In the DESY/MIT group’s matchbox-sized electron gun (shown in lower-right-hand corner), a UV pulse (blue) back-illuminates the gun photocathode, producing a high-density electron bunch inside the gun. The bunch is accelerated by ultra-intense terahertz pulses—whose field strength is enhanced by subwavelength confinement—to energies approaching 1 keV. [Image: Courtesy of W. Ronny Huang]
In the past few years, research labs have garnered headlines by creating ultracompact, photonically driven linear accelerators small enough for the lab bench. These advances have raised the possibility of bringing some of the power of femtosecond X-ray sources, such as large free-electron lasers, into ordinary academic and industry labs. But reaching that goal requires another component: a similarly compact, highly stable electron gun that can fire into the accelerator the high-quality stream of electrons that’s used to create the X-ray pulses.
Now, in a recent study (Optica, doi: 10.1364/OPTICA.3.001209), a team led by OSA Fellow Franz Kärtner has developed an all-optical, THz-driven prototype electron gun that can fit between the thumb and forefinger of a person’s hand—and that can achieve surface fields greater than those of car-sized RF electron guns, with zero timing jitter. The combination of compactness, field strength and timing stability has, according to the researchers, “the potential to transform accelerator-based science,” and could mark another step on the road to bringing ultrafast X-ray science into small-scale labs.
Power and jitter
Free-electron lasers, such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Center in the United States, work by injecting a stream of high-energy electrons, created by a car-sized, radio-frequency (RF) electron gun, into a kilometer-long linear accelerator. The electrons, passing between an array of magnets known as an “undulator,” are gathered in bunches and build up powerful X-ray fields that are eventually emitted in extremely short (1 to 5 fs) X-ray pulses (see “Sources and Science of Attosecond Light,” OPN, May 2015).
A year ago, Kärtner’s team demonstrated that it was possible to shrink the accelerator part of the setup to tabletop scale using optically generated terahertz pulses (Nat. Commun., doi: 10.1038/ncomms9486). But getting to a benchtop X-ray source also requires shrinking the other key component, the giant electron gun, to a more manageable size.
And making a compact gun brings its own suite of hurdles to clear. One is the sheer power necessary to drive an RF gun—which, even given that high power input, is limited to creating surface fields of less than 200 MV/m. An even bigger issue is the timing uncertainty, or jitter, between the RF source used to create the accelerating field and the UV radiation source used as a phototrigger to kick the electrons into the field itself. That jitter can exceed 100 fs—a very meaningful slice of time uncertainty in ultrafast experiments.
Subwavelength confinement
To get to a smaller, less power-hungry gun, Kärtner’s team, including scientists at the Deutsches Electronen Synchrotron (DESY) and the University of Hamburg, Germany, and the Massachusetts Institute of Technology, USA, turned to the THz band. The group’s setup (previously proposed in several other papers) begins with a 1-kHz, 1030-nm pump laser, pulses from which are split, using several nonlinear techniques, into THz and UV pulses. The THz pulses, traveling in free space, are then focused into a copper, parallel-plate waveguide with a subwavelength spacing of 75 μm, creating the gun’s electron-accelerating field.
Simultaneously, the UV pulse, acting as a phototrigger, hits the copper film forming the bottom surface of the waveguide, kicking out electrons that are accelerated by the field from the confined THz radiation. The electrons exit the gun through a slit in the top surface of the waveguide.
Matchbox-sized gun
The result of the scheme is a matchbox-sized electron gun powered by an ordinary, few-millijoule laser—yet one that’s capable, because of the subwavelength confinement, of delivering surface fields in excess of 300 MV/m, better than RF guns thousands of times bigger. And, because the field-creating (THz) pulse and the phototrigger (UV) pulse come from the same laser, the intrinsic timing jitter drops to virtually zero.
The device is very much a proof of concept; for example, the width of the electron bursts it produces at present, on the order of several hundred femtoseconds, is not much different from the best existing RF guns. But the team believes that, with the concept now proved out, the researchers can materially improve on burst width and other parameters with additional work on the setup details.
That work, according to the researchers, could ultimately help bring a compact, ultrafast X-ray light source to a lab near you. The paper’s first author, Ph.D. student Ronny Huang, says the team is “building a tool for chemists, physicists and biologists who use X-ray light sources or the electron beams directly to do their research.”