| by Wang Lishi Liu Fang
In the first episode of the TV series version of "The Three-Body Problem", we saw the "China No. 2 High-Energy Accelerator Base" in the TV series, and followed the series to see the process of Dr. Yang Dong's particle collision experiment in "The Three-Body Problem":
A still from the collider of the "China No. 2 High-Energy Accelerator Base"
Of course, I also saw Tomoko's interference with the experiment. Why are Trisolarans so afraid of the Earthlings' collider? The collider is actually a type of particle accelerator. According to the setting of the original book, the Trisolarans are worried that humans will use accelerators to reveal the nature of matter, achieve technological leaps, and become their powerful rivals.
This setting is very scientific! Particle accelerators do play a pivotal role in the history of human science and technology. By artificially accelerating charged particles, scientists can use accelerators to do high-level physics experiments, explore the basic composition of the material world, and promote the development of basic science; It is also possible to detect flaws in materials, handle delicious food, and even diapers that babies can't do without!
What exactly does an accelerator look like and how does it work? Today, we're going to introduce you to three common accelerators.
Linear accelerator – probably your old TV
As the name suggests, a linear accelerator (Linac) is an accelerator that accelerates charged particles in a straight line. As early as 1924, the British physicist Easing proposed the concept of linear accelerator - using multiple acceleration electric fields to make charged particles obtain higher energy. In 1928, Widro from Norway developed Ising's principle and built the world's first linear resonance accelerator.
The popular "big ass" TV set in the eighties and nineties was actually a linear accelerator, which produced an image by hitting the luminous paint on the inside of the screen through the electron beam generated by the cathode ray tube.
Accelerator in the TV (Source: CERN)
Linear accelerators are currently one of the most common accelerators. When you're munching on a supermarket-bought pickled pepper claw, chances are you're enjoying the convenience of an accelerator. Linear accelerator-based irradiation processing technology (radiation irradiation) is an efficient and safe sterilization technology that can extend the shelf life of pepper claws from 3-5 days to several months without adding preservatives.
The principle of a linear accelerator is not complicated: as long as the particles are given an accelerated electric field, the particles can move in a straight line. If a suitable alternating electric field is used, it is possible to continuously accelerate the particles.
However, the general alternating electric field has positive and negative, how to ensure that the speed of particles only increases but does not decrease? The scientists cleverly used a structure called a drift tube. When encountering a negative voltage, the particles happen to be able to hide in the drift tube, so that the particle beam (often called the beam) can brave in the straight segment!
Drift tube linear accelerator principle (source: Ver Wang Lishi modified)
Schematic diagram of a drift-tube linear accelerator (Source: William A. Barletta, USPAS)
The red-red-green-green cylinder in the picture above is the drift tube. Observant readers may notice that these cylinders become longer and longer because, as the energy of the particles increases and the speed increases, the distance traveled per unit of time also increases, so the length of the corresponding drift tube gradually lengthens.
Internal structure of a drift tube linear accelerator (Source: CERN)
According to this principle, if you want to get higher energy particles, the longer the length of the linear accelerator will be, which will significantly increase the cost of the accelerator. In order to avoid building excessively long accelerators, cyclotrons came into being.
Cyclotron – probably like a sliced cake
American physicist Lawrence opened his brain on the basis of linear accelerators and designed and manufactured the first cyclotron in 1932, which changed the linear orbit of accelerated particles into the form of spirals by applying an external magnetic field. This pioneering achievement solved the problem of inefficient acceleration caused by too long an accelerator, for which Lawrence won the Nobel Prize in Physics in 1939.
Lawrence with his cyclotron (Source: LBNL)
How does a cyclotron work? If you look at the following figure, you will have a preliminary understanding:
Schematic diagram of classical cyclotron (Source: Wang Lishi)
As shown, particles are injected from the center of the cyclotron and accelerated through the D-box slit. Since the particles accelerate each time they pass through the D-box slit, the radius of each circle becomes larger under the action of a fixed magnetic field, and the trajectory of motion forms a spiral orbit.
Subsequently, in order to meet the experimental requirements of nuclear physics and particle physics, the classical cyclotron underwent a series of upgrades, such as improving it into a fan-focused cyclotron with a helix angle. The "focusing" here can be understood in this way: we can think of the particle beam as a parallel beam of sunlight, which is concentrated into smaller and more dazzling points of light by the action of a lens (in this case, a magnetic field with a gradient), which can improve the collision efficiency of particles in the material.
Focusing structure with helix angle (Source: Mike Seidel, CAS, Cyclotrons, 2019)
Since the 70s of last century, as a reasonable extension of the fan-focused cyclotron, scientists have developed a separate fan cyclotron, which further increases the voltage required for acceleration and improves operating efficiency. Especially at that time, in order to meet the needs of heavy ion physics research, the separation fan cyclotron was a very good choice.
Looking at the separation fan cyclotron built by the Institute of Modern Physics of the Chinese Academy of Sciences in 1988, is it quite like four slices of cake that have been cut?
A separation fan cyclotron built by the Institute of Modern Physics of the Chinese Academy of Sciences. (Source: Institute of Modern Physics)
Ring accelerator – how do you direct particles to run circles?
In "The Three-Body Problem", teacher Liu Cixin also imagined such a circular accelerator:
This grand idea actually corresponds to the third type of accelerator we want to mention - the ring accelerator. Coincidentally, the great physicist Fermi once proposed the idea of a "global accelerator", which is also based on a ring accelerator.
