A 210 MeV SLAC-type electron linac is currently under
construction at BNL as part of the Source Development Laboratory.
A 1.6 cell RF photoinjector is employed as the high brightness
electron source which is excited by a frequency tripled Titanium:Sapphire
laser. This linac will be used for several source development
projects including a short bunch storage ring, and a series of
FEL experiments based on the 10 m long NISUS undulator. The FEL
will be operated as either a SASE or seeded beam device using
the Ti:Sapp laser. For the seeded beam experiments; direct amplification,
harmonic generation, and chirped pulse amplification modes will
be studied, spanning an output wavelength range from 900 nm down
to 100 nm. This paper presents the project's design parameters
and results of recent modeling using the PARMELA and MAD simulation
codes.
The National Synchrotron Light Source has been engaged in the development of an FEL facility operating in the ultra-violet for more than five years. The Source Development Lab (SDL) has been established to pursue critical experiments on the path to short wavelength FELs, including development of high brightness beams, bunch compression and transport to high energy, and a broad range of SASE and seeded single pass FEL experiments. These FEL experiments will include study of startup, optical guiding, saturation, linewidth and fluctuations. The SDL is comprised of three major programs:
The electron beam development will be devoted to producing high brightness beams with peak current of 1 kA, normalized RMS emittance of 1 mm-mrad, and subpicosecond bunch lengths. The effects of coherent synchrotron radiation[1] and space charge forces[2] are expected to be significant with these parameters. Beam experiments will be devoted to studying their effect on emittance.
The electron source[5] is a radio-frequency photocathode
developed by a collaboration from BNL, SLAC, and UCLA. It consists
of a 1.6 cell RF structure driven at 2856 MHz. The maximum gradient
has been measured at 140 MV/m, yielding an exit energy of approximately
8 MeV. Improvements over the original BNL design[6] include elimination
of the side coupling into the half-cell to reduce emittance growth
due to the TM110 mode, installation of a removable
cathode allowing different cathode materials (e.g. magnesium)
to be used, and increasing the half-cell length to increase RF
focusing and decrease the peak field on the cell-to-cell iris.
The emittance correction solenoid has also been improved with
the addition of a re-entrant iron flux return to produce a more
uniform magnetic field with little fringing. PARMELA runs[5,7]
indicate that this RF gun is capable of producing electron bunches
with 7 ps flat top, 1 nC charge, and normalized RMS emittance
of 1.3 mm-mrad. The laser system used to excite the RF photocathode
is based on a wide-band Ti:Sapp oscillator[8] mode-locked to the
35th subharmonic of the RF frequency. Phase jitter is less than
1 ps. The light output from the oscillator enters a multipass
amplifier that stretches the pulse, amplifies it, and recompresses
to produce 10 mJ in a final pulse length of 150 fs. Up to 0.4
mJ of the 266 nm third harmonic of the amplified pulse is then
stretched to a final pulse length adjustable from 300 fs to 20
ps. An aberrated telescope is used to produce an elliptical beam
for a square transverse intensity profile and 65 degree wavefront
tilt to match the incidence angle on the RF photocathode. The
square intensity profile is optimal for emittance correction.
The wide bandwidth of the Ti:Sapp laser allows for longitudinal
pulse shaping so that nonlinear emittance correction may be investigated.
The linac (Figure 1) currently consists of four SLAC-type constant-gradient linac tanks operating at 2856 MHz, with provision for installation of a fifth section.
The first two linac tanks are used to accelerate
the beam to approximately 84 MeV. They also produce an energy
chirp on the electron beam in preparation for bunch compression
in a magnetic chicane. The compressor may be operated at any value
from zero field to full strength. A phosphor flag and collimator
are installed at the point of maximum dispersion for use as energy-spread
diagnostics, and for slice emittance[9] studies. Tracking studies
with MAD indicate the bunch should compress by a factor of 12,
from an RMS length of 600 m to 50 m. Following the chicane the
beam is accelerated through two more linac tanks to a maximum
energy of 230 MeV.
.
Following the linac is the 10 m long NISUS wiggler[10]
originally built by STI, Inc. for Boeing Aerospace (see Table
1). This wiggler is constructed of vanadium-permendur poles and
samarium-cobalt magnets, with iron shims added for error reduction.
The gap is remotely adjustable, has a maximum field strength of
0.56 T, and can produce a compound taper to improve efficiency
for high gain FELs. There are 256 periods, each of length 3.89
cm.
Electron Beam | |
Max. Energy | 230 MeV |
Peak Current | 1 kA |
Bunch Length | 200 fs < z < 20 ps |
RMS Emittance | < 2 mm-mrad |
E/E | 0.5% |
Length | 10 m |
Period | 3.89 cm |
Peak Field | 5.6 kG |
Num. Poles | 256 |
aw | 1.44 max |
Min. Gap | 1.44 cm |
Energy Taper | < 20% |
Wavelength | 80 nm < < 1000 nm |
Peak Power | 70 MW |
The vacuum pipe through the wiggler is constructed
of 8 independent sections. Each section has two ports for pop-in
phosphor screens, two ports for pick-up electrodes, and two sets
of steer/focus wires that can produce external dipole and quadrupole
fields. The wiggler poles are canted to produce focusing in both
planes.
