shown in Fig. 1.
Two un-SLEDed 5045 klystrons will be used: one to
power the gun and the first two accelerator sections and
the other for the last two accelerator sections. High power
rf attenuators and phase shifters will allow independent
control of the rf power, phase and amplitude for the gun
and each of the first two accelerating sections.
Many diagnostics have been designed into the injector
to characterize the beam and to aid in tuning the injector.
In the drift section from the gun to the first accelerator
there will be a beam position monitor (BPM) a current
monitor, and a profile screen/Faraday cup pop-in device. A
mask in the laser beam into the photocathode can be used
to produce beamlets in various patterns to study emittance
trends from the gun, or just as a tuning aid to optimize
the steering. After each accelerator section there is a BPM,
* Work supported by Department of Energy contract
DE-AC03-76SF00515
a current monitor, and a screen. After the second
accelerator section there is a bunch length monitor which
is an x-band cavity outside the beamline near a ceramic
gap in the beamline [5]. While this monitor will not
characterize the longitudinal bunch profile, it will be a
useful tuning aid to minimize the bunch length.
A full transverse emittance diagnostic section is
included prior to the dog-leg which consists of four wire-
scanners [6] separated by 45˚ of x and y betatron phase
advance. The matched input beam produces an x and y rms
beam size of 67
µ
m at each wire. A fifth wire scanner in
the dog-leg allows the energy spread measurement.
Emittance and energy scrapers are also planned in this
region.
2 SIMULATION
Emittance growth and compensation in the LCLS
injector from the gun to the end of the fourth accelerator
section has been thoroughly studied using PARMELA
simulations. The electric field map of the rf gun was
obtained with SUPERFISH and directly used in
PARMELA. SUPERFISH was used to simulate the fields
in the traveling wave accelerator sections and space
harmonics were calculated for use in PARMELA. The rf
fields were assumed to be cylindrically symmetric. Much
care has been taken to symmetrize the dominant dipole rf
fields in the gun [2] thus assuming cylindrical symmetry
in the rf fields is not unreasonable.
A magnetic field map for the solenoid magnets at the
gun was produced using POISSON and passed to
PARMELA. Single coils were used to represent magnetic
fields from solenoids between accelerator sections.
Space charge effects were included assuming cylindrical
symmetry. For these simulations, the thermal emittance
at the cathode and the emittance growth due to multipole
electric fields were ignored. The magnetic and electric
fields in the gun and the accelerator region were optimized
to minimize the emittance at 150 MeV at which point the
normalized emittance is virtually constant.
The peak electric field in the gun is 150 MV/m at the
cathode and the laser is injected at 23 degrees ahead of the
rf crest. In the first and second accelerator sections the
centroid of the beam is about 5 degrees ahead of the crest
and the gradient is 7 MV/m. This introduces about a 1.5%
energy spread for the full beam at the exit of the second
accelerator but helps slightly in emittance compensation.
This energy spread is removed by the third accelerator
section by phasing the rf such that the beam arrives

