Abstract. A brief review of the history of power compression over a range encompassing approximately 40 orders of magnitude places laser-nuclear interactions roughly at the logarithmic midpoint of the scale at approximately 10 20 W/cm 3 . The historical picture also motivates four conclusions, specifically, that (1) foreseen developments in power compression will enable laser-induced coupling to all nuclei, (2) conventional physical mechanisms will encounter a limit of Ωα ∼ 10 30 − 10 31 W/cm 3 , a value approximately 10 10 above the presently demonstrated capability, (3) the key to reaching the Ωα limit is the generation of relativistic/chargedisplacement self-trapped channels with multikilovolt X-rays in high-Z solids, a concept named "photon staging," and (4) penetration into the 10 30 − 10 40 W/cm 3 zone, the highest range known and the region represented by processes of elementary particle decay, will require an understanding of new physical processes that are presumably tied to phenomena at the Planck scale.
A nonlinear optical phenomenon, relativistic cross-phase modulation, is reported. A relativistically intense light beam ͑I = 1.3ϫ 10 18 W cm −2 , = 1.05 m͒ is experimentally observed to cause phase modulation of a lower intensity, copropagating light beam in a plasma. The latter beam is generated when the former undergoes the stimulated Raman forward scattering instability. The bandwidth of the Raman satellite is found to be broadened from 3.8-100 nm when the pump laser power is increased from 0.45-2.4 TW. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW. The experimental cross-phase modulated spectra compared well with theoretical calculations. Applications to generation of high-power single-cycle pulses are also discussed.
The detailed numerical and experimental studies of the stability of relativistic and ponderomotive selfchanneling [1] have demonstrated that unstable modes of propagation can be converted into ultra-powerful stable channels using a two-stage process of self-channeling, with the initial phase involving an appropriate gradient in the longitudinal electron density profile [2][3][4]. Recent calculations have revealed that two-stage multi-PW relativistic selfchanneling results in optimized power compression and the formation of stable multi-PW relativistic channels with the trapped power exceeding 104 critical powers and peak channel intensity in the 1023 W/cm2 range at 248 nm. At the present time, more than a decade after the first experimental observation [5], the relativistic and ponderomotive mechanism of self-channeling stands as the first rank method for the controlled power compression of high intensity laser pulses in plasmas. Major applications associated with these relativistic channels include (a) the generation of coherent x-rays, (b) particle acceleration, and (c) the fast ignition of fusion targets.Stable relativistic self-channeling of ultra-powerful laser pulses in plasmas is one of the key processes for recently developed ultra-bright multi-keV coherent x-ray source from gaseous cluster targets. The observed [6,7] strong (_106) enhancement and narrowing of certain Xe(L) lines in the X -2.71-2.93 A region indicate the strong amplification of these lines in stable relativistic channels. Our latest experimental results have provided an evidence of the tunability of the developed ultra-bright multi-keV coherent x-ray source.Current experimental studies of the 4.5 keV x-ray source are focused on the properties of single-pulse Xe(L) spectra recorded with a von Haimos spectrometer equipped with a mica crystal and a linear CCD array. Observations [7] of single-pulse Xe(L) spectra under conditions of strong amplification in the channel have indicated the presence of spectral hole-buming on the Xe34+ and Xe35+ 3d-+2p transition arrays in the k -2.84-2.88 A range.The developed x-ray source has the properties necessary to be a key component of a new form of biological microimaging technology applicable to high spatial resolution studies of living matter. References 1. A.
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