Several of the important beam parameters for the FEL experiments have an unusually large range of adjustment. The pulse length and energy spread can be varied and optimized with both the drive laser and the magnetic chicane. The initial pulse length may be varied by nearly two orders of magnitude via the Ti:Sapp laser alone. Recent magnetic compression studies[11] have shown that increasing the initial pulse length can lead to shorter final pulses. This is because the more intense wakefields and higher space charge of an initially short bunch increase the nonlinear distortion in the energy-phase correlation used for compression[12]. Finally, one can optimize the bend angle in the compressor so that the nonlinear effects of the longitudinal wakefield and RF are partially canceled by the effect of the nonlinear dependence of path length on the energy deviation.
Very short, high current bunches propagating through bends can experience significant transverse emittance growth through two distinct effects, the longitudinal coherent space charge force (CSCF), and coherent synchrotron radiation (CSR). Emittance growth due to CSCF scales as Q (a/)2 where Q is the bunch charge, a is the bunch radius, and is the bunch length. Similarly, CSR scales as Qa/4/3. Simulations with a version of MAD modified by one of us (TR) to include CSR indicate that the emittance grows from 1.2 mm-mrad to 2.0 mm-mrad in the final bend of the compressor when compressed to a final bunch length of 0.6 ps. Greater compression is possible, but results in larger emittance. The bunch length, transverse size, and charge will all be varied in order to study the magnitude and scaling of these effects. The compressor vacuum pipe has a radiation port to capture synchrotron radiation which will be used as a diagnostic for bunch length and beam size. The emittance may be measured immediately before and after the compressor to isolate the effects of CSR and CSCF.
At high energy, the bunch length will be verified
through two methods. By passing the beam through a foil, coherent
optical transition radiation (OTR) will be generated at wavelengths
comparable to the bunch length. An experiment is planned following
the linac which measures the coherent OTR spectrum. Additionally,
the final linac section and bend may be used to produce an energy
chirp which can then be "streaked" on a phosphor screen
to measure bunch length. This profile measurement combined with
charge measurements in the Faraday cup or BPMs will give the bunch
charge profile. The drive laser pulse is approximately 1mm long
by 1mm in radius. The very short longitudinal profile significantly
affects the minimum emittance achievable via solenoidal emittance
correction[13]. By varying this profile, we will study the relative
strength of the nonlinear terms in the emittance correction. Measurements
of the slice emittance as developed at BNL's Accelerator Test
Facility will be used in these studies.
The FEL development program for the SDL can be broken
into stages based on machine requirements and modifications. In
the first stage, a normalized emittance of 6.5 mm-mrad at a beam
energy of 130 MeV is required. SASE experiments will be conducted
at roughly 1m wavelength with an anticipated peak power of 70
MW. Tapering and harmonic content will be investigated. The first
seeded beam operation will be at the Ti:Sapp fundamental (900
nm). Chirped pulse amplification experiments at this wavelength
will yield photon pulses as short as 10 fs. After adding an energy
modulation wiggler and dispersive section at a later date, the
FEL output wavelength will be pushed to 200 nm using harmonic
generation. A 400 kW beam from the Ti:Sapp at 400 nm will bunch
the electron beam, which will then lase on the 2nd
harmonic, producing 70 MW at 200 nm. With the addition of a 5th
linac section increasing the beam energy to 310 MeV, and emittance
of 1 mm-mrad, FEL operation below 100 nm should be possible, including
the demonstration of CPA at 80 nm with a 5 fs pulse duration.
Thanks to J. Gallardo of BNL and D. Palmer of Stanford
for PARMELA input for the linac, to L. DiMauro of BNL for discussions
on the Ti:Sapp laser, to L.-H. Yu of BNL for the FEL parameters,
and to B. Carlsten of LANL for PARMELA simulations of the RF gun.
[1] C. L. Bohn, "Beam-Induced Fields and Emittance Growth in Bends", Proc. Micro-Bunches Workshop at BNL, Upton, NY, USA (1995).
[2] B.E. Carlsten, T.O. Raubenheimer, "Emittance
Growth of Bunched Beams in Bends", Phys. Rev. E51 (1995)
p1453-1470.
[3] L.-H. Yu, "Generation of intense uv radiation
by subharmonically seeded single-pass free-electron lasers",
Phys. Rev. A44, (1991) p5178-5193.
[4] L.-H. Yu et al, "Femtosecond free-electron
laser by chirped pulse amplification", Phys. Rev. E49, (1994)
p4480-4486.
[5] D.T. Palmer et al, "Microwave measurements
of the BNL/SLAC/UCLA 1.6 Cell Photocathode RF Gun", Proc.
IEEE Particle Accelerator Conf. (1995) p982-984.
[6] K. Batchelor et al, "Performance of the
Brookhaven Photocathode RF Gun", NIM A318 (1992) p372-376.
[7] B.E. Carlsten, private communication.
[8] Spectra Physics Lasers, Inc., Tsunami Model 3960
L1S.
[9] X. Qiu et al, "Demonstration of Emittance
Compensation through the Measurement of the Slice Emittance of
a 10-ps Electron Bunch", Phys. Rev. Lett. 76 (1996) p3723-3726.
[10] D.C. Quimby et al, "Development of a 10-Meter
Wedged-Pole Undulator", NIM A285 (1989) p281-289.
[11] B.E. Carlsten, "Subpicosecond compression
of 0.1-1 nC electron bunches with a magnetic chicane at 8 MeV",
Phys. Rev. E53 (1996) p2072-2075.
[12] B.E. Carlsten, "Nonlinear subpicosecond
electron-bunch compressor", Los Alamos Nat. Lab, LA-UR-95-3933
(1995).
[13] B.E. Carlsten, "Space Charge Induced Emittance Compensation in High Brightness Photoinjectors",Part. Accel 49 (1995) p27-65.