4–97
8253A1
Vac valves Attenuator
Phase ShifterSolenoid
BPM,
profile
monitor
BPM, current mon,
Faraday cup, screen
Steering
coils
Screen
Toroid
E scraper
Matching & Diagnostics
7 Quads,
BPMs,
steering
Main
linac
Klystron
Streak camera, screen,
BPM, Toroid
Beam stopper
Wire scanner
Toroid, BPM,
screen, BLM
RF
gun
K K
Figure 1. Beamline layout from rf gun to injection into the SLAC main linac. The ‘matching & diagnostics’ section
includes matching quadrupoles, 4 wire scanners for transverse diagnostics, 4 phase space scrapers, a toroid, BPMs and a
streak camera station for bunch length measurements. The two dog-leg inflector bends follow the diagnostic section.
slightly behind the crest. The gradient of the third and
fourth accelerator sections is 17 MV/m.
The magnetic field at the gun is generated by two
solenoids placed symmetrically up and downstream of the
cathode such that the axial magnetic field at the cathode is
zero. The field then rises sharply after the cathode to about
3 kG with an effective length of about 20 cm. The axial
magnetic field between accelerator sections is slightly
greater than 3 kG with an effective length of 70 cm. Fig.
2 shows the axial magnetic fields along the length of the
beamline where s = 0 is the cathode location.
Figure 2. Solenoidal magnetic field along the beamline
from the cathode (s = 0) to the 150 MeV point.
At the cathode, the edge beam radius is about 0.9 mm
dictated by emittance compensation—a balance between
space charge, magnetic field, and electric field effects. At
the entrance of accelerator-section-3, however, radial rf
fields have a more dominant effect on emittance growth
than space charge since the beam energy is nearly 50 MeV
In addition, the accelerating gradient in section-3 is more
than twice that of sections 1 and 2. Thus the beam radius
is drastically reduced at the entrance of the third accelerator
section to reduce the emittance growth due to radial
electric field effects.
The transverse distribution at the cathode is assumed to
be uniform, but the temporal shape is assumed to be a
truncated Gaussian achieved by using a Gaussian pulse
with an rms of 4.4 ps which is then truncated at ±2
σ
. The
overall rms length is then 3.8 ps. In fact, a uniform
temporal distribution is more desirable for optimal
emittance compensation, however a truncated Gaussian
results in a bunch-compression system which is much
less sensitive to injection timing jitter [7].
It should be noted here that simulations indicate a 40%
shorter beam at 150 MeV than at the cathode due to
bunching because the laser is fired 23 degrees ahead of the
rf. However there is experimental indication of bunch
lengthening in the 1.6 cell S-band rf gun at these current
densities which is not manifested in PARMELA
simulations [8]. Thus the expected bunch length at 150
MeV may be longer than indicated in simulation by
possibly 30-40%. Fig. 3 shows the temporal and energy
distributions and phase space as well as an x-y particle
scatter plot of the simulated bunch at 150 MeV.
Figure 3. Temporal distribution (a), x-y space (b),
longitudinal phase space (c), and energy distribution (d) at
150 MeV for Table 1 parameters.
Fig. 4 is a plot of the horizontal normalized emittance
(
βγε
x
, where
β
v/c) along s for an optimized parameter
set of the injector beamline shown in Fig. 1. Bunch
parameters at the end of the fourth accelerator section are

