Investigator: Wayne D. Kimura (e-mail), STI Optronics, Inc.
Location: Beamline #1 of
Accelerator Test Facility (ATF), Brookhaven
and Main Achievements:
electron accelerators (laser linacs) offer the potential for enabling
much more economical and compact devices with very high acceleration
gradients.However, the development of practical laser linacs requires
accelerating a large ensemble of electrons together (“trapping”)
while keeping their energy spread small (“monoenergetic”).This has
never been realized before for any laser acceleration system until
the STELLA experiment.We have demonstrated
for the first time efficient, monoenergetic trapping and acceleration
of electrons via laser acceleration.
basic concept utilized by STELLA is to first group the electrons
within the electron-beam (e-beam) into microbunches (see
Principle of Electron Microbunch
Formation for more details).Briefly, the initial e-beam
energy is distributed uniformly over all phases of the accelerating
electromagnetic wave (i.e., laser beam optical field).A sinusoidal
energy modulation is imparted onto the e-beam using the intense
electric field of the laser beam.This accelerates some of the electrons
and decelerates others.The fast electrons are allowed to catch up
with the slow ones resulting in grouping (“bunching”) of the electrons
into tiny clusters (“microbunches”). These microbunches can then
be efficiently trapped and accelerated by a second laser acceleration
device while maintaining a narrow energy spread.This process of
trapping and acceleration by a second device is fundamental to staging
multiple laser acceleration sections, which is necessary for enabling
high net energy gains.Microbunching and staging were first demonstrated
in an earlier precursor experiment -.
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basic STELLA concept can be applied to many different laser acceleration
schemes.For experimental convenience, inverse free electron lasers (IFEL) were
chosen as the laser acceleration mechanism.An IFEL is a free electron
laser operating in reverse.The laser beam co-propagates with the
e-beam within the gap between a pair of parallel-facing magnet
arrays called an undulator.Depending on the sign of the laser field
seen by an electron, it will be accelerated or decelerated.
1 shows a schematic layout of the experiment.It consists of two
IFELs driven by a single laser beam from the ATF CO2 laser.The first IFEL (IFEL1)
is called the buncher and causes the sinusoidal energy modulation.The
second IFEL (IFEL2) is referred to as the accelerator. This IFEL traps and accelerates the microbunches.A
bunch compressor or chicane is located between IFEL1 and IFEL2.
The chicane has both a fixed magnetic field (i.e., permanent
magnets) and a variable magnetic field (i.e., electromagnets).The
fixed field forces the electrons
to travel through a “V-shaped” trajectory in which the faster electrons
generated by IFEL1 traverse a shorter path than the slower ones,
thereby causing the electrons to bunch at the entrance to IFEL2. (See Principle of Electron Microbunch
Formation for more details.) This creates a train of microbunches separated by 10.6 mm with individual bunch lengths of »1 mm (equivalent to »3 fs). Control
of the phase delay between the microbunches and the laser field
in IFEL2 is achieved by adjusting the variable field of the chicane. This enables resynchronizing the microbunches
with the accelerating portion of the laser field by slightly delaying
when the electrons exit the chicane.
Usage of the chicane also makes the entire system more compact
with a total length of 1.2 m from the entrance of IFEL1 to the exit
of IFEL2. At the end of the experiment is an energy spectrometer
for measuring the electron energy spectrum. See Photographic Tour of STELLA Experiment
for more details of the experimental hardware.
1. Schematic plan-view of
IFEL1 the gap separation is uniform along the undulator. IFEL2 uses a tapered undulator where the gap
separation decreases by 11% towards the end of the undulator. Tapering is important for achieving high trapping
efficiency and sufficient energy gain to separate the accelerated
electrons from the background electrons.
2 shows an example of the energy spectrum of the e-beam only
(red curve) along with the model prediction (gray histogram).
Plotted on the abscissa is the energy shift from the initial
e-beam energy (»45 MeV).
The ordinate represents the number of electrons where the
areas under the data and model curves have been normalized to each
other. The intrinsic energy
spread of the e-beam is very small (0.03%), thus the energy
spread shown in Fig. 2(b) is actually due to the finite spectrometer
resolution. The model includes
the finite spectrometer resolution and demonstrates very good agreement
with the data. (For more
details of the model see Ref.
2(a). Comparison of e-beam-only spectrum (red curve) with
2(b). Enlarged view of spectrum shown in (a).
the laser beam driving both IFELs, there is a dramatic change in
the e-beam spectrum. Figure 3 gives an example of high trapping
efficiency by IFEL2. The
data (red curve) shows a large number of electrons gaining 7-9 MeV;
in fact, an integration of the data curve indicates that »80% of the electrons have been trapped and accelerated
with an energy spread of »1.2% (1s). The model
prediction (gray histogram) agrees well with the data showing a
similar amount of trapping efficiency.
