New Scientist Extract: Downsize Particle Accelerators

The age of giant particle accelerators may be over

Enormous particle colliders may become obsolete

PHYSICS 24 July 2019

Jon Cartwright  

(apologies for following edits, no intent to change content JimO)

For the best of physics, we've always thought that "size matters"; maybe that's not true anymore. Consider a well known example of "growth physices".  The first particle smashers of the early 1960s were little wider than a dining room table. Sometime later, the Tevatron, a circular collider in the US, had a circumference of 6 kilometers. Today’s largest machine, the Large Hadron Collider (LHC), has one four times as long. Now, we plan to build a collider 100 kms in circumference (about the size of New York City).

These are enormously expensive undertakings. Learning new subatomic secrets has always meant accelerating particles over increasingly longer distances; then, smashing them together. But a new shortcut is emerging, consider plasma  (stuff of stars, collection of charged particles, ions). Inject particles into plasma, and they accelerate a much, much faster.

Such plasma accelerators have been advancing steadily over the past few decades, and while they have yet to pose a serious threat to the dominance of conventional facilities, that might be changing. Several recent developments suggest that plasma accelerators could soon give big beasts like the LHC a run for their money. Ultimately, the hope is that these small machines will let us tackle some of the biggest questions in physics: why our universe is filled with matter and not antimatter, for instance, or what constitutes dark matter. It seems the ironclad rule of particle physics is about to be broken.

Typical technology seems to undergo inevitable miniaturization; HOWEVER,  conventional particle accelerators seem to suffer from an intrinsic growth. Complicated control systems contain numerous metal pipes. which individually kick particles forward by carefully coordinating numerous short and strong electric fields.  To make the overall accelerator more powerful, you need even more pipes.

This physical limit led to circular vs. linear, particle smashers: in a circle, particles can keep orbiting until they reach the desired energy. (Within reason.) HOWEVER, even circular accelerators must be very big; otherwise, particles simply shed their energy as radiation – or else get flung out of the ring as they skid around tight corners. Hence the LHC. Only a circular collider 27 kilometres in circumference could smash opposing beams of protons with enough energy – up to 13,000  GeVs – to produce the famous Higgs boson. Many particle physicists now wonder what's next; many scientists thought 27 kms was the max,”  

Some researchers pursue a cheaper, smaller alternative.  

Plasmas are the fourth state of matter: an ethereal mix of electrons and the positively charged atomic nuclei, or ions, from which they were stripped. As the electrons and ions move around, tiny electric fields are created and destroyed, making plasma the perfect medium for carrying charged particles.  

Plasma acceleration originated  at the University of California, Los Angeles, (UCLA) The idea was to fire a laser into a gas of atoms, creating a plasma and dividing its electrons from its positively charged ions. In the laser's wake, this division of negative and positive charges would create a greatly enhanced electric field. An properly injected electron would follow the plasma wake to accelerate over a thousand times faster than in a conventional accelerator.

Chandrashekhar Joshi put this into practice, accelerating injected electrons by 7 MeVs in only a few millimeters (mms). Instead of a laser, he used a pulse of electrons from a conventional accelerator to create the plasma, divide it and accelerate it.

In 2005, California's Stanford Linear Accelerator Center (SLAC) used its existing accelerator to turbo-boost electrons by 3 GeVs in about 10 centimeters (cms). In 2007,  it demonstrated 15 times this energy gain in under a meter – nearly 10,000 times the rate of acceleration at the LHC.  

Note a few issues. 
1) Consistency.  Give all particles the same boost. The spread of energies now generated by plasma accelerators is currently 10 times too broad for ready interpretation of particle collisions. 
2) Reliability. Particle accelerators must work 24/7 continuously for weeks at a time. 

Thus, we must link and align a series of laser or electron pulses to reach the highest energies. 

Possible Breakthrough.  AWAKE, an international collaboration at CERN, is experimenting with protons, almost 2,000 times the size of electrons. Send protons into a plasma to generate a huge wake to fling injected electrons forwards in one fell swoop. This eliminates several stages, which can complicate a beam line. 

Plasma accelerators may have immediate practical benefits. With no need for collisions, compact accelerators could make advanced types of radiation therapy for cancer more widely available. They could also probe cutting-edge materials, or enable security staff to check for hidden explosives. In fact, plasma accelerator spin-offs like these could be just five or 10 years away.

Using recent laser technology, Berkeley Lab recently used a laser pulse with a power of 850 trillion watts to achieve electron energies of nearly 8 GeVs over 20 c,s in a plasma accelerator.