given in Table 1. The core emittance (excluding a 7.7%
halo) at 150 MeV is 1.08 mm-mrad for the truncated
Gaussian temporal bunch shape.
If a uniform distribution with rise and fall of less than
1 ps is used at the gun, the emittance at 150 MeV is
0.95 mm-mrad (excluding a 3% halo). We have also
simulated the 1 nC, uniform transverse and Gaussian
temporal bunch truncated at ±4
σ
(i.e. nearly a full
Gaussian pulse). In this case the core emittance at 150
MeV is 1.07 mm-mrad with the exclusion of 8.1% halo.
Figure 4. Normalized emittance (solid) and rms beam size
(dash) along the beamline from cathode (s = 0) to 150 MeV.
The step at s ~ 16 m is a 7.7% halo cut (in simulation only).
Table 1. Simulated electron beam parameters at 150 MeV.
Longitudinal distribution Gaussian
Transverse distribution uniform
Bunch charge nC 1.0
Halo population % 7.7
rms pulse length psec (mm) 2.3 (0.69)
Energy MeV 150.5
rms relative energy spread % 0.13
Norm. rms core emittance mm-mrad 1.08
3 DOG-LEG INFLECTOR
The function of the dog-leg inflector is to transport the
150 MeV electron beam from the new injector linac into
the existing SLAC linac. It is possible to design the dog-
leg as a first bunch compression stage, but this
necessitates a large incoming correlated energy spread of 1-
2% and the chromaticity of the quadrupole magnets within
the dog-leg—required for linear achromaticity—generate
large second order dispersion which requires sextupole
compensation. Due to this, and also the need to tune the
momentum compaction of the compressor (not natural in
a dog-leg), the dog-leg is designed as a simple transport
line, not a compressor.
The design requirements of the line are: 1) should
provide a horizontal beamline inflection of ~1 meter over
a few meters distance, 2) should not alter the bunch length
(isochronous), and 3) should introduce no significant
transverse emittance dilution. The inflection may also be
made in the vertical plane or a rolled plane. However,
there is no strong motivation to do so. A simple system
which satisfies these conditions is composed of two dipole
magnets of opposite strength separated by a +I optical
transformer (seven quadrupoles) to produce a linear
achromat. The dipoles are rectangular bends.
The beamline is 4 meters long with two 14˚, 20-cm
long bends providing the ~1 meter inflection. This
produces a momentum compaction, R
56
, of 4 mm. An
extreme electron which is off energy by 1% will move
axially by 40
µ
m which is small compared to the ~1 mm
bunch length. The effect of the second order momentum
compaction, T
566
, is even less. Note, the incoming rms
energy spread from the injector is 0.13%. The system is
therefore, for all practical purposes, isochronous with no
significant chromatic emittance dilution.
4 SUMMARY
The injector design for the LCLS takes advantage of
the present development of photocathode rf gun
technology to produce 150 MeV, 1 nC, 3 ps rms electron
bunches with a normalized rms emittance of 1.1 mm-
mrad. The necessary beamline and rf systems have been
defined to produce such beams and the design includes
sufficient diagnostic components to aide in the production
and characterization of the electron bunches.
5 ACKNOWLEDGMENTS
We are grateful to Dr. Luca Serafini of INFN-Milan in
Italy and Dr. James Rosenzweig of UCLA for many
helpful discussions on the production and maintenance of
the high brightness beams from rf photocathode guns.
6 REFERENCES
[1]. J. S. Fraser, et. al., "Photocathodes in Accelerator
Applications", Proceedings of the IEEE Particle
Accelerator Conference, Wash. DC, 1987, p 1705.
[2]. D. T. Palmer, et. al., "Microwave Measurements of
the BNL/SLAC/UCLA 1.6 Cell Photocathode RF
Gun", Proceedings of the 1995 IEEE Particle
Accelerator Conference, Dallas, 1995, p982.
[3]. B. E. Carlsten, "New Photoelectric Injector Design
for the LANL XUV FEL Accelerator", NIM, A285,
1989, p313.
[4]. L. Serafini and J. B. Rosenzweig, "Envelope
Analysis of Intense Relativistic Quasi-Laminar
Beams in RF Photoinjectors: A Theory of Emittance
Compensation", to be published in Phys. Rev. E.
[5]. R. H. Miller, "Proposed Bunch Monitor", SLAC
TN-63-65, August 1963.
[6]. M. C. Ross, et. al., "Wire Scanners for Beam Size
and Emittance Measurements at the SLC",
Proceedings of the IEEE Particle Accelerator
Conference, San Francisco, 1991, p 1201.
[7]. V. K. Bharadwaj, et. al., “Linac Design for the
LCLS Project at SLAC”, these proceedings.
[8]. D. T. Palmer, et. al., "Emittance Studies of the
BNL,/SLAC/UCLA 1.6 cell Photoacthode rf gun",
these proceedings.
Citations
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Journal ArticleDOI
TL;DR: In this article, a 1.6-cell S-band rf gun with an optical spot size at the cathode of a radius of {approximately} 1 mm and a pulse duration with an rms sigma of { approximately}3 ps.
Abstract: We report on the design of the rf photoinjector of the Linac Coherent Light Source (LCLS). The rf photoinjector is required to produce a single 150 MeV bunch of {approximately}1 nC and {approximately}100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance of {approximately}1 {pi} mm-mrad. The design employs a 1.6-cell S-band rf gun with an optical spot size at the cathode of a radius of {approximately}1 mm and a pulse duration with an rms sigma of {approximately}3 ps. The peak rf field at the cathode is 150 MV/m with extraction 57 {degree} ahead of the rf peak. A solenoidal field near the cathode allows the compensation of the initial emittance growth by the end of the injection linac. Spatial and temporal shaping of the laser pulse striking the cathode will reduce the compensated emittance even further. Also, to minimize the contribution of the thermal emittance from the cathode surface, while at the same time optimizing the quantum efficiency (QE), the laser wavelength for a Cu cathode should be tunable around 260 nm. Following the injection linac the geometric emittance simply damps linearly with energy growth. PARMELA simulations show that this design willmore » produce the desired normalized emittance, which is about a factor of two lower than has been achieved to date in other systems. In addition to low emittance, we also aim for laser amplitude stability 1% in the UV and a timing jitter in the electron beam of 0.5 ps rms, which will lead to less than 10% beam intensity fluctuation after the electron bunch is compressed in the main linac.« less

