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Scientific Assessment of High-Power Free-Electron Laser Technology 2 State of the Art A BRIEF HISTORY OF THE FREE-ELECTRON LASER FOR NAVY APPLICATIONS Although others had previously conceived of similar devices,1 the history of the free-electron laser (FEL) for Navy applications begins in 1972, with the conception (and name) of the free-electron laser by John Madey. In 1976, he and a team of early-career physicists at Stanford experimentally demonstrated gain at 10.6 micrometers (μm) using a CO2 laser probe. A short time later they succeeded in achieving oscillation at 3 μm.2 Serious military interest in FELs began in 1978, when the Defense Advanced Research Projects Agency (DARPA) concluded that no other high-power laser could achieve the optical beam quality necessary to focus the beam on a distant (thousands of kilometers) target. The responses to the call for proposals included a conceptual design from Los Alamos for a 10 MW FEL. That design included energy recovery and a very long optical resonator to reduce the irradiance on the mirrors. Although there have been important technological advances since then, especially in superconducting accelerators and injectors, and we would now use one linear accelerator (linac) for both the acceleration and the energy recovery, the design bears a strong resemblance to the designs considered today; considerable experience has been obtained in the intervening 30 years. The Los Alamos National Laboratory (LANL) design also identified the critical problems in injectors and mirrors. These challenges remain today. During the late 1970s and 1980s, the Navy developed the technology for high-energy laser (HEL) weapons systems based on scaling deuterium fluoride (DF) gas lasers to the megawatt class. These devices produced radiation distributed over a series of lines from 3.6 μm to 3.9 μm. At low power, these lines would transmit through the sea-level maritime environment fairly efficiently with relatively low total extinction. Unfortunately, at high power the molecular absorption component of the atmospheric extinction was determined to cause an unacceptably high level of thermal blooming to be useful for self-defense. (Thermal blooming results from a small amount of heating due to atmospheric absorption in the middle of the laser wavefront, causing beam spreading.) In the early 1990s, a search for improved (low absorption and low-to-moderate scattering) wavelengths was initiated. Using HI-TRAN modeling and experimental measurements, three wavelength regions were found that were far better than the 3.6 μm to 3.9 μm band and appeared to be adequate for megawatt propagation. These fairly narrow spectral bands were near 1.045 μm, 1.6 μm, and 2.2 μm. Unfortunately, there were no obvious lasers that held the promise of scaling to the megawatt level at those wavelengths. For this reason and the Navy’s desire for electric (nonchemical) lasers, FELs were selected to explore their scalability to the megawatt level. In 1983, the Strategic Defense Initiative (SDI) began, and tremendous progress was made in high-power FELs. In particular, the radio-frequency (RF) photoelectric injector was invented at LANL and substantial advances were
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Scientific Assessment of High-Power Free-Electron Laser Technology made in optics, both in the FEL program and in other (especially chemical) laser programs. Unfortunately, when the SDI program ended in the early 1990s, much of the progress in optics for FELs was lost. Industrial experience with high-power coatings atrophied, and understanding of relevant optical architecture was lost. On the positive side, injector development continued for other FEL programs, and substantial progress has been made in superconducting accelerators, so that these are now the preferred technology. Thus the Navy proposal to construct a high-power FEL is based on a long history of progress. This history includes both significant successes, such as the 14 kW continuous-wave (cw) FEL at the Thomas Jefferson National Accelerator Facility, known as the Jefferson Laboratory, or JLab, and enduring challenges in injectors and optics. Table 2.1 lists demonstrated