Relativistic electron beam interaction and Ka - generation in solid targets Page: 4 of 10
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The main obstacle to realizating a hard X-ray laser is, of course, the extremely high intensity acquired to
pump it. Simple scaling laws2 derived from the Einstein relations lead to intensities around 1020 W/cm2 for a laser
at 1 keV. Fortunately, in recent years, considerable progress in the development of ultrashort-pulse!lasers" in
particular the invention of the CPA technique6 has made it possible to generate intensities even exceeding this
value. Thus, it seems feasible to reconsider keV X-ray lasers taking into' account the new generation of
ultrashort-pulse lasers. -
It has been known for some time that at intensities of about 10" W/cm2 (at wavelengths around 1 atm) a new
quality of interaction of laser pulses with a plasma is induced, owing to the fact that the electrons quiving in the
laser field become relativistic. These relativistic effects include self-focusing of the laser pulse, subsequent
generation of channels of propagation and generation of directed electron beams."' We recently poin out that
a combination of these effects can be used to alleviate considerably the problem of pumping a keV X -ay laser."
The favorable features considered include an increase in the applied intensities due to self-focusing, fijnation of
a relatively long channel in which the energy is deposited and application of the relativistic elobtrons for
traveling-wave excitation with the velocity of light.
In the present paper we first discuss the concept of a relativistically supported X-ray laser, gid present
simulations to lay down the pumping requirements. In the second part of the paper we report experirn nts which
investigate basic features of hot electron generation and propagation in cold material.
2. CONCEPT OF A RELATIVISTIC PLASMA-PUMPED X-RAY LASER
At intensities 1e z 10"' W pm2 /cm2 the velocity of the electrons oscillating in the laser field becoi*s so high
that a significant mass increase takes place. The resulting reduction in the time-averaged plasma freqdncy leads
to an increase of the plasma refractive index for the laser pulse. Thus, for a laser pulse with a* intensity
maximum on axis a self-focusing mechanism takes place which overcomes the beam spread due to direction at
a critical power given by'4
P. = 17 Nc/N [GW], (1)
where N and N. are the critical electron density and the plasma electron density respectively. PIC # ulations
show that relativistic self-focusing leads to a pulse propagation channel only a few wavelengths v$de with a
length of many diffraction distances.
A second relativistic effect consists in acceleration of electrons due to the Lorentz force, as given ba v,. x BL,
where v. is the quiver velocity of the electrons in the laser field and B is the magnetic fld of the
electromagnetic wave. It is well known that for a plane wave the Lorentz force will not eventually lead o forward
acceleration of the electrons, since all of the energy will be returned to the field after the laser p.'se." PIC
simulations show, however, that under realistic conditions, i.e. for a laser pulse with an intensity prQoe peaking
on axis, or in a plasma, a number of mechanisms lead to net acceleration of the electrons. CollimaM~d electron
beams with a Boltzmann distribution with energies of several MeV are predicted?-12
Reviewing X-ray laser schemes which might operate under the conditions outlined above, one has $ conclude
that the ones most successful in the soft X-ray region, viz. recombination pumping and electron: collisional
excitation, are very difficult to implement with a relativistic plasma. The high electron temperatu4s and the
corresponding low cross-sections for atomic interactions preclude the generation of a high pumping rate-
Innershell photopumping, however, seems to be well suited to application of relativistic effects.
I t
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Eder, D C; Eidman, K; Fill, E; Pretzler, G & Saemann, A. Relativistic electron beam interaction and Ka - generation in solid targets, article, June 1, 1999; California. (https://digital.library.unt.edu/ark:/67531/metadc619662/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.