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Proceedings ArticleDOI
12 May 1997
TL;DR: The Linac Coherent Light Source (LCLS) at SLAC is being designed to produce intense, coherent 0.15-nm X-rays by a single pass of a 15 GeV bunched electron beam through a long undulator.
Abstract: The Linac Coherent Light Source (LCLS) at SLAC is being designed to produce intense, coherent 0.15-nm X-rays. These X-rays will be produced by a single pass of a 15 GeV bunched electron beam through a long undulator. Nominally, the bunches have a charge of 1 nC, normalized transverse emittances of less than 1.5/spl pi/ mm-mr and an rms bunch length of 20 /spl mu/m. The electron beam will be produced using the last third of the SLAC 3-km linac in a manner compatible with simultaneous operation of the remainder of the linac for PEP-II. The linac design necessary to produce an electron beam with the required brightness for LCLS is discussed, and the specific linac modifications are described.

8 citations

References
More filters
Journal ArticleDOI
TL;DR: In this paper, the Los Alamos National Laboratory XUV FEL accelerator injector has been redesigned to provide more charge to the wiggler, and the new design can deliver 8nC of charge within 20 ps with a normalized 90% emittance of π mm mrad.
Abstract: The injector for the Los Alamos National Laboratory XUV FEL accelerator has been redesigned to provide more charge to the wiggler. The new design can deliver 8nC of charge within 20 ps with a normalized 90% emittance of π mm mrad to the wiggler at an energy of 200 MeV. In addition to the new design of the injector, we analyze the emittance growth and subsequent reduction through the injector, including the different mechanisms for emittance growth and the methods used to eliminate the correlated emittance.

304 citations

Journal ArticleDOI
TL;DR: In this paper, an analytical description for the transverse dynamics of relativistic, space-chargedominated beams undergoing strong acceleration, such as those typically produced by rf photoinjectors is provided.
Abstract: In this paper we provide an analytical description for the transverse dynamics of relativistic, space-chargedominated beams undergoing strong acceleration, such as those typically produced by rf photoinjectors. These beams are chiefly characterized by a fast transition, due to strong acceleration, from the nonrelativistic to the relativistic regime in which the initially strong collective plasma effects are greatly diminished. However, plasma oscillations in the transverse plane are still effective in significantly perturbing the evolution of the transverse phase space distribution, introducing distortions and longitudinal-transverse correlations that cause an increase in the rms transverse emittance of the beam as a whole. The beam envelope evolution is dominated by such effects and not by the thermal emittance, and so the beam flow can be considered quasilaminar. The model adopted is based on the rms envelope equation, for which we find an exact particular analytical solution taking into account the effects of linear space-charge forces, external focusing due to applied as well as ponderomotive RF forces, acceleration, and adiabatic damping, in the limit that the weak nonlaminarity due to the thermal emittance may be neglected. This solution represents a special mode for beam propagation that assures a secularly diminishing normalized rms emittance and it represents the fundamental operating condition of a space-charge-compensated RF photoinjector. The conditions for obtaining emittance compensation in a long, integrated photoinjector, in which the gun and linac sections are joined, as well as in the case of a short gun followed by a drift and a booster linac, are examined. @S1063-651X~97!10706-1#