relativistic FELs in 2008. A location or institution, followed by the FEL’s name in parentheses, identifies each FEL. (In the location/name column, KAERI is the Korea Atomic Energy Research Institute, Nihon refers to Nihon University in Japan, RIKEN is a natural sciences research institute in Japan, and DESY is the German Electron-Synchrotron Research Center.) The first column following the FEL name lists the operating wavelength, λ, or the wavelength range. The longer wavelengths are listed at the top with short x-ray wavelength FELs at the bottom of the table. The large range of operating wavelengths, seven orders of magnitude, indicates the flexible design characteristics of the FEL mechanism. In the next column, σz is the electron pulse length divided by the speed of light, c, and ranges from 25 ns to short subpicosecond pulse timescales. The expected optical pulse length in an FEL oscillator can be three to five times shorter or longer than the electron pulse depending on the optical cavity Q, the FEL desynchronism, and the FEL gain. The optical pulse can be up to 10 times shorter in the high-gain FEL amplifier. Also, if the FEL is in an electron storage ring, the optical pulse is typically much shorter than the electron pulse. Most FEL oscillators produce an optical spectrum that is Fourier transform limited by the optical pulse length. The electron beam energy, E, and peak current, I, are listed in the third and fourth columns, respectively. The next three columns list the number of undulator periods, N, the undulator wavelength, λ0, and the root mean square (rms) undulator parameter, K = eBλ0/2πmc2 (cgs units), where e is the electron charge magnitude, B is the rms undulator field strength, and m is the electron mass. For an FEL klystron undulator, there are two undulator sections as listed in the N column; for example, 2 × 33. The FEL klystron configuration uses two undulators separated by a drift space or dispersive section in order to increase the FEL gain in weak optical fields, but at the expense of extraction in strong optical fields. Some undulators used for harmonic generation have multiple sections with varying N, λ0, and K values as shown. Most undulators are configured to have linear polarization. Some FELs operate at a range of wavelengths by varying the undulator magnetic field, as indicated in the table by a range of values for K. The FEL resonance condition, λ = λ0(1 + K2)/2γ2, provides a relationship that can be used to relate the fundamental wavelength, λ, to K, λ0, and E = (γ − 1)mc2, where γ is the relativistic Lorentz factor. Some FELs achieve shorter wavelengths by using harmonics. The last column in Table 2.1 lists the accelerator types and FEL types, using the abbreviations defined at the bottom of the table. For the conventional oscillator, the peak optical power can be estimated by the fraction of the electron beam peak power that spans the undulator spectral bandwidth, 1/(2N), or P ≈ EI/(8eN). For the FEL using a storage ring, the optical power causing saturation is substantially less than this estimate and depends on ring properties. For the high-gain FEL amplifier, the optical power at saturation can be substantially greater than 1/(2N). The average FEL power is determined by the duty cycle, or spacing between the electron micropulses, and is typically many orders of magnitude lower than the peak power. The infrared FEL at the Jefferson Laboratory has now reached an average power of 14 kW with the recovery of the electron beam energy in superconducting accelerator cavities. In the FEL oscillator, the optical mode that best couples to the electron beam in an undulator of length L = Nλ0 has a Rayleigh length z0 ≈ L/121/2 and a mode waist radius of w0 ≈ N1/2γλ/π. The FEL optical mode typically has more than 90 percent of the power in the fundamental mode described by these parameters. In 2008, the DESY FLASH FEL reached the shortest wavelength ever for an FEL, λ ≈ 6.5 nm. There was one other new lasing at Kyoto (KU-FEL) at λ ≈ 11-14 μm. Countries worldwide participate in FEL development as a tool for scientific research. More than 10 countries from Europe, North America, and Asia are represented, with more than half of the FELs located in the United States and Japan.
