We examine the interaction of intense, femtosecond laser radiation with the large ͑50-200 Å͒ clusters produced in pulsed gas jets. Both experiment and simulation show that the plasmas produced during these interactions exhibit electron temperatures far in excess of that predicted by above-threshold ionization theory for a low-density gas. Efficient heating of the clusters by the laser is followed by rapid expansion of the clusters and long-lived x-ray emission from hot, decaying, underdense plasma.

collimation and subsequent con®nement of the dust plume can be explained with simple models of the wind±wind interaction 2 , or whether more detailed three-dimensional calculations are required, such as those of Walder 20 . Spectral studies of the plume should reveal the thermal and chemical evolution of the dust as it is swept outwards into the interstellar medium, and should also yield information on processes underlying the binary-mediated dust-creation mechanism. With a few dusty WR systems open to study with this method for the detection of binary stars, wider questions of dust formation in this class of objects can now be addressed. M

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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.