We may not realize it, but we live in a world with particle accelerators all around us. While, the most commonly well known accelerator, The Large Hadron Collider, which recently went operational, is capable of moving protons and even entire atomic nuclei at speeds approaching relativity, we’ve been doing the same thing with electrons without many of us realizing it. From Dental X-Rays, to Security Scanning Devices and Medical Resonance Imaging (MRI), atomic particles moving at the speed of light are being harnessed for a variety of practical tasks all around us not including research in physics and many other sciences dependent on imaging technologies.
As anyone who’s ever had an MRI knows, devices dependent on particle accelerators can be large, complicated, and incredibly expensive to manufacture and operate. However, a team of scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have devised a device that can accelerate electrons 10 times faster than conventional methods with materials built on the nano-scale level.
It’s so effective that the researchers believe they can match accelerative forces of their two mile long SLAC accelerator in a mere 100 meters using this technology. Such an advance has far reaching implications in a variety applications, from portable X-ray equipment to more affordable medical devices.
Electrons are accelerated using electromagnetic radiation. The oscillating electric field generated by this radiation can be used to apply force on the charged electrons and accelerate them forward. Current devices use radiation at radio frequency (which have a period several meters long). However these researchers have developed a new method that employs commercially available TI: sapphire lasers that emit infrared radiation. This represents a wave length seven to eight orders of magnitude smaller than radio waves used by conventional methods.
Electrons are accelerated up in two phases. In the first phase, the electric field actually speeds up the particles as the approach the speed of light, once the electrons reach this physical limit, the forces imparted on them don’t actually speed them up anymore but do increase their energy; this second phase is the most challenging for particle accelerators.
In this study, electrons are sped up to relativity using a conventional accelerator. This devices creates a concentrated beam of electrons only a few micrometers in radius. These electrons are shot in between two fused silica wafers with a small vacuum tube — less than a micrometer in radius — between them. The lasters are aimed at this silica wafer, along the length of the vacuum tube.
However, a challenge that has stumped researchers has been actually accelerating the stream of electrons in a normal vacuum tube. The electric field generated by the electromagnetic radiation from the lasers oscillates, which means part of the time the electric field will point in a direction that would speed up the electrons in the tube. However, when the electromagnetic wave alternates the direction of the field it would in turn actually slow the electrons down again. When you apply this radiation to a normal cylindrical aperture, there is actually no net acceleration at all.
To handle this conundrum, researchers actually invented a novel approach to the design of the vacuum tube itself. The scientists used optical lithography and reactive ion etching techniques to design the structure of the vacuum tube on a nanoscale level. Optical lithography is a technique similar to the process used to manufacture modern circuit boards. Light is used to shine a specific geometric pattern on light sensitive substrate (in our case the silicon wafer).
Then chemically reactive high energy ions (plasma) is used to remove the etched material. This process allows for incredible precision on the nanoscale. Using these techniques the researchers designed a vacuum aperture with a pattern of ridges; the width of the aperture alternates periodically with narrow and wider regions.
This alteration in gap width actually affects the direction of the electric field so that the electric field is weaker in the wider portions of the channel. This is important because this means if u time the electron’s path just right, you can get it so that some of the electrons are in the narrow section of the channel (stronger electric field) while the field is pointing forward and in the wider section (weaker electric field) when the field is pointing backwards, resulting in a net forward force.
All of the electrons are that are sent through this accelerator won’t actually speed up, only the ones that are closely synched with oscillation of electric field will approach the desired energy.
Many others won’t get any faster and others will actually decelerate after entering accelerator; thus, the net effect on all of the electrons is still essentially zero, but the portion of electrons that are accelerated can be harnessed and used to do useful work.This strategy for accelerating particles is much simpler than current techniques, and it could allowed particle accelerating technologies to be used in a form factor and price range quite smaller than existing methods.
However, to ultimately realize this dream we need to similarly scale down the part of the accelerator that actually brings the electrons up to speed in the first place. Nonetheless the work presented by these researchers represents an important step forward.
