Abstract
By introducing a sub-terawatt (TW) laser pulse into a high-density gas target, the self-focusing effect and the self-modulation instability can greatly enhance the laser peak power to a level capable of driving the laser wakefield acceleration (LWFA) of electrons. A particle-in-cell model has been developed to study the scheme in which 1030-nm pulses produced from a diode-pumped laser system are introduced into a gas cell with a flat-top density profile, allowing the LWFA to be operated at high frequencies. Because 1030-nm lasers are typically produced with a long duration >200 fs, a spectral broadening technique can be applied to reduce the pulse duration, from which a greater ponderomotive force is acquired to drive LWFA. To understand the dependence of LWFA performance on the driving pulse duration, selected durations, ranging from 200 fs to 10 fs, are assigned for 0.5-TW, 1030-nm pulses in a series of simulations. Results show that a duration around 50 fs can provide the optimal LWFA results, as a compromise between the weak ponderomotive force available from a long pulse >100 fs and the depletion effect which can rapidly diminish a short pulse <25 fs in a dense plasma. When a low laser peak power of 0.25-TW is available, the pulse depletion can be significant at a high target density and render LWFA ineffective. Using a laser pulse with a longer wavelength >2 μm represents a viable route to realize the LWFA with a low laser peak power; in this way, an appropriately selected target density which allows the laser peak power PL ∼ 1.25Pcr of self-focusing critical power is favourable for realizing an efficient LWFA process.
Highlights
The advance of laser-driven particle acceleration research primarily relies on the employment of novel laser technology, such as the chirped-pulse laser amplification scheme invented since the mid-1980s.1 In the laser wakefield acceleration (LWFA), pulses with an increased pulse energy EL and/or a shorter pulse duration τL are always desirable, because the ponderomotive force that a laser pulse can exert for driving a plasma wave is proportional to the laser peak power PL ∝ EL/τL
Reducing the duration of these 1030-nm pulses will be a straightforward route to enhance the ponderomotive force FN that they can exert to the plasma; when a spectrum broadening technique is involved to reduce the pulse duration, a wider bandwidth achieved for a pulse will result in a smaller pulse energy EL available after the conversion process
When a sub-TW laser pulse is incident into a high-density gas target, the combination of the self-focusing effect and the self-modulation instability contribute to the achievement of a successful LWFA process
Summary
The advance of laser-driven particle acceleration research primarily relies on the employment of novel laser technology, such as the chirped-pulse laser amplification scheme invented since the mid-1980s.1 In the laser wakefield acceleration (LWFA), pulses with an increased pulse energy EL and/or a shorter pulse duration τL are always desirable, because the ponderomotive force that a laser pulse can exert for driving a plasma wave is proportional to the laser peak power PL ∝ EL/τL. Among the new technologies, diodepumped ytterbium (Yb) laser represents a viable route to realize TW-scale, high-average-power (kHz operation) ultrafast lasers,[24,25] including associated optical parametric amplification (OPA)/optical parametric chirped-pulse amplification (OPCPA) architectures for producing fs, midinfrared (IR) laser pulses.[26] While being optically pumped by 980-nm diodes, recent results have demonstrated that Yb:CaF2 amplifiers can produce 1030-nm pulses with energies 50 - 110 mJ at the repetition rates 20 - 100 Hz.[27] These 1030-nm pulses are considered as potential driving forces to promote LWFA performance at kHz-level frequencies;[28] they are typically produced with relatively long durations in the range 150 - 200 fs, making them less efficient to drive plasma waves due to the smaller ponderomotive force FN ∝ a2 ∝ λ2L E2, where E is the electric field amplitude, they can provide at the front edge than that of 800-nm pulses usually
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