Gravity: magnets in particle accelerators


Accelerator magnets can be as long as a pickup, sometimes longer, and can weigh tons. It usually takes months to create each. They bend and focus particle beams, correct imperfect focusing, and even increase beam stability. Credit: Reidar Khan, Fermilab

In 1820, Hans Christian Oersted held an electricity demonstration for an advanced student class at the University of Copenhagen in Denmark. Using an early battery prototype, he looked at how the electric current would affect the compass, and since he did not have time to test his experiment in advance, the result was as unknown to him as his students. When he completed the circuit by attaching one wire to both ends of the battery, the resulting current caused the compass needle to line up with the wire, indicating that electricity and magnetism were two aspects of the same phenomenon.


In building electricityOersted created a temporary magnet – an electromagnet. Physicists continued to develop electromagnets for their experiments, and today they are everywhere: in MRI scanners, loudspeakers, transformers, electric motors – and particle accelerators,

Accelerator magnets bend and form beams of subatomic particles as they move at speeds close to the speed of light. Experts are designing magnets so they can use the beam correctly to get the physics they aim for.

Accelerator Magnets – How Do They Work?

The movement of charged particles, such as protons and electrons, creates a magnetic field. In the same way, magnetic fields affect the movement of charged particles. This is the relationship that Oersted helped open up 200 years ago, and later scientists will come to a definition: electricity and magnetism two sides of the same coin.

This is a phenomenon that mankind uses to change the world. The electrical network that powers the device that you use to read this comes from understanding the magnetism-electricity relationship.

Elementary particle physicists use electromagnetism to study the origin of our universe by controlling particle beams in accelerators, breaking them into targets and producing even more particles for scientists to study.

By passing an electric current through a spiral wire, accelerator Experts produce a temporary magnet with a north and south pole. These spiral wires form the poles of the electromagnets used in accelerators. They can be placed not only in bipolar electromagnets, but also in magnets with four, six or even more poles.

Make no mistake: they are not like your home magnets. Accelerator magnets can be as long as a pickup, sometimes longer, and can weigh tons. It usually takes months to create each.

Regardless of the materials from which they are made, accelerating magnets can be classified by the number of poles. Most of them are of one of four types: dipole magnets bend the beam, quadrupoles focus the beam, sextupoles correct the imperfect focus of the quadrupoles, and octupoles can help increase the stability of the stored particle beams. In the language of the accelerator, these are different magnetic "multipoles" that scientists use to control the rays in these opening engines.

Dipoles are not just steering beams

Dipoles most often consist of two separate spiral wires, their north and south poles facing each other. When current flows through the coils, a unidirectional magnetic field forms in the gap between the poles.

Gravity: magnets in particle accelerators

When a positively charged particle hits the page and passes through a dipole magnet, it deviates to the left by an angle proportional to the magnitude of the force applied by the magnet. Courtesy: Gerald Pinson

“Scientists and accelerator engineers can use this field to bend charged particle beams along a curve,” said Jonathan Jarvis, Fermilab Research Fellow. “Simply put, dipoles are our main way to get the rays where they need to go.”

If you were on a proton directed directly to a magnetic field directed down, you and your proton would have shifted to the left by an amount proportional to the magnetic field strength. The stronger the magnetic field, the more you and your proton will pull to the left. For vertical magnetic fields, the path you would follow is a horizontal arc.

Dipole magnets are commonly used to bend particle beams. In a circular accelerator, for example, several dipole magnets lined up along the beam path. The particle beam moves one after another, pushing in the same direction with each passage, so that it follows a curve.

High-speed dipoles can also be used to “knock out” particle beams into or out of the main beam of a circular accelerator.

Quadrupoles – stay focused

Magnets using unidirectional force work well to bend particle beams in a certain direction, but they are not able to maintain the shape of the beam.

“If we leave the beam to our own devices in dipoles, it will fall apart,” Jarvis said. “Just like a collection of gas molecules, a particle beam has a temperature, and this random energy will cause the particles to naturally disperse in the accelerator. If the beam particles do not come together, they will fall into the walls of the vacuum tubes where they circulate. "

Therefore, scientists use quadrupole magnets to refocus wayward particles and return them back to the crease.

As the name implies, quadrupoles have four alternating poles. They create a special magnetic field that can bring particles together, just like lenses can bend light rays to a point.

