Laser-plasma accelerators of only a centimetre's length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt ...energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.
We measure characteristics of plasma bubbles in GeV-class laser-plasma accelerators (LPAs) using Faraday rotation diagnostics. We extend these techniques, previously demonstrated for LPAs in ...atmospheric density plasmas (electron density \(n_e >10^{19}\) cm\(^{-3}\)), to LPAs in low-density plasmas (\(n_e \approx 5\times10^{17}\) cm\(^{-3}\)), in which plasma bubbles are \(\sim 5\) times larger, and correspondingly easier to visualize in detail. The signals show \(\approx 0.5^\circ\) rotation streaks of opposite sign separated by \(\sim50\) \(\mu\)m, consistent with bubble diameter; no on-axis rotation; streaks length consistent with transverse probe pulse duration (\(180\) \(\mu\)m for \(500\) fs pulse length, and \(600\) \(\mu\)m for \(2\) ps pulse length). We utilized an anamorphic imaging system to obtain a wide longitudinal field of view (\(>1\) cm) and a high transverse resolution (\(<9\) \(\mu\)m). We also demonstrated that Faraday rotation signals are sensitive to the stages of acceleration processes using extended 2D Finite Difference Time Domain (FDTD) simulation.
We present an in-depth experimental-computational study of the parameters necessary to optimize a tunable, quasi-monoenergetic, efficient, low-background Compton backscattering (CBS) x-ray source ...that is based on the self-aligned combination of a laser-plasma accelerator (LPA) and a plasma mirror (PM). The main findings are: (1) an LPA driven in the blowout regime by 30 TW, 30 fs laser pulses producesnot only a high-quality, tunable, quasi-monoenergetic electron beam, but also a high-quality, relativistically intense (a0~1) spent drive pulse that remains stable in profile and intensity over the LPA tuning range. (2) A thin plastic film near the gas jet exit retro-reflects the spent drive pulse efficiently into oncoming electrons to produce CBS x-rays without detectable bremsstrahlung background. Meanwhile anomalous far-field divergence of the retro-reflected light demonstrates relativistic "denting" of the PM. Exploiting these optimized LPA and PM conditions, we demonstrate quasi-monoenergetic (50% FWHM energy spread), tunable (75 to 200 KeV) CBS x-rays, characteristics previously achieved only on more powerful laser systems by CBS of a split-off, counter-propagating pulse. Moreover, laser-to-x-ray photon conversion efficiency ~6e12 exceeds that of any previous LPA-based quasi-monoenergetic Compton source. Particle-in-cell simulations agree well with the measurements.
Atomic oxygen profile measurements Henderson, Watson R.
Journal of Geophysical Research,
1 September 1974, Letnik:
79, Številka:
25
Journal Article
Recenzirano
A series of atomic oxygen profiles are presented that show evidence that silver film sensors provide reliable measurements of atomic oxygen density in the 90‐ to 120‐km altitude range. Measurements ...were made at Wallops Island, Virginia, and Point Barrow, Alaska. The much lower atomic oxygen density observed for the high‐latitude winter and spring shots is discussed in terms of the local production of atomic oxygen. Transport of atomic oxygen poleward in winter at altitudes of 120 km and above is implied by these measurements. Currently available production processes and rate constants for the 557.7‐nm airglow feature were used to calculate expected airglow intensity from these atomic oxygen profiles. Reasonable airglow values were found by using the mid‐latitude atomic oxygen profile, but airglow values much lower than the accepted observational values were inferred from the high‐latitude profiles.
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