Quasi-monoenergetic electron bunches with energies peaked in 10–20 MeV are generated from laser wakefield acceleration (LWFA) by focusing few-TW laser pulses onto a sub-mm gas jet of dense nitrogen. A 152-μm diameter orifice is used to produce transient (≤20 ms), free-flow nitrogen jets, while the plasma electrons with a 860-μm wide Gaussian density profile and a density up to ∼2.8 × 1019 cm−3 enable self-focusing effect and self-modulation instability to develop on the pump pulse, resulting in a high intensity to drive the LWFA. Meanwhile, this Gaussian nitrogen plasma facilitates ionization-induced injection and density down-ramp injection throughout the acceleration process and consequently improves the energy and charge stabilities of output electrons. When 40-fs, 3.2-TW, 810-nm pump pulses are applied, output electrons with a peak energy ∼11 MeV and a charge ∼20 pC are routinely generated with ≤20% energy and charge stabilities, ∼20 mrad divergence, and ∼10 mrad pointing variation. A large electron energy spread is attributed to the dominant mechanisms of ionization and down-ramp injections. This scheme represents a viable approach for implementing a high-repetition-rate LWFA, from which stable tens-of-MeV electrons can be generated with less than 150 mJ of on-target laser energy.
By focusing conventional 1-TW 40-fs laser pulses into a dense 450- μm-long nitrogen gas cell, we demonstrate the feasibility of routinely generating electron beams from laser wakefield acceleration (LWFA) with primary energies scaling up to 10 MeV and a high charge in excess of 50 pC. When electron beams are generated with a charge of ≈30 pC and a beam divergence of ≈40 mrad from the nitrogen cell having a peak atom density of [Formula: see text] cm−3, increasing the density inside the cell by 25%—controlled by tuning the backing pressure of fed nitrogen gas—can induce defocusing of the pump pulse that leads to a twofold increase in the output charge but with a trade-off in beam divergence. Therefore, this LWFA scheme has two preferred regimes for acquiring electron beams with either lower divergence or higher beam charge depending on a slight variation of the gas/plasma density inside the cell. Our results identify the high potential for implementing sub-millimeter nitrogen gas cells in the future development of high-repetition-rate LWFA driven by sub-TW or few-TW laser pulses.
Spectral broadening and compression of a sub-terawatt (TW) laser pulse can be achieved by tightly focusing the pulse into a thin, dense gas target; in this way, the excited plasma wave drives self-phase modulation in the pulse and causes a coupled spatial-temporal evolution of field envelope. Through three-dimensional particle-in-cell simulations, selected focal positions of incident pulse, gas species, and target peak densities are assigned to investigate the performance of pulse compression. When a 0.25-TW, 40-fs, 810-nm pulse is incident into a hydrogen target with a 120-μm wide Gaussian density profile and a peak density of 8×1019 cm−3, a shortest output duration of ≈ 20 fs is acquired when the pulse is focused to a size of 4 μm with a position 50 μm before the density peak. Under the same rest of parameters, using a nitrogen target inhibits the pulse compression due to undesired ionization-induced defocusing. Moreover, using a high peak density of 1.2×1020 cm−3 for hydrogen target allows the 0.25-TW pulse to be self-focused to a high intensity capable of exciting a strong plasma wave, which, in turn, modulates and compresses the pulse to ≈7 fs, along with a significantly broadened spectral bandwidth ≈200 nm. This widely expanded spectrum supports a transform-limited pulse duration ≈2.8 fs and allows the output pulse to reach a TW-level peak power when appropriate post-compression is applied.
By using a thin, high-density gas cell, subterawatt laser wakefield acceleration (sub-TW LWFA) of electrons can be driven by few tens of megajoule pulses from diode-pumped lasers operated at high repetition rates. When a 0.5-TW, 1030-nm pulse interacts with a dense plasma, the self-focusing effect and the self-modulation instability are induced to enhance the pulse intensity to a level capable of exciting plasma bubbles. Through particle-in-cell simulations, this study investigates the sub-TW LWFA in which a H2-N2 mixture is applied for the gas target; in this fashion, the nitrogen doping ratio ρN can be varied to improve the output energy and the charge of accelerated electrons with the addition of ionization-induced injection. The results show that the acceleration efficiency is limited when using a pure hydrogen target, since the self-injection of electrons rarely occurs in the first plasma bubble having the highest accelerating field. By doping the hydrogen target with nitrogen, free electrons generated when the pulse peak ionizes the N5+ and N6+ ions can be injected into the first bubble. The optimal performance of sub-TW LWFA can be acquired with a nitrogen doping ratio between ρN = 1% and 3%, from which electrons can be produced with a maximum energy of > 40 MeV and a total charge ∼6 pC for the high-energy component (>20 MeV). Using a relatively high doping ratio, ρN≥ 5% will significantly degrade the properties of the output electrons, primarily because of the manifest ionization defocusing encountered by the driving pulse.
We demonstrate the feasibility of using 1-TW, 40-fs laser pulses to generate electrons with peak energy ≈ 9.4 MeV and charge ≈ 32 pC through the laser wakefield acceleration in a dense, 450-µm long nitrogen gas cell.
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