One quadrupole focuses the beam in one plane. For example, a quadrupole can squeeze the sides of the beam inward as it passes through the accelerator, but – just like the Play-Doh lump reacts when you squeeze its sides together – the beam will defocus in the other direction.

The solution is to link several quadrupoles together with alternating orientations. A beam passes through one and contracts in the horizontal direction. Then it goes through the next one and contracts in the vertical direction. With each subsequent increase, he becomes focused.

The net effect is a steady stream of particles rattling back and forth as they spin around the accelerator.

Similarly, quadrupoles can also defocus beams. When particles pass through the accelerator, there are situations when the beam needs to be slightly less densely packed, reducing the likelihood that the particles will interfere with each other. When the rays pass through quadrupoles with a weaker magnetic force, they are allowed to propagate first in the up-down direction, then in the left-right direction, and so on, until they are properly defocused.

Gravity: magnets in particle accelerators

Quadrupoles have four magnetic poles. In a particle accelerator, poles push particles together if they deviate too far from the centralized beam. The quadrupoles focus only in one plane, therefore, to compress the accelerator beam on both sides, these magnets usually fold one after the other, each of which is rotated 90 degrees relative to the previous one. Thus, the beam particles move together in both directions as they pass through successive magnets. Courtesy: Gerald Pinson

Sextupoli – color grading

Just as dipole magnets can bend a beam, but are not able to keep it in focus, quadrupoles can focus particles, but not all in the same place.

The particles that make up the beam have slightly different energies.

“Unfortunately, the quadrupoles do not behave the same for all beam energies,” said Jarvis. “A particle with a higher energy is less affected by the quadrupole a magnetic field than a particle with lower energy. "

As a result, particles with high and low energy are focused at different points of the beam path. It is like droplets of water bending light of different colors, creating an amazing rainbow.

For quadrupoles, this “chromatic aberration” causes differences in how quickly particles bounce back and forth in an accelerator, a phenomenon known to accelerator scientists as color.

“In many cases, in order to see the physics we want, we need to correct the color, and we do it with sextuples,” Jarvis said.

When properly placed in the accelerator, these six-pole magnets cause particles with higher energy to align with the rest of the beam.

Octupoli – Mixing

We all had such a moment: you walk along the corridor when someone turns a corner and is right on your way. You both maneuver in one direction, then the other, then back again, trying to avoid a collision, a collision that can last forever. The reason it is so hard to get around another person is the result of your similar speeds. If one person moved slower or simply kept to a course, then this behavior would be suppressed.

Particle beams can exhibit similar types of collective behavior if they all oscillate at the same frequency.

To stabilize the situation, eight-pole magnets called octupoles can be used to mix particle frequencies. Scientists call the resulting stabilization "Landau damping," and it provides the particle beam with natural immunity against some unstable behaviors.

Gravity: magnets in particle accelerators

Quadrupole magnets are not able to focus particles with different energies at a certain point, so scientists use sextupole magnets to correct this chromatic aberration. Courtesy: Gerald Pinson

Unfortunately, the increased stability and improved focusing created by higher-order multipolar magnets is expensive.

“These magnets can create harmful resonances and reduce the overall range of positions and energies that particles can have,” said Jarvis. "If the particles fall outside this range of the so-called" dynamic aperture ", then they will be lost from the accelerator."

Integrated optics and more

Accelerator scientists around the world are working to create more productive particle beams in their quest for physics, which underlies the universe.

One way to do this is to increase the intensity of the beam — the number of particles that they pack into the beam. But there is one snag: as the intensity increases, ray behavior can become much more complex, which limits the extent to which traditional magnets can limit them.

To pave the way for the next generation of particle physics, accelerator scientists at Fermilab are considering fundamentally new types of magnets that can cope with the ever-increasing beam intensity.

“These non-linear magnets are, in fact, special combinations of many multipoles, and they can significantly improve Ray stability without compromising inherent in simple octupoles, "said Jarvis.

As scientists continue to expand the boundaries of magnetic technology, we can look deeper into the subatomic world – by discovering exotic particles that exist only in the most extreme conditions, observing the mysterious transformation of neutrinos and the decay of muons, and ultimately come for a better understanding of that how the universe began.

It is amazing to think that a humble magnet is our gateway to some of the deepest secrets of the universe, but then again, it is a force of attraction.


Waltz from LHC magnets started


citation:
Gravity: magnets in particle accelerators (2020, March 20)
retrieved March 21, 2020
from https://phys.org/news/2020-03-power-magnets-particle.html

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