Backward Raman compression of x-rays in metals and warm dense matters

Abstract

Experimentally observed decay rate of the long wavelength Langmuir wave in metals and dense plasmas is orders of magnitude larger than the prediction of the prevalent Landau damping theory. The discrepancy is explored, and the existence of a regime where the forward Raman scattering is stable and the backward Raman scattering is unstable is examined. The amplification of an x-ray pulse in this regime, via the backward Raman compression, is computationally demonstrated, and the optimal pulse duration and intensity is estimated.PACS numbers: 52.38.-r, 52.59. Ye, 42.60.Jf Coherent intense x-rays would enable various potential applications [1,2], however, it is very difficult to compress an x-ray pulse, or even in the UV regime. Some progress has been made in this direction, based on the recent advances in the areas of free electron laser [3] and the inertial confinement fusion [4]. It is shown that the backward Raman scattering (BRS) [5,6], where two light pulses and a Langmuir wave interact one another to exchange the energy, is one promising approach to create an ultra short light pulse (down to a few attoseconds [7,8]); a pulse gets compressed and intensified via this laser-plasma interaction. The BRS is successfully used to create intense pulses of the visible light frequencies [9,10]. It is natural to consider the same technique for the x-ray compression [7,8]. However, some physical processes in the regime where x-rays might be compressible are considerably different from those in the visible light regime. There are new physical processes to be considered, such as the Fermi degeneracy and the electron quantum diffraction [11][12][13][14], to mention a few.We consider one key aspect for the BRS in the x-ray compression regime, i.e., the damping of the Langmuir wave. In an ideal plasma, the decay rate of the Langmuir wave increases rapidly as the wavelength decreases. It poses a concern for the BRS, as the plasmon from the forward Raman scattering (FRS), having a lower wave vector than the BRS, depletes the pump. There have been various attempts to suppress the FRS in the visible light regime [15]. The situation is different in metals and the warm dense matters that we consider for the x-ray compression. The interaction of electrons via the inter-band transition (the Umklapp process) [16][17][18][19][20][21][22] becomes the dominant plasmon decay process, in contrast to the Landau damping in an ideal plasma. As a consequence, the decay rate of the long wavelength plasmon * Electronic address: seunghyeonson@gmail.com † Electronic address: sku@cims.nyu.edu ‡ Current address: Citigroup, 388 Greenwich St. New York, NY 10013 is much higher than the prediction of the prevalent dielectric function theory. In this paper, we show that a parameter regime where the BRS is unstable and the FRS is stable does exist in metals and warm dense matters, as the plasmon from the FRS strongly decays. Here, we consider metals in room temperature, and show that an x-ray pulse can be compressed in this regime. We estimate an optima...

Hard x-ray or gamma ray laser by a dense electron beam

“…The development of such a light source is under increased attention, due to recent advances in the inertial confinement fusion [4][5][6] and the free electron laser (FEL) [7][8][9][10]. In conventional FEL, an ultrarelativistic electron beam interacts with the periodically laid-out strong magnets (so-called the wiggler or the undulator) and subsequently emits coherent photons [11].…”

Two-stream instabilities in degenerate quantum plasmas

“…We also have shown that the unstable Langmuir wave due to the two-stream instability can have the frequency as large as 1.7 ω pe , which is impossible in the classical plasmas. Theses findings in this paper can have major implications on the Backward Raman scattering [15][16][17][18][19][20][21][22] and the beam stopping by the dense background plasmas.…”

Numerical modeling of quasitransient backward Raman amplification of laser pulses in moderately undercritical plasmas with multicharged ions

“…This may be particularly useful for modeling BRA of powerful optical pulses in ionized low-density solids, or for BRA of intense x-ray pulses in denser plasmas, such as at solid densities. 14,15 While many of the BRA features were described computationally in previous studies (see for instance, Refs. [16][17][18][19][20][21][22][23][24][25][26][27][28], it is only here that a simple computational model is employed to study the major physical processes that are associated specifically with the QBRA regime.…”