Letters

The high-energy horizon

Further considerations suggest that we revise and expand the third paragraph of our article "The light-pulse horizon", where we addressed the particle-acceleration schemes using lasers (CERN Courier March 2009 p22).

Enrico Fermi contemplated a 1 PeV (10⁹ MeV) accelerator girdling the Earth. While an energy of peta-electron-volts is in general held to be too ambitious for currently available technology, we see it on the horizon. Laser acceleration may allow us to reach this energy in a device with a size of the order of 1 km employing a sub-picosecond 15 MJ laser. On the way towards this goal, we can test the ultrahigh-gradient acceleration theory at 10 TeV, which we hope can be achieved with a laser of 15 kJ and a 50 fs pulse. Such an intense laser pulse is not yet available, but the proposed Extreme Light Infrastructure (ELI) should offer an opportunity to explore this physics. The peak power of ELI will be in the exawatt (10¹⁸ W) region– that is 100,000 times the power of the global electricity grid, albeit only over several femtoseconds.

Controlling a beam of even relatively few electrons at such energies may allow us to calibrate high-energy cosmic-ray detectors, study large Lorentz-contraction phenomena such as the Landau–Pomeranchuk–Migdal effect, and test fundamental laws in this new domain, including an exploration of the validity and limits of relativity itself.

Gérard Mourou, Johann Rafelski and Toshiki Tajima.

Clarification

Commenting on the small resistive zone in an LHC bus connection that led to the incident on 19 September 2008, a recent article (CERN Courier January/February 2008 p6) stated that "in less than a second the zone led to a resistive voltage of 1 V at 9 kA. The resistance was small – 200 nΩ – dissipating of the order of 10 W at high current intensity."

A quick application of Ohm's law suggests that these two statements are incompatible. What happened is that the resistance evolved with time as the current was ramped up. In tests on 15 September at 7 kA the resistance was indeed around 200 nΩ and had it stayed so small there would have been no incident.

However as the current was ramped up to 8.7 kA, localized heating increased the resistance, leading to thermal runaway. Dissipation was nearly 9 kW by the time that the detection threshold of 1 V was reached.