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The papers listed below constitute Volume 7, Number 1 of the Journal of Directed Energy, a CUI special issue on Free Electron Lasers. Copies of these papers are available to those having necessary credentials by contacting Kat@deps.org.
| Free Electron Laser Issue Introduction | |
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Summary of Results from the Active Denial Biological Effects Research Program
F. Krawczyk, S. Brandt, C. Buechler, B. Carlsten,* J. Lewellen, D. Nguyen, and R. Wheat, Accelerator Operation and Technology, Los Alamos National Laboratory The free-electron laser (FEL) has been identified as an enabling technology to achieve
megawatt (MW)-class laser-power devices for the battlegrounds of the future. Because of a
lasing process in vacuum, FELs are scalable to power levels significantly beyond the typical
~100 kW of power achievable by more traditional high-power laser designs. They do not depend
on interaction with crystalline laser media and thus are continuously tunable to an arbitrary
lasing frequency, and they do not require the presence of dangerous chemicals, as are needed
in MW-class chemical lasers. The development of high-power FELs was based on the successfully
operated high-average-power accelerators originally intended for science applications, such
as the Jefferson Laboratory energy recovery linear accelerator (ERL), which was used to
demonstrate greater than 10-kW average power from an FEL. These initial developments led to
the Navy funding an Innovative Naval Prototype (INP) project for a 100-kW prototype system
scalable to the power levels needed for a practical weapon. A key realization from this work
was that while the standard ERL technology had several crucial advantages, overall this
architecture was limited to about 100-kW output power. This report presents the subsequent
development of the next-generation high-power FEL accelerator architecture that addresses the
shortcomings of the traditional ERL and leads to a true MW-class FEL laser system. |
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Megawatt-Traceable, High-Power Free-Electron Laser Physics Design with Start-to-End Simulations
B. E. Carlsten,* F. L. Krawczyk, J. W. Lewellen, Q. R. Marksteiner, and D. C. Nguyen, Accelerator Operation and Technology, Los Alamos National Laboratory We report on the results of a 180-kW free-electron laser (FEL) point design study, with >50%
conversion efficiency from electron-beam power to laser power at 2.2 µm. This is achieved
with a relatively high FEL efficiency (~3.5%) by using a 55-MeV, 0.1-A electron beam and
extracting over 97% of the residual electron beam's power after the FEL interaction using
same-cell energy extraction. The special design features include a short-gap superconducting
photoinjector, ballistic bunching instead of magnetic compression of the electron beam to
minimize the effects of requiring a large energy slew and coherent synchrotron radiation, and
dual-aperture superconducting cavities to allow a larger energy acceptance in the energy recovery
line while eliminating large power flow between separated cavities. Efficient lasing was obtained
with a relatively low electron-beam peak current (250 A) through excellent slice emittances
(about 10 µm) and energy spread (about 0.05%). Tapering was used to increase the extracted power
and was limited by slippage in the wiggler. The dual-aperture spoke cavities introduced strong
transverse radio frequency (RF) fields, and careful tuning was necessary to avoid significant
transverse emittance growths. These cavities currently have a beam-breakup threshold of about
100 mA, which limited the output power in this theoretical study to <200 kW; importantly,
however, this design architecture is traceable to megawatt (MW) levels with engineering
improvements in the spoke cavity design. |
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SRF Injectors for MW-Class Free-Electron Lasers
B. E. Carlsten, F. L. Krawczyk, J. W. Lewellen,* Q. R. Marksteiner, and D. C. Nguyen, Accelerator Operation and Technology, Los Alamos National Laboratory Free-electron laser (FEL) design has evolved to the point where they are viable candidates
for MW-class shipboard self-defense laser systems, and point-to-theater air defense systems for
land bases. Los Alamos National Laboratory has developed a point design for such an FEL and the
accelerator required to drive it.1 Such accelerators require electron-beam sources capable of
generating good-quality beams with average currents on the order of an ampere. Depending upon
the overall design of the MW-class FEL, the beam source will also have specific requirements for
the peak current, energy spread, and so on, that it generates. This article describes one
approach to MW-class FEL injector design, based around the use of quarter-wave resonator
cavities for beam generation and acceleration, and a novel 3-cell buncher section to obtain
the required peak current. |
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Superconducting, Dual-Aperture Spoke Cavities for High-Efficiency Energy Recovery
F. Krawczyk, B. Carlsten* J. Lewellen, D. Nguyen,Accelerator Operation and Technology, Los Alamos National Laboratory, and B. Rusnak, Physics Division, Lawrence Livermore National Laboratory The use of superconducting radio frequency (SRF) technology and specifically SRF structures
with high mechanical stability and good beam quality control are crucial for battleground
megawatt (MW)-power free-electron lasers (FELs). Spoke resonators have been identified as the
most suitable technology; they are rugged, provide good gradient, and can integrate higher-order-mode
(HOM) damping in-line in each resonator. They also provide simplified cryogenic operation, as they can
operate at lower frequency and thus higher temperature, 4 K, than traditional elliptical SRF resonators.