3. Example of high-trapping efficiency data
and comparison with model.
4 is an example of very narrow energy spread.
The accelerated electrons (red curve) still gained >7
MeV, but with an energy width of »0.36%
and a smaller trapping efficiency of »14%. The lower trapping efficiency appears to be
due to nonsymmetric focusing of the e-beam within IFEL2 and
a slight transverse offset between the e-beam and laser beam
inside IFEL2, which causes some of the electrons to be partially
accelerated. This is reflected
in the model simulation (gray histogram), which included e-beam
misfocusing and beam offset.
4. Example of narrow energy spread data and
comparison with model.
W. D. Kimura, A. van Steenbergen, M. Babzien, I. Ben-Zvi,
L. P. Campbell, C. E. Dilley, D. B. Cline, J. C. Gallardo, S. C.
Gottschalk, P. He, K. P. Kusche, Y. Liu, R. H. Pantell, I. V. Pogorelsky,
D. C. Quimby, J. Skaritka, L.C. Steinhauer, and V. Yakimenko, “First
Staging of Two Laser Accelerators,” Phys. Rev. Lett. 86,
2) W. D. Kimura, L. P. Campbell, C. E. Dilley,
S. C. Gottschalk, D. C. Quimby, A. van Steenbergen, M. Babzian,
I. Ben-Zvi, J. C. Gallardo, K. P. Kusche, I. V. Pogorelsky, J. Skaritka,
V. Yakimenko, D. B. Cline, P. He, Y. Liu, L. C. Steinhauer, and
R. H. Pantell, “Detailed Experimental
Results for Laser Acceleration Staging,” Phys. Rev. ST Accel.
Beams 4, 101301 (2001).
References Related to STELLA Program:
Zhou, D. B. Cline, and W. D. Kimura, “Beam Dynamics Analysis
of Femtosecond Microbunches Produced by the Staged Electron Laser
Acceleration Experiment,” Phys. Rev. ST Accel. Beams 6,
N. E. Andreev,
S. V. Kuznetsov, A. A. Pogosova, L. C. Steinhauer, and W. D. Kimura,
“Modeling of Laser
Wakefield Acceleration at CO2 Laser Wavelengths,”
Phys. Rev. ST Accel. Beams 6, 041301 (2003).
W. D. Kimura,
M. Babzien, I. Ben-Zvi, L. P. Campbell, D. B. Cline, C. E. Dilley,
J. C. Gallardo, S. C. Gottschalk, K. P. Kusche, R. H. Pantell, I.
V. Pogoresky, D. C. Quimby, J. Skaritka, L. C. Steinhauer, V. Yakimenko,
and F. Zhou, “STELLA-II: Staged Monoenergetic Laser Acceleration – Experiment
Update,” in Advanced Accelerator
Concepts, Jun. 23-28, 2002, Mandalay Beach, CA, AIP Conference
Proceedings No. 647, C. E. Clayton and P. Muggli, Eds., (American
Institute of Physics, New York, 2002), p. 269-277.
L. C. Steinhauer,
W. D. Kimura, N. E. Andreev, S. V. Kuznetsov, and A. A. Pogosova,
of Laser Wakefield Acceleration Using ATF CO2 Laser,” in Advanced Accelerator Concepts, Jun. 23-28,
2002, Mandalay Beach, CA, AIP Conference Proceedings No. 647, C.
E. Clayton and P. Muggli, Eds., (American Institute of Physics,
New York, 2002), p. 751-759.
L. P. Campbell,
C. E. Dilley, S. C. Gottschalk, W. D. Kimura, L. C. Steinhauer,
M. Babzien, I. Ben-Zvi, J. C. Gallardo, K. P. Kusche, I. V. Pogorelsky,
J. Skaritka, A. van Steenbergen, V. Yakimenko, D. B. Cline, P. He,
Y. Liu, and R. H. Pantell, “Inverse Cerenkov Acceleration and Inverse
Free Electron Laser Experimental Results for Staged Electron Laser
Acceleration,” IEEE Trans. Plasma Science 28, 1143-1151 (2000).
L. C. Steinhauer
and W. D. Kimura, “Longitudinal Space
Charge Debunching and Compensation in High Frequency Accelerators,”
Phys. Rev. ST Accel. Beams 2,
W. D. Kimura,
G. H. Kim, R. D. Romea, L. C. Steinhauer, I. V. Pogorelsky, K. P.
Kusche, R. C. Fernow, X. Wang, and Y. Liu, “Laser Acceleration of
Relativistic Electrons Using the Inverse Cerenkov Effect,” Phys.
Rev. Lett. 74, 546-549