A high-intensity accelerator beam is fashioned of trillions of particles that race at lightning speeds down a system of highly effective magnets and high-energy superconductors. Calculating the physics of the beam is so advanced that not even the quickest supercomputers can sustain.
Nevertheless, a milestone achievement by accelerator physicists on the Division of Vitality’s (DOE’s) Oak Ridge Nationwide Laboratory (ORNL) has enabled beam characterizations to be studied in extraordinary new element. They used a newly developed measurement approach to higher perceive beam loss—stray particles that journey outdoors the confinement fields of the accelerator. Mitigating beam loss is paramount to realizing extra highly effective accelerators at smaller scales and decrease prices.
“It’s an issue that’s been haunting us for greater than 20 years,” mentioned ORNL accelerator physicist Alexander Aleksandrov. “Beam loss might be the most important challenge for high-intensity accelerators, just like the Massive Hadron Collider at CERN and the Spallation Neutron Supply (SNS) right here at Oak Ridge.”
Working at 1.4 megawatts, SNS is certainly one of DOE’s flagship analysis services that leverages neutrons to review power and supplies on the atomic scale. Neutrons are created at SNS by propelling bunches, or pulses, of protons at virtually 90 p.c the pace of sunshine down the power’s linear accelerator—or linac. On the finish of the linac, the proton beam pulses smash right into a steel goal vessel full of swirling liquid mercury at a fee of 60 instances a second.
The atomic collisions create spalls of neutrons—about 20 neutrons per proton. The neutrons then fly by means of power moderators and vacuum chambers to surrounding devices the place scientists use them to review how a cloth’s atoms are organized and the way they behave. Primarily, growing the accelerator energy will increase the variety of neutrons created, which in flip will increase the power’s scientific productiveness and allows new kinds of experiments.
“Ideally, we wish all of the particles within the beam to be concentrated right into a single, very compact cloud. When particles stray away, they kind low-density clouds, referred to as a beam halo. If the halo will get too large and touches the partitions of the accelerator, that leads to beam loss and might create radiation results and different issues,” mentioned Aleksandrov.
As an alternative of creating the measurements at SNS, the crew used a duplicate of the SNS linac at ORNL’s Beam Check Facility. Utilizing a duplicate allows researchers to conduct superior physics research on the accelerator with out interrupting experiments on the precise neutron manufacturing facility.
The superior measurement approach is predicated on the identical strategy the researchers utilized in 2018 to make the . Whereas 3D area contains factors on the x, y, and z axes to measure place, 6D area has three further coordinates to measure a particle’s angle, or trajectory.
“The approach is definitely fairly easy. We take a block of fabric with various slits that we use to chop out small samples of the beam. That gives us with a beamlet containing a smaller, extra manageable variety of particles that we are able to measure, and we are able to transfer that block round to measure different sections of the beam,” mentioned Aleksandrov.
The beam samples have been extracted from one of many linac’s major accelerating parts referred to as the medium power beam transport line, or MEBT. The duplicate MEBT, round 4 meters lengthy, features a beam scraper to scale back early beam halo and offers more room than typical MEBTs for different diagnostic instruments.
“However, as a substitute of reducing out 6D part area, this time we solely lower out samples in two-dimensional part area,” he mentioned. “Mainly, should you can measure in six dimensions with cheap decision, then you possibly can measure in decrease dimensions with a lot greater decision.”
Utilizing the 6D measurements as a baseline strategy, measuring in 2D unlocked a radically improved stage of decision of 1 half per million. One half per million is critical to trendy accelerators for two causes, in response to Aleksandrov. It’s the most allowable density at which beam halo is manageable, and it’s the stage of decision, or dynamic vary, essential to validate and construct extra correct laptop modeling simulations of the beam halo impact.
“Prior to now, beam modeling at this stage was sort of an unimaginable job as a result of computer systems weren’t able to calculating billions of particles; and now they’ll, however it could possibly’t be carried out precisely with out these preliminary beam distributions,” mentioned Kiersten Ruisard, a Clifford G. Shull postdoctoral analysis fellow at ORNL. “There isn’t any mannequin that we all know of that predicts the beam loss patterns which can be measured in the true accelerator. Testing our fashions with this unprecedented stage of precision is important to construct extra sturdy simulations that may assist us mitigate these losses.”
Measuring the beam at a comparatively low power of two.5 megaelectronvolts offered the researchers with insights into learn how to mannequin the beam at greater energies. Aleksandrov mentioned they’re already engaged on the following approach enchancment, which is able to contain utilizing lasers to measure the beam at a considerably greater power of 1 gigaelectronvolt. That improve is a couple of years out.
The crew’s analysis outcomes are printed within the scientific journal Nuclear Devices & Strategies in Physics Analysis. Along with Aleksandrov, Cousineau, and Ruisard, the paper’s authors embrace ORNL’s Alexander Zhukov.
“Though we might make 100 megawatt-class accelerators now, it’s simply not sensible. They’d be too large and too costly,” mentioned physicist Sarah Cousineau, the science and expertise part head in ORNL’s Analysis Accelerator Division. “Enhancing the decision of the measurement to greater ranges not solely permits us to make progress in understanding and simulating beam halo, nevertheless it additionally advances our understanding of learn how to make accelerators extra highly effective, at smaller scales, and at way more cheap prices.”
Reference: “First measurement of a 2.5 MeV RFQ output emittance with 1 part-per-million dynamic vary” by A. Aleksandrov, S. Cousineau, Ok. Ruisard and A. Zhukov, 2 November 2020, Nuclear Devices & Strategies in Physics Analysis.
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