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Scientific Assessment of High-Power Free-Electron Laser Technology TABLE 2.1 Relativistic Free-Electron Lasers in 2008 Location (Name) λ(μm) σz (ps) E (MeV) I (A) N λ0 (cm) K (rms) Type Frascati (FEL-CAT) 760 15-20 1.8 5 16 2.5 0.75 RF,O UCSB (mm FEL) 340 25,000 6 2 42 7.1 0.7 EA,O Novosibirsk (RTM) 120-230 70 12 10 2 × 33 12 0.71 ERL,O KAERI (FIR FEL) 97-1,200 25 4.3-6.5 0.5 80 2.5 1.0-1.6 MA,O Osaka (ISIR,SASE) 70-220 20-30 11 1,000 32 6 1.5 RF,S Himeji (LEENA) 65-75 10 5.4 10 50 1.6 0.5 RF,O UCSB (FIR FEL) 60 25,000 6 2 150 2 0.1 EA,O Osaka (ILE/ILT) 47 3 8 50 50 2 0.5 RF,O Osaka (ISIR) 32-150 20-30 13-19 50 32 6 1.5 RF,O Tokai (JAEA-FEL) 22 2.5-5 17 200 52 3.3 0.7 RF,O Bruyeres (ELSA) 20 30 18 100 30 3 0.8 RF,O Dresden (U-100) 18-230 5-20 20-40 25 38 10 2.8 RF,O Osaka (FELI4) 18-40 10 33 40 30 8 1.3-1.7 RF,O LANL (RAFEL) 15.5 15 17 300 200 2 0.9 RF,O Kyoto (KU-FEL) 11-14 2.0 25 17 40 4 0.99 RF,O Darmstadt (FEL) 6-8 2 25-50 2.7 80 3.2 1 RF,O Osaka (iFEL1) 5.5 10 33.2 42 58 3.4 1 RF,O BNL (HGHG) 5.3 6 40 120 60 3.3 1.44 RF,A Beijing (BFEL) 5-20 4 30 15-20 50 3 1 RF,O Dresden (ELBE) 4-22 1-10 34-16 30 2 × 34 2.73 0.3-0.7 RF,O,Kl Tokyo (KHI-FEL) 4-16 2 32-40 30 43 3.2 0.7-1.8 RF,O Nieuwegein (FELIX) 3-250 1 50 50 38 6.5 1.8 RF,O Orsay (CLIO) 3-53 0.1-3 21-50 80 38 5 1.4 RF,O KAERI (HP FEL) 3-20 10-20 20-40 30 2 × 30 3.5 0.5-0.8 RF,O,Kl Osaka (iFEL2) 1.88 10 68 42 78 3.8 1 RF,O Nihon (LEBRA) 0.9-6.5 <1 58-100 10-20 50 4.8 0.7-1.4 RF,O UCLA-BNL (VISA) 0.8 0.5 64-72 250 220 1.8 1.2 RF,S JLab (IR upgrade) 0.7-10 0.15 120 400 30 5.5 3 ERL,O BNL (ATF) 0.6 6 50 100 70 0.88 0.4 RF,O Duke (OK-5) 0.45 0.1-10 270-800 35 2 × 32 12 0-4.75 SR,O,Kl Dortmund (FELICITAI) 0.42 50 450 90 17 25 2 SR,O Osaka (iFEL3) 0.3-0.7 5 155 60 67 4 1.4 RF,O Orsay (Super-ACO) 0.3-0.6 15 800 0.1 2 × 10 13 4.5 SR,O,Kl BNL (SDL FEL) 0.2-1.0 0.5-1 100-250 300-400 256 3.9 0.8 RF,A,S,H Okazaki (UVSOR) 0.2-0.6 6 607 10 2 × 9 11 2 SR,O,Kl Tsukuba (NIJI-IV) 0.2-0.6 14 310 10 2 × 42 7.2 2 SR,O,Kl Trieste (ELETTRA) 0.2-0.4 28 1,000 150 2 × 19 10 4.2 SR,O,Kl Duke (OK-4) 0.193-2.1 0.1-10 1,200 35 2 × 33 10 0-4.75 SR,O,Kl RIKEN (SCSS Prototype) 0.03-0.06 1 250 300 600 1.5 0.3-1.5 RF,S DESY (FLASH) 0.0065 0.025 1,000 2,000 984 2.73 0.81 RF,S NOTE: λ, optical wavelength; σz, pulse length; E, beam energy; I, beam peak current; N, number of undulator periods; λo, undulator period; K, undulator parameter; RF, radio-frequency linac; EA, electrostatic accelerator; ERL, energy recovery linac; MA, microtron accelerator; SR, electron storage ring; A, FEL amplifier; O, FEL oscillator; Kl, FEL klystron; S, self-amplified spontaneous emission (SASE) FEL; H, high-gain harmonic generation (HGHG) FEL. SOURCE: W.B. Colson, J. Blau, J.W. Lewellen, B. Wilder, and R. Edmonson, “Free Electron Lasers in 2008,” Proceedings of the 30th International FEL Conference, Gyeongju, Korea, in press, Table 1. Available at www.JACoW.org.