274 citations

01 Jan 1987
TL;DR: In this paper, Fraser et al. showed that the acceleration of short bunches in an rf cavity that dense s lp ace charge the external rf field lead to a degradation of beam quality and, therefore, to a loss of brightness.
Abstract: PHOTOCATHODES IN ACCELERATOR APPLICATIONS* J. S. Fraser, R. L. Sheffield, E. Gray, P. M.Giles, R. W. Springer and A. Loebs, MS-H825 Los Alamos National Laboratory, Alamos, NM 87545 Abstract Some electron accelerator applications require bursts of short ulses at high microscopic repetition rates and high pea E brightness. A illuminated by a mode-locked laser, is well suited to filling this need. The intrinsic brightness of a photoemitter beam is high; experiments are under way at Los Alamos to study the brightness of short bunches with hi h s ace charge after acceleration. A laser-illuminated E s,S t photoemitter is located in the first rf cavity of an injector linac. Diagnostics include pep er-pot emittance analyzer, a magnetic spectrometer, an 2 a streak camera. Introduction Electron accelerator applications that require a beam of high peak brightness as well a high pulse-repetition rate put severe demands on the electron source and injector. The conventional solution to this problem is to use a pulsed thermionic emitter of low peak brightness followed by a bunching, or phase compression, system that increases the peak current by a large factor. Ideally, the FJ eak brightness should increase in proportion to the unching factor; in however, the result always falls short of the ideal. An additional shortcoming; in the conventional buncher is that a repetition rate in excess of a few tens of megahertz is beyond the state of the art electronically swrtching a thermionic triode electron gun. There have been several reports of diode and triode guns operated in an rf cavity to produce electron pulses of width somewhat less than one-quarter of the rf period.1-3 The ultimate upper limit to this technique for triode guns may be the power dissi ation in the grid. In current app F lcations of rf-driven free-electron lasers, trains of over 300-A, 16-ps-wide (5nC charge) pulses are being used.4 Recent success 5 with the production of high peak currents from a laser-illuminated photocathode shows that high repetition rates in peak-brightness electron beams may be possible. At the Los Alamos National Laboratory, a program is under way to develop a high peak brightness, average current photoelectron injector. The Los Alamos Photoinjector Program 1985, the achievement of hi!h peak currents from a Cs,Sb uhotocathode was reported. It has been shown that thg laser-driven photocathode produces an intrinsically bright beam.‘j It remains to be demonstrated that short bunches can be accelerated to relativistic energies without loss of brightness. It is evident from simulation studies70f the acceleration of short bunches in an rf cavity that dense s lp ace charge the external rf field lead to a degradation o beam quality and, therefore, to a loss of brightness. Although pulses of only a few picoseconds can be produced in a photocathode, it now seems advisable to generate pulses that initially are about 100 ps long and thin, after acceleration to about 10 MeV, bunch them magnetically.8 Acceleration of the longer bunches is best done in a low-frequency linac at subharmonic of the main linac frequency.g Nevertheless, there is a strong incentive to accelerate the bunches as rapidly as possible, a condition that can be met only with high-frequency rf fields.‘O A study of the envelope equation Ref. 11 reveals that, for continuous beams, the dominance of space charge over emittance is adiabatically damped as Y-“~. A *Work performed under the auspices of U.S. Department of Energy and supported by the U.S. Army Strategic Defense Command. paper in these proceedinps’2 shows that for bunched beams, the damping dependence on energy is much stronger, namely y- . Therefore. the reauirement of maximum acceleratin gradient (hence a hiih frequency) to minimize the in if uence of space charge must be balanced against a conflicting need to accept long pulses (hence a low frequency to reduce the emittance growth associated with rf fields). The conceptual design of a subharmonic injector linac with an rf gun electron source has been described earlier.g The initial rf gun experiments, however, are being carried out at a high frequency because powerful klystron was available for use at 1300 MHz. A schematic diagram of the Los Alamos injector experiment is shown in Fig. 1. OtlnRTz SCHkCN -j Fig. 1. Plan view of the photoinjectorexperiment. Photocathode Design In recent years, photocathodes for sources have been made from wafers of B olarized electron aAs.13*14 Curre?! densities as high 180 A/cm2 have been reported. Photoemitters of Cs,Sb are less demanding o system cleanliness15 than are those of GaAs. An additional advantage of a positive electron affinity semiconductor like Cs,Sb lies in rapid emissio? of Jhe photoelectrons. l6 By contrast, the intrinsic emission-time uncertainty of GaAs has been measured in the range from 71 ps for active layers between 50 nm and 2 pm in thickness.16 A cesium antimonide (Cs Sb) photocathode was chosen for its ease of preparation within the vacuum environment linac and for its relative tolerance vacuum conditions in the injector linac.15 A photoinjector linac must be bakeable in its entirety to about 200°C ;nd be capable of maintaining a preferably lo-‘Otorr. If a Cs, b hotocathode LS damaged ,rrre below . lo- torr, in use, the damage can be erased y heating to 4OO”C, then a new one prepared in situ. The spectral response17 of Cs Sb extends from a quantum energy of 1.8 eV (A = B90 nm) to energies greater than 3.8 eV (A < 320 nm). Therefore, a Cs,Sb F hotocathode can be used with Nd:YAG laser doubled (A = 532 nm) or tripled (A = 355 nm). ?&%;;‘lG laser can readilv be mode-locked to deliver trains of 60-ps pulses at a microscopic repetition rate in a range from 120 MHz. Rf Gun Cavity Design The highest cavity is limited l!l ossible acceleration rate in rf .gu+n y the sparking breakdown characterlstlc of the cavity. A typical rf cavity, optimized for maximum effective shunt im edance ZT2, will have a ratio of peak surface electric fie acceleration gradientlo of about 6. A lower ratio, a larger maximum 1705 CH2387.9:87!000(- $ I .oO Q IEEE

