Plasma Physics Reports, Vol. 29, No. 3, 2003, pp. 207–210. From Fizika Plazmy, Vol. 29, No. 3, 2003, pp. 231–235. Original English Text Copyright © 2003 by Tajima. NEW TRENDS IN PLASMA PHYSICS Fundamental Physics with an X-ray Free Electron Laser1 T. Tajima Advanced Photon Research Center, JAERI, Kizu, Kyoto, 619-0215 Japan Received August 19, 2002 Abstract—Of late, laboratories around the world are considering building X-ray free electron lasers based on high energy electron accelerators (with energies exceeding 10 GeV) to produce bright coherent X rays with wavelengths on the order of 1 Å. Because of the extremely small wavelength and high brilliance of these coherent X rays, there is an unprecedented opportunity to explore new applications of what is sometimes called the fourth generation of light sources. Here, we point out that in addition to the anticipated applications to material science and biology, a number of applications to fundamental physics may become possible in the fields of extreme high-energy accelerating gradients, ultrahot matter creation, coherent γ-ray generation, violent acceleration and “horizon physics,” and nonlinear field theory (quantum electrodynamics). Intensive development in technical areas, particularly that of X-ray optics, will be needed, however, in order to achieve ultrahigh intensity X-rays that can allow these applications. © 2003 MAIK “Nauka/Interperiodica”. 1 1. X-RAY FREE-ELECTRON LASER The rapid growth in the demand for synchrotron radiation and the successful employment of wiggler devices in light sources have allowed the development stage often called the “third generation” of acceleratorbased light source facilities to be reached, following the first- and second-generation sources that used bending magnets with electron bunches from storage rings. Third-generation X-ray sources are capable of generating radiation in the 1- to 10-nm wavelength range. On the drawing board for X-ray free-electron lasers (FEL) driven by a high energy electron accelerator, however, are devices that will deliver even shorter wavelength X-rays with much higher brightness, often called the fourth-generation sources. These include the plan for the Linac Coherent Light Source (LCLS), being designed by the Stanford Linear Accelerator Center, the Lawrence Livermore National Laboratory, and several other collaborating institutions, as well as the DESY plan of Germany. For example, the LCLS calls for a wavelength of 1.5 Å, pulse duration of 230 fs, peak power of 9 GW, and peak brightness of 1033 photons/(s mm2 mrad2) in a bandwidth of 0.1%. Clearly, these impressive parameters are orders of magnitude higher in the energy and brightness of X rays than those of existing facilities [1]. As impressive as these are, Chen and Pellegrini [2] have ventured the opinion that an even higher performance might be theoretically possible by increasing the FEL energy extraction (by the tapered undulator technique) and a yet unspecified method of diffraction-limited X-ray focusing. Chen and Pellegrini [2] suggested that X-ray laser intensities of 1024–1028 W/cm2 are theoretically possible. At present, available technology does not allow such fantastic intensities, mostly because the 1 This article was submitted by the author in English. current X-ray optics do not allow diffraction-limited focusing. With some development effort, intensities of 1017 W/cm2 can be reached [3] and, with some major effort, intensities of 1023 W/cm2 might be possible [3, 4]. This prospect, however remote it might be from becoming reality, would represent a major demarcation from the previous generations of light sources. The reason is that, in addition to applications to material sciences and life sciences, an X-ray FEL could be employed to probe some of the questions that touch on fundamental physics issues. These are high-field science applications of the X-ray FEL, taking advantage of not only the high energy and coherency of photons, but also their extreme high fields, which interact with matter in a unique way. In this regard, there has been recent theoretical suggestion of creating intense incoherent petawatt X rays by irradiating intense petawatt laser on a metal [5]. If and when the intensity of FEL X-rays can be increased as some of the above scenarios indicate, there will be unprecedented opportunities to use these intense X-rays in order to explore some of the issues of fundamental physics that have eluded man’s grasp so far. In the present article, we list a few examples of these applications. If optical laser physics is any guide, remarkable new phenomena emerge when the laser intensity approaches the relativistic regime. In the optical wavelength range, the so-called “high-field science” regime begins to occur at intensities of 1014– 1015 W/cm2, where some new atomic physics and plasma physics phenomena have been observed. At around 1018 W/cm2, relativistic effects fully enter into the dynamics. Since the frequency of the X-ray FEL of LCLS is four orders higher than that of an optical laser, the corresponding intensities would be 1022– 1023 W/cm2 and 1026 W/cm2, respectively. These inten- 1063-780X/03/2903-0207$24.00 © 2003 MAIK “Nauka/Interperiodica” 208 TAJIMA sities are very high as compared with the prospective high intensity anticipated at LCLS, namely, 1017 W/cm2. We point out that even though progress to achieve such a lofty goal is rather slow and laborious, the rewards that may be derived in this unique regime are so extraordinary that looking into LCLS’s extension of X-ray FEL to this regime merits serious consideration. 