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Scientific Assessment of High-Power Free-Electron Laser Technology FREE-ELECTRON LASER DESCRIPTIONS The FEL schematic diagram in Figure 2.1 shows an energy recovery linac (ERL)-based FEL amplifier or oscillator. The electron beam path, shown in red, is in a vacuum pipe. At the beginning of the path, a cathode drive laser excites a sequence of electron pulses from the cathode surface into the electron gun and booster, acting as the injector system. The electron pulses leave the injector with energy Ei and move into the merge, where they enter the superconducting linear accelerator (linac). The electron pulses entering at RF phases suitable for acceleration reach an average energy E0 at the end of the accelerator before entering the 180° bend. The electron pulses then are directed into one of the undulators, where a small percentage (ΔE/E0) of their energy is converted into light. In the case of the FEL oscillator, the optical pulses are bouncing between the cavity mirrors of an open optical resonator. Care must be taken to synchronize the sequence of electron pulses triggered by the cathode drive laser into the correct phase of the RF cycles and to overlap with the stored optical pulses at the entrance to an undulator. In the case of the FEL amplifier, there is no optical resonator; a seed laser sends optical pulses synchronized to overlap the electron pulses as they enter the undulator. After the undulator, the electron beam continues at reduced average energy (E0 – ΔE) around a second 180 degree bend to the merge, where the electron pulses re-enter the linac. Here they are interleaved with the accelerating electrons, but at RF phases that reduce their energy. Their kinetic energy is converted (recovered) to RF energy, substantially reducing the external drive power required by the linac RF cavities and also reducing the ionizing-radiation shielding required. After deceleration, the low-energy electron beam is separated from the path of the high-energy beam and directed into the beam dump at energy Ed. Not shown are various bending and focusing magnets along the electron beam path. FIGURE 2.1 Representative schematic diagram of a free-electron laser (FEL) energy recovery linac (ERL) system illustrating both major genres of FELs (oscillators and amplifiers). This illustration captures the major elements of the FEL ERL and is not to scale. An FEL ERL need not operate in both genres. SOURCE: W.B. Colson, Naval Postgraduate School.