45 citations

Proceedings ArticleDOI
01 May 1995
TL;DR: In this paper, the longitudinal accelerating field E/sub z/ has been measured as a function of azimuthal angle in the full cell of the cold test model for the 16 cell BNL/SLAC/UCLA 3 S-band RF gun using a needle rotation/frequency perturbation technique.
Abstract: The longitudinal accelerating field E/sub z/ has been measured as a function of azimuthal angle in the full cell of the cold test model for the 16 cell BNL/SLAC/UCLA 3 S-band RF gun using a needle rotation/frequency perturbation technique These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short Two different waveguide to full cell coupling schemes were studied The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the /spl theta/-coupling scheme The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques The Panofsky-Wenzel theorem is used in estimating the transverse emittance due to the multipole components of E/sub z/

43 citations

Proceedings ArticleDOI
06 May 1991
TL;DR: In this article, the authors describe the design, construction, commissioning and ultimate uses of wire scanners in the SLAC Linear Collider (SLAC) focusing on the linear accelerator and upstream systems scanners.
Abstract: The authors describe the design, construction, commissioning and ultimate uses of wire scanners in the SLC (SLAC Linear Collider), focusing on the linear accelerator and upstream systems scanners. Of particular interest is the interaction between the wire and the scattered radiation from the wire with the extreme electric field of the beam. As this field reaches the level of several volts/angstrom, as it does easily at the SLC interaction point (and may in upstream parts of SLC), field emission from the wire may occur. A key feature of SLC operation is the degree of high level active control required to keep it optimized. Advanced high level control software allows the use of wire scanner data in feedback and beam optimization procedures. Non-invasive scans are performed almost continually and the results are logged so that long term trends in emittance can be examined. >

35 citations

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Q1. What are the contributions in "A proposed injector for the lcls linac" ?

In this paper, an injector beamline has been designed using the SUPERFISH, POISSON, PARMELA, and TRANSPORT codes in a consistent way to simulate the beam from the photocathode rf gun up to the entrance of the main accelerator linac where the beam energy is 150 MeV.