2. EXTREME HIGH-ENERGY ACCELERATING GRADIENT For the X rays of LCLS, the X-ray frequency is so high that even the density of metallic electrons is subcritical. As has been pointed out in [6, 7], short X-ray bursts in a metal are capable of exciting a wake field inside the metallic electrons. The wake field may be accentuated by the seeding technique [8]. The wake field strength is proportional to the square root of the electronic density: 1/2 2 E = ( n/n 18 ) a 0 GeV/cm, (1) where n18 is the density in units of 1018 cm–3, a0 is the normalized vector potential eE0 /mω0c of the X-ray FEL, E0 and ω0 are the electric field and frequency of the X-ray laser, e is the electron charge, and m is the electron rest mass. When the laser becomes relativistic, a0 approaches unity. If the electron density of a metal is n = 1024 cm–3, the wake field electric field amounts to 2 E = a 0 TeV/cm. (2) Let us consider one example of how we can take advantage of this tremendous accelerating gradient induced by LCLS X rays. By creating a nanohole in a metal, the electron density in the nanohole is slightly below the surrounding density. If the metal is chosen such that the electron density is 1023 cm–3, then the dephasing length of an electron with the X-ray wake field is on the order of 10 cm, because the plasma wavelength is on the order of 100 nm. If X rays are focused into the nanohole and kept focused either by the self-focusing mechanism or by the geometrical optics of the nanohole itself, then, over the length of a metallic slab of thickness d, the energy gain by this X-ray wake field through the slab nanohole is deE. If we choose the slab thickness to be 2 10 cm, the electron energy gain can be as great as 3 a 0 TeV. If the laser is relativistic, the energy gain is 3 TeV through a 10-cm slab. One may be able to study beam dynamics under channeling conditions (and dechanneling conditions) [9]. For example, Huang et al. [10] suggested the possibility of beam cooling due to radiation channeling. In addition, the longitudinal component of intense X-ray fields can itself be employed as the accelerating gradient [11] if the crystal is appropriately designed with the use of emerging nanotechnology. 3. ULTRAHOT MATTER The irradiation of relativistic X-rays on a metallic target should exhibit a remarkable phenomenon. As is well known, hard X rays, such as those from the X-ray LCLS, will penetrate any material over a long distance. However, when the X-ray intensity is raised to the extent of becoming relativistic, the absorption of X rays suddenly becomes very efficient. This phenomenon was discovered in the laser–cluster interaction [12–15]. The effective cluster radius for laser absorption is 10 nm–1 µm. We now apply this knowledge from relativistic laser–cluster interaction to the interaction of a relativistic X-ray laser with atoms. For an optical laser interacting with nano- and microclusters, the physics of strong interaction between the electromagnetic (EM) waves and clusters was investigated in [16]. When the applied EM field is sufficiently strong [17], the electron wave function is skewed in the tilted nuclear Coulomb potential and the electron can tunnel through the potential barrier and become ionized. This is Coulomb-barrier suppression ionization. This process creates ionized itinerant electrons instantly (within a femtosecond). These free electrons create a large polarization field, because ions in the cluster are immobile over this short time scale. Due to the combined effects of the large cluster polarization and the strong laser EM fields, the orbits of the itinerant electrons become chaotic (orbital stochasticity) [16]. This stochasticity of the electrons that carry the bulk of the laser energy implies the instantaneous absorption of the laser energy. Particle-in-cell simulations show this process and indicate extraordinarily high laser absorption by clusters [16]. With a four orders of magnitude reduction in the scales, the cluster size of 1 µm transforms into 1 Å, the size of an atom. The size of the inner-shell electron orbitals is a fraction of this. Thus, metallic atoms become very efficient absorbers of X rays, just as clusters are efficient absorbers of optical lasers. Typically, relativistic laser light is absorbed by several layers of clusters. Thus, we expect that relativistic X rays will be absorbed by several atomic layers. The range of highenergy X rays in a metal is quite long (a matter of fractions of mm). However, we expect the range to be a 2 strong function of the X-ray intensity, a 0 . As a0 increases and approaches unity, the range rapidly decreases toward nm. The typical absorption rate by several layers exceeds 50%. If 1 J of X-ray energy is suddenly absorbed by a 1-µm2 square area of thickness 1 nm, this amounts to a typical energy deposition of a couple of GeV per particle (or nucleon). The physics of this regime of X-ray laser–matter interaction may be called a driven quantum liquid. The matter so created is extraordinarily hot. Such matter not only generates copious positrons, but perhaps gives rise to exotic matter such as quark–gluon plasma. PLASMA PHYSICS REPORTS Vol. 29 No. 3 2003 FUNDAMENTAL PHYSICS WITH AN X-RAY FREE ELECTRON LASER 4. GAMMA RAYS Similar irradiation of strong X-rays on a metal target should lead to the