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Scientific Assessment of High-Power Free-Electron Laser Technology HIGH-ENERGY LASER TRADE-OFFS Trade-offs between competing systems concepts and technologies form the basis for decision making. Ultimately, if HEL weapons are to be deployed, they must show a competitive advantage over other system concepts, such as the well-developed defense-in-depth missile and gun systems currently deployed by the Navy. A trade-off analysis with missile systems is not within the scope of this study, but it is worth mentioning the threat scenario in which a missile-based defense has limitations and HEL weapons have potential strengths. In particular, these emerging antiship threats include sea-skimming missiles with very low signatures, high supersonic maneuvering sea-skimming missiles, and antiship ballistic missiles. In each of these cases, the time lines for reaction from threat detection to ship impact are very short, and in the second two threat cases, defensive missile agility (pulling enough g’s) is severely stressed. The HEL speed-of-light delivery of energy and the ability to slew a beam to track a target at high rates is a potential counter to these advanced threats. In addition, the advanced optical systems in HEL devices could highly augment conventional detection and tracking systems for low-signature targets. This section discusses trade-offs between different HEL concepts (all of which share these advantages) rather than between HEL and alternative weapon systems. Trade-offs within FELs form the basis for the FEL technical assessment in Chapter 3. The following list, based on information provided by the Navy and the knowledge base of the committee, forms the decision space for Navy HEL trade-off analysis: Potential to scale to megawatt power levels with multisecond continuous operation; Ability to provide optimized wavelength for propagation in the marine layer (modeling and experimental data point to wavelengths of 1.045 μm, 1.62 μm, and 2.2 μm); Beam quality to maximize energy on target; Size and complexity of entire HEL system, Energetic chemicals vs. electric power, Complexity of optical system, Requirements for cooling, cryogenics, and vacuum, Sensitivity to shock and vibration, Need for ionizing-radiation shielding and other safety factors; and Technology maturity. Three different classes of HEL are evaluated accordingly: chemical lasers, slab and fiber solid-state lasers, and free-electron lasers. Chemical lasers. These have already been scaled to the megawatt level with adequate beam quality and operational optical trains. The DF laser operates at a wavelength over a series of lines from 3.6 μm to 3.9 μm, while the chemical oxygen iodine laser (COIL) operates at 1.315 μm. The showstopper for chemical lasers for naval applications is propagation in the marine boundary layer, where even modest absorption (principally by aerosols) leads to thermal blooming that reduces the energy on the target to subcritical levels for head-on engagements no matter how much energy is provided at the laser output aperture. No chemical laser systems have been identified to operate at the wavelengths mentioned above for optimized propagation in the marine layer. Solid-state lasers (SSLs). These are at a similar state of technical maturity as FELs in terms of scale-up but have distinct attributes and technology issues. The principal potential advantage of SSLs over FELs is in overall system size and complexity for moderately high power. While the device does require cooling (the major issue in scale-up), it does not require cryogenics or a hard vacuum; furthermore, SSLs should be relatively insensitive to shock and vibration, as well as being compact compared to FELs. The SSL devices are electrically pumped; however, there is not a significant ionizing-radiation hazard. A principal obstacle to scale-up to megawatt power levels is the removal of waste heat from the solid-state gain medium. As the gain medium heats, optical quality is lost as the medium distorts, and eventually heating will kill the gain.
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Scientific Assessment of High-Power Free-Electron Laser Technology Another obstacle to scale-up to very high powers for the SSL comes from the limitations in combining compensated and phased amplifier chains from multiple slabs or fibers. At power levels above ~100 kW, the projected size and complexity of SSLs begin to exceed those of the projected FEL. Solid-state (slab or fiber) lasers can approach some of the requirements for propagation at 1.045 μm, depending on the SSL gain medium, but not easily at 1.62 μm or 2.2 μm. For naval applications the SSL does not appear to be an attractive alternative for megawatt applications, but it may be an attractive alternative for requirements below 100 kW. Free-electron lasers. These have several natural advantages for high-power weapon applications. At least in an oscillator configuration, the small Fresnel number of the resonator assures that the optical beam quality will be good. Experience has proved that this is true in amplifiers as well as in oscillators. In addition, the high speed of the gain medium (the electron beam) assures that the waste heat is rapidly removed from the optical system. Called the “garbage-disposal” principle, this is the principal