Thursday, July 28, 2016

Accelerating Electrons by Slingshot

You load a rock into a small pouch, pull it back until the bands are stretched tightly, and let the rock fly. A few seconds later, a window shatters into a million pieces.

Slingshots-5.
Image Credit: Stephanie Sicore (CC BY 2.0)
Slingshots are much more closely associated with troublemakers and target practice than cutting edge scientific research and medical treatments. However, research published last week describes how something called the “slingshot effect” (not the gravitational kind) could lead to a new kind of table-top particle accelerator with wide-ranging applications in science, medicine, and even security.

The slingshot effect was proposed just a couple of years ago by researchers from the National Institute for Nuclear Physics in Italy and the University of Naples Federico II. It describes how, under the right conditions, you can accelerate electrons to nearly the speed of light through a process reminiscent of accelerating a rock with a slingshot. You can think about the process in two main steps.

  1. Loading: To accelerate a rock in a slingshot, first you put the rock in the pouch and pull it back.
  2. Shooting: To shoot the slingshot, you let the pouch go from its pulled-back position and the rock takes off in the opposite direction.

The slingshot effect has the same steps, although the details are very different.

  1. Loading: To accelerate an electron through the slingshot effect, first you shoot a very short, very intense laser pulse into a plasma. Often called the fourth state of matter, a plasma is a hot gas-like substance made of positively charged ions and negatively charged electrons. Under the right conditions for the slingshot effect, an incoming laser pulse will generate electric and magnetic forces within the plasma that propel some of the electrons in the same direction that the pulse is traveling.
  2. Shooting: Shortly after the electrons are propelled forward, they do an about-face and take off in the opposite direction. This 180-degree turn is the result of positively charged ions pulling on the electrons and the negatively charged electrons pushing each other away. The force is so strong that many of the electrons just keep on going, breaking right out of plasma at super high speeds.

In the latest development, Gaetano Fiore, one of the researchers who proposed the slingshot effect, along with Sergio De Nicola from the SPIN Institute and the National Institute for Nuclear Physics in Italy performed an in-depth analysis of the experimental conditions under which this effect is likely to occur. Their work was published last week by the American Physical Society in the journal Physical Review Accelerators and Beams.

So far the slingshot effect hasn’t been tested in the lab, but the authors say that experimental tests seem to be at hand. Through mathematical work and numerical simulations, they determined a wide range of conditions under which the slingshot effect seems likely to occur. According to their results, it should be possible to verify their predictions with lasers that are readily available and with experimental facilities that already exist. In fact, the researchers used their model to predict features of the resulting electrons that experiments are likely to discover.

You might call the slingshot effect a cousin of another plasma-based acceleration technique, one farther along in testing and developing that has provided exciting results. Laser wakefield acceleration* is a process in which electrons are accelerated out of a plasma by an incoming laser pulse through a different mechanism. In this case, the electrons are accelerated in the same direction that the pulse travels.

Huge particle accelerators such as the Large Hadron Collider at CERN get a lot of well-deserved attention, but many people don’t realize that there are more than 30,000 tabletop particle accelerators around the world. Used to diagnose medical conditions, treat cancer, manufacture computer chips, inspect cargo, and perform scientific research, among many other applications, these accelerators provide extremely valuable services.

One of the main challenges of current particle accelerator technology is that you need a lot of space to accelerate particles up to high energies. Plasma-based acceleration techniques cut way down on this requirement. Experiments indicate that they are a promising way to pack more acceleration into each inch—which could make powerful accelerators more widely accessible, beneficial, and capable of breaking even more barriers.

Kendra Redmond

*Note: this link leads to SMBC Comics, some of which contain material that is not appropriate for all ages.

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