Fast Compression of Laser Beams to Highly Overcritical Powers

Abstract

Laser beams can be strongly compressed in a plasma by stimulated Raman backscattering in a time short compared to the time scale for filamentation instabilities to develop. Such a compression should make feasible multi-MJ multi-exawatt-laser pulses technologically challenging by other means. The compression efficiency can reach nearly 100% at the Langmuir wave breaking limit. [S0031-9007(99) PACS numbers: 52.40.Nk, 42.65.Yj, 52.35.Mw Achievable laser intensities have increased remarkably during recent years mainly due to the method of chirped pulse amplification (CPA). Petawatt (PW 10 15 W) powers and kJ energies have been attained by stretching, amplifying, and then recompressing laser pulses [1]. Certain modern applications, however, require even higher powers and energies. An example of such an application is the fast igniter approach to inertial confinement fusion [2]. Higher power pulses could have applications for radiography of dense materials. Important applications are already envisioned for exawatt (EW 10 18 W) MJ pulses. The higher total power in the pulse presents a practical difficulty to CPA techniques, because very large gratings would be needed to handle large power and heat loads.The rationale of the CPA technique of stretching and recompressing is to avoid nonlinear effects which tend to spoil the beam quality. These nonlinear effects appear in the refraction coefficient at power densities ϳGW͞cm 2 . Longitudinal recompressing by ϳ10 3 times allows CPA to reach power densities ϳTW͞cm 2 , without focusing in vacuum. For kJ 0.5 psec pulse, the cross section of the amplifier and compressor must be ϳ2000 cm 2 , so that ϳ50-cm-diameter beam and ϳ1-m-diameter gratings are required [1]. A higher recompression factor of 10 4 would allow CPA to get a 10 kJ, 0.5 psec pulse using gratings about a meter in diameter. For higher energies, larger, more expensive gratings are required.This Letter examines an alternative to the CPA approach, wherein the laser beams are amplified to power densities much higher than a GW͞cm 2 , even without stretching-recompressing and focusing outside the amplifying medium. The medium capable of bearing such high power densities and heat loads is plasma, but intense pulses in plasmas are subject to numerous instabilities. In particular, the modulational instability can lead to filamentation of the laser beam followed by ponderomotive expulsion of electrons from the filaments. The critical power for such a cavitation is P cr ഠ 17͑v͞v p ͒ 2 GW, where v is laser frequency and v p is plasma frequency [3]. Larger P cr occurs for larger v͞v p , but then the laser-plasma coupling becomes poor and hence the compression efficiency decreases. For a reasonably large frequency ratio, say v͞v p 10, one has P cr ഠ 1.7 TW.Highly overcritical powers are needed, but instabilities stand in the way of reaching those powers.The difficulties could be overcome by ultrafast compression of laser beams, such that highly overcritical laser powers are attained before the instabilities develop. Stimulat...

Generation of ultrahigh intensity laser pulses

“…Also, in this regime, all short enough and intense enough initial seeds will asymptotically reach the -pulse solution. 11 That a self-similar solution, depending only on the ratio z/t, exists can be seen from a scaling of the equations. Essentially, the Raman depletion of the pump by the seed takes place in a distance that varies inversely with the seed pulse amplitude.…”

“…As shown in Ref. 11, within several growth times of the deleterious instabilities, the pulse grows to amplitudes far exceeding the instability thresholds, i.e., to overcritical powers. Pump depletion quickly ensues, so that the efficiencies are limited in principle only by the so called ''Manley-Rowe'' relations; in other words, since a pump photon is converted to a counterpropagating pulse photon downshifted by the plasma frequency, the fraction of pump energy that will be left in the plasma wave is p / .…”

“…However, recently, it was suggested that Raman amplification and compression in a plasma at high power commands particularly favorable effects. [11][12][13] In the transient high-power, pump-depletion regime, which supports the so-called '' -pulse regime'' solutions, a counterpropagating short pulse is resonantly amplified even as it compresses further within the plasma. The pulse is amplified rapidly, before competing deleterious effects, such as modulational or other deleterious instabilities, 14,15 have a chance to develop.…”

Interplay of Laser-Plasma Interactions and Inertial Fusion Hydrodynamics

“…Space plasmas show important interplay of waves and energetic particles, and ionosphere modification experiments by radar transmitters have shown anomalous absorption by decay into Langmuir waves (LWs) [3]. This Letter focuses on laboratory LPI, specifically in inertial confinement fusion (ICF), but is also relevant to parametric-amplifier [4] and beam-combining schemes.…”

Reaching the Nonlinear Regime of Raman Amplification of Ultrashort Laser Pulses

“…A further increase of the pulse intensity using the CPA technique would require even larger gratings in order not to exceed the material damage threshold. Such a system would be very difficult to implement in universityscale laboratories.In order to overcome the CPA material limit at ultrahigh intensities, different backscattering coupling techniques were proposed in plasma, including Compton scattering [5], resonant Raman backscattering [6], and Raman backscattering at an ionization front [7]. The experiments reported here utilize the resonant Raman mechanism [6], where a short seed laser pulse is amplified by a counterpropagating long pump pulse, with their frequencies satisfying the resonance relation, !…”