restriction on the power of solid-state lasers. Finally, by their nature, FELs are wavelength tunable by simply adjusting the electron energy or the undulator field strength via a gap change. Although this tunability is restricted by optical coatings, it is always true that the wavelength is at least selectable for the application (a marine atmosphere, in the Navy application) when the FEL is designed. FELs also have significant scale-up issues, which are described in detail in Chapter 3. Not described in this report but a significant hurdle for the FEL is the assumed overall system size and complexity. The system will require significant isolation from ship-induced shock and vibration, a hard vacuum, and cryogenic cooling. Moreover, the dumping of high-energy electrons will require ionizing-radiation shielding. This trade-off discussion on laser alternatives to reach megawatt power levels at wavelengths of interest to the Navy supports the conclusion that the FEL has clear and significant advantages over other types of lasers to meet these laser device-level requirements. It does not, of course, address whether the FEL will meet system-level requirements for a weapon system or provide trade-offs with kinetic energy weapons systems. RELATION TO SCIENTIFIC FREE-ELECTRON LASERS The scientific opportunities presented by free-electron lasers and other advanced coherent light sources were studied in a 1994 report by the Committee on Free Electron Lasers and Other Advanced Coherent Light Sources, organized by the Board on Chemical Sciences and Technology and the Board on Physics and Astronomy of the National Research Council (NRC) with the support of the Department of Energy and the Office of Naval Research.3 Due to cost and benefit considerations, the report was organized according to spectral regions, but it was recognized that the same physical principles govern the design of free-electron lasers in all wavelength regions. The most compelling scientific case for a free-electron laser facility was found to be in the far infrared, the region between 1,000 and 10 μm. The research advantage of the FEL in this context is its wide tunability and its flexible pulse structures, with the possibility of using chirped pulses being a capability that is unavailable with conventional lasers in this wavelength region. In addition, the report pointed out that research and development aimed at improving FELs in a specific wavelength region may be important to the improvement of FELs in all wavelength regions. Since that NRC report in 1994, significant advances in the development of FELs have been propelled by scientific utilizations of next-generation light sources capable of producing coherent photons continuously tunable from the terahertz (THz) to the hard x-ray regimes. In particular, the committee notes the emergence of the x-ray FEL and its connection with advances made in the emittance of nanocoulomb charge beams from RF electron guns, the development of superconducting RF guns, and the development of energy recovery linacs, advanced by the synchrotron radiation community and electron cooling technology. Crucial to the realization of turnkey, high-average-power FELs in the wavelength region of interest for Navy applications is the synergy between the advances in hardware and software simulations that have occurred during the past decade. The synchrotron radiation sources of the past and present can be defined as follows. First-generation machines are electron synchrotrons and storage rings that were built for other purposes—for example, for high-energy and nuclear physics—but their bending magnet radiation was parasitically used by synchrotron radiation users. This
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Scientific Assessment of High-Power Free-Electron Laser Technology radiation covered many wavelength regimes due to the nature of the bending magnet emission. In addition, the machines produced rather large photon source sizes as the electron beam emittance was large and not intended for (or ideal for) synchrotron radiation applications. Second-generation machines are dedicated machines for synchrotron radiation users and employ bending magnets as the primary source of synchrotron radiation. The beam emittances were designed by the machine architects to be smaller in order to provide users with a smaller source size and greater brilliance. Third-generation machines are dedicated for synchrotron radiation users and were designed to accommodate many so-called insertion device magnets, such as undulator and wiggler magnets. Undulator magnets generate narrow spectral lines, which enhances the overall photon brilliance. Next-generation light sources involve an optical gain mechanism, with the goal of transverse and longitudinal optical coherence such as in an FEL. NOTES 1. A.J. Balkcum, D.B. McDermott, R.M. Phillips, and N.C. Luhmann, “High-Power Coaxial Ubitron Oscillator: Theory and Design,” IEEE Transactions on Plasma Science 26: 548-555 (1998). 2. D.A.G. Deacon, L.R. Elias, J.M.J. Madey, G.J. Ramian, H.A. Schwettmann, and T.I. Smith, “First Operation of a Free-Electron Laser,” Physics Review Letters 38: 892-894 (1977). 3. National Research Council, Free Electron Lasers and Other Advanced Sources of Light: Scientific Research Opportunities (Washington, D.C.: National Academy Press, 1994).