Fermi's envisioned "Globe" accelerator (Source: Enrico Fermi: The Master Scientist)
In 1952, Leeuvenstone of Brookhaven National Laboratory (another accelerator boss) broke the focusing structure of traditional accelerators (the same focusing structure for a week, that is, weak focus synchrotron), alternately arranging magnets with focusing and defocusing (that is, focusing-defocusing-focusing-defocusing-... ), the result turned out to be unexpectedly good (the overall effect is focused)! But Levinstone was skeptical of this result and asked his colleague Colant to do the calculations. Cowland re-studied the results and proposed the principle of alternating gradient focusing, also known as the strong focusing principle, with Schneider and others. The principle of strong focusing and the principle of automatic phase stabilization later became the two cornerstones of modern accelerators.
The proton accelerator (Cosmotron) built by Brookhaven National Laboratory in the fifties of the last century, which is the world's first weakly focused synchrotron. (Source: BNL)
The Alternating Gradient Synchrotron, built by Brookhaven National Laboratory in the fifties of the last century, is the world's first strongly focused synchrotron. (Source: BNL)
A ring-type accelerator built by the Institute of Modern Physics - cooling storage ring (Source: Institute of Modern Physics)
The orbit of particles in a ring accelerator is much simpler than that of a cyclotron, which is a closed circular orbit. The particles are deflected by a magnetic element called a two-pole magnet to form a circular orbit and accelerate in a cavity similar to a linear accelerator. At the same time, in order to ensure that the beam will not "run apart" during the movement, the ring accelerator also uses elements such as quadrupole magnets to adjust its formation, so that the particle beam moves in an orderly manner under the command of the magnet.
Ring accelerator principle (Source: Wang Lishi)
Ring accelerators are able to better store accelerated particles and can effectively adjust the mass of particle beam clusters. Nowadays, most large accelerators use ring accelerators as the main part, and linear accelerators and cyclotrons are generally used as injectors for ring accelerators. For example, the world's largest particle accelerator - the Large Hadron Collider, as well as domestic synchrotron radiation light sources, spallation neutron sources, etc., their main bodies are ring accelerators.
Schematic diagram of the structure of the high-flow heavy ion accelerator (HIAF) built by the mainland in Huizhou, Guangdong. This accelerator is composed of linear accelerators and ring accelerators. (Source: Institute of Modern Physics)
Why do we need accelerators?
Why do we need accelerators so much?
Exploring the microscopic world has always been a long-cherished dream of scientists. Rutherford, the "father of nuclear physics," once said: "For a long time, I have hoped to obtain a large number of atoms and electrons for research, which have far more energy than particles from radioactive objects."
From the earliest experiments in which Lutherfoli bombarded gold leaf with α particles to open the door to atomic physics research, to the discovery of the Higgs particle by the Large Hadron Collider in 2013, accelerators are undoubtedly the most important and cutting-edge tools for scientists to carry out material science research. We can use it not only to discover new elementary particles, but also to synthesize new isotopes, and even to experience the feeling of a creator - to synthesize new elements!
The high-energy particle collider collides with a new understanding of the world. (Source: CERN)
The contribution of accelerators to basic research is not limited to the fields of nuclear and particle physics. Using the electromagnetic radiation emitted by charged particles when they move along a curved orbit under the action of electromagnetic fields, scientists have built synchrotron radiation light sources and carried out related experiments in materials science, structural biology and other disciplines. These experiments have helped humans better understand the structure of the microscopic world, and play an important role in analyzing the structure of avian influenza, Ebola, Zika and the new coronavirus, and screening antiviral drugs and antibodies.
Schematic diagram of the composition of Beijing high-energy synchrotron radiation light source. (Source: Institute of High Energy Physics)
In addition to basic scientific research, scientists can carry out a variety of applied research by accelerating different beams of charged particles or ions, such as electrons, heavy ions, etc. The application of accelerators has penetrated into many aspects of human life:
We can use accelerators to treat cancer. Heavy ion radiotherapy is currently internationally recognized advanced radiotherapy method, heavy ion beam is like a precision missile, can directly hit the lesion, concentrated release of energy, sanitize cancer cells.
A carbon ion therapy device developed by the Institute of Modern Physics, this accelerator is composed of a cyclotron and a ring accelerator. (Source: Institute of Modern Physics)
Even the production of delicious chocolate and baby diapers is related to accelerators! Synchrotron radiation light source technology can make the chocolate produced without frosting, improving the texture, taste and appearance of chocolate. Lawrence Berkeley Laboratory in the United States has used X-rays generated by advanced light sources (ALS) to help chemists at Dow Company understand and improve superabsorbent polymeric materials in diapers, thus changing the way parents care for their babies.
Accelerator applications are much more than that, security inspection systems, non-destructive testing equipment, irradiation mutagenesis breeding, sewage treatment, aerospace... are all closely related to accelerator technology. No wonder, the Trisolarans are so afraid of the accelerator of the Earthlings! If you can come to the site to visit, I believe your shock will not be worse than reading "The Three-Body Problem"!
Author: Wang Lishi Liu Fang
Author's affiliation: Institute of Modern Physics, Chinese Academy of Sciences
The reprinted content only represents the views of the author
It does not represent the position of the Institute of Physics of the Chinese Academy of Sciences
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Source: Science Compound