Published on Apr 02, 2024
Antimatter rockets are what the majority of people think about when talking of rockets for the future. This is hardly surprising as it is such an attractive word for the writers of science fiction.
It is, however, not only interesting in the realm of science fiction. Make no mistake; antimatter is real. Small amounts, in the order of nanograms, are produced at special facilities every year. It is also the most expensive substance of Earth; in 1999 the estimated cost for 1 gram of antimatter was about $62.5 trillion.
The reason it is so attractive for propulsion is the energy density that it possesses. Consider that the ideal energy density for chemical reactions is 1 x 107 (10^7) J/kg, for nuclear fission it is 8 x 1013 (10^13) J/kg and for nuclear fusion it is 3 x 1014 (10^14) J/kg, but for the matter-antimatter annihilation it is 9 x 1016 (10^16) J/kg. This is 1010 (10 billion) times that of conventional chemical propellants.
This represents the highest energy release per unit mass of any known reaction in physics. The reason for this is that the annihilation is the complete conversion of matter into energy governed by Einstein's famous equation E=mc2, rather than just the part conversion that occurs in fission and fusion.
Antimatter is exactly what you might think it is -- the opposite of normal matter, of which the majority of our universe is made. Until just recently