copious emission of γ rays. The energy conversion rate from X rays to γ rays is very high, probably in excess of 50%. The X-ray laserdriven quantum liquid, as we mentioned above, gives rise to temporary coherent periodic bremsstrahlung emission of γ photons. With a clever manipulation of the target and the laser frequency, it may be possible to change these γ rays into coherent γ rays. Since X rays are so intense and their conversion efficiency so high, one would expect extremely brilliant γ rays. These γ rays may be utilized to investigate various nuclear physics issues. These issues may include (i) the less trodden paths of (γ, n) and (γ, p) processes (rather than neutron capture processes) [18], (ii) the excitation of giant resonances of nuclei, (iii) excitation of isomers such as Hf and Ta (and their relevance to the cosmic synthesis of heavier elements) [19–21], (iv) the measurement of cross sections of photonuclear reactions [22], and (v) the possible detection of parity nonconservation [17, 23]. 5. NONLINEAR QUANTUM ELECTRODYNAMICS AND THE RECREATION OF ASTROPHYSICAL CONDITIONS With a sufficiently strong X-ray laser, the electric field of the laser begins to polarize the vacuum, causing the vacuum to “split.” This causes spontaneous generation of copious electron positron pairs. Nonlinear field theory may then be studied. An X-ray FEL in the relativistic regime is able to access astrophysical conditions that, so far, are only dreamt of, such as conditions not so far removed from those at the site of a gamma-ray burst (GRB). The above techniques can give rise to the X-ray irradiance so high that it resembles that of a GRB. In the case of a typical GRB, it is believed that γ rays (about 100-keV energy) have a flux of 1030 W/cm2. This intensity corresponds to the field strength of the Schwinger field, ES = mc2/ λC, where λC is the Compton wavelength h/mc. At or near the Schwinger field, the fluctuating vacuum polarization may be ripped open, so that the copious production of electron–positron pairs occurs. The fact that the most intense astronomical emitter of energy, GRB, has this field intensity is therefore not a coincidence. Also, γ rays are just about relativistic at this energy, yielding an a0 of about unity for 100-keV γ rays. The intensity of FEL X rays (10 keV for LCLS), if relativistic, also has an a0 of on the order of unity. Thus, the nonlinearity of photons in a GRB and that of FEL X rays are in the same ballpark. There are other astrophysical extreme conditions that may be recreated by X-ray FELs, such as LCLS. See more examples in [24]. PLASMA PHYSICS REPORTS Vol. 29 No. 3 2003 209 6. VIOLENT ACCELERATION, GRAVITY, AND HORIZON PHYSICS The acceleration by the electric field of a relativistic laser is so enormous that an electron may be accelerated sideways with a gradient similar to that near the surface of a black hole horizon. As was demonstrated in [25], proper setup of a standing X-ray laser will cause such a large acceleration that the electron in the accelerated frame of reference feels a tremendous equivalent gravity. This causes the horizon of the electron to emerge at a finite distance. This is the fundamental reason why Unruh radiation arises, which is a sister phenomenon to Hawking radiation from a black hole horizon. Perhaps, we could peek into the physics at a horizon, just like that at a black hole horizon. Some of the interesting questions to ask include the following: (i) Can we confirm the predicted existence of Unruh radiation? (ii) What is the spectrum of Unruh radiation? Is it blackbody radiation or that of Chen–Tajima [25]? Is there a deficit of radiation due to new physics such as the leakage into extra dimensions [26]? Because a violently accelerated electron sees the horizon shrink to a finite distance in the direction opposite to that of the acceleration, the gravitational (or equivalent accelerating) field within the distance to the horizon may obey a law different from that of our daily Gauss’s law. This different law may be manifested in the spectrum of Unruh radiation. 7. CONCLUSION In conclusion, we have speculated about various possibilities for applying the upcoming X-ray FELs to high-field science. If and when the technology of X-ray optics and other associated X-ray FEL systems is advanced to make the intensity of X rays to become nearly relativistic, there will be a new class of applications that can investigate fundamental physics issues from a novel perspective. In addition, a similar deployment of undulators for a future high energy electron accelerator such as an NLC (Next Linear Collider) can stretch the X-ray (or γ-ray) energy up to ~100 keV. A technique for generating ultrashort electron bunches will facilitate such a deployment in the future. Although we have focused on high-energy X-ray FELs in the regime of keV (or beyond), most of the applications mentioned here also are relevant for FELs with photon energies in the 10- to 100-eV range (with the appropriate parameters adjusted accordingly). ACKNOWLEDGMENTS This paper was originally presented as a plenary talk at the Mini-Workshop on High-Field, High-Intensity Physics with LCLS organized by C. Pellegrini and P. Chen (SLAC, Stanford, CA, 2000). Since then, papers [27–29] have appeared on similar subjects. Discussions with Drs. P. Chen, A. Toor, R. Tatchyn, R. Ruth, and C. Pellegrini are appreciated. 210 TAJIMA REFERENCES 1. 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