This article covers a generic description of the performance and advantages of spoke resonators for MW-FEL
applications as compared to elliptical resonators. It then introduces a novel variant of spoke resonators
that preserves the operational advantages of standard spoke resonators, while extending the configuration
to dual-aperture geometries. This modification is a crucial technology step that, when mature, will enable
compact FEL systems to operate at full MW-power level. This article also includes an overview of the
development of auxiliary subsystems, including power couplers and HOM dampers. |
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High-Average-Current Normal-Conducting RF Photoinjector for a High-Power Free-Electron Laser Demonstration
K. Bishofberger,* H. L. Andrews, J. Bradley, S. M. Brandt, C. Buechler, M. Caffrey, B. E. Carlsten, L. D. Duffy, F. L. Krawczyk, S. S. Kurennoy, J. W. Lewellen, D. Lizon, J. Lovato, P. Marroquin, F. A. Martinez, N. A. Moody, D. C. Nguyen, R. M. Renneke, W. Roybal, R. Shurter, M. Tacetti, W. Tuzel, and R. Wheat, Accelerator Operation and Technology, Los Alamos National Laboratory The electron beam needed for megawatt (MW)-class free-electron lasers (FELs) requires a continuous wave (CW) photoinjector. While some technology existed based on high-duty factor normal-conducting radio frequency (NCRF) photoinjectors when the Navy began investigating MW-class FELs, there was barely any experience on using superconducting radio frequency (SRF) injector technologies. Additionally, preliminary studies indicated the feasibility of an NCRF injector for a 100-kW intermediate step. The Navy's decision to develop a high-average-current NCRF injector provided a challenging, but faster, track to high-power FEL technology demonstration than the utilization of the mostly unknown SRF injector approach. In this article, we report on the design, fabrication, and testing of a CW NCRF photoinjector. Its design
was a collaboration between Los Alamos National Laboratory (LANL) and Advanced Energy Systems (AES), and it
was installed and successfully operated at LANL. Two successful stages of operation met a range of performance
criteria. In the first stage (RF and thermal testing), all performance criteria were achieved, demonstrating
record performance of a high-gradient NCRF structure in CW mode. During the second stage (beam testing), the
expected accelerated beam energy and one of the highest bunch charges and average beam currents of any CW
injector to date were achieved. Limits in long-term performance and cathode lifetime were determined and
understood. Subsequent mitigation of these issues and higher operating performance were limited by early
termination of the experimental program. Importantly, an in situ conditioning approach was demonstrated
that adds a potential general increase in capability to long-life operation of alkalide cathodes. Based
on its performance, the NCRF injector was selected as the baseline injector technology for the 100-kW
Innovative Naval Prototype FEL demonstration project. |
Volume 7, Number 1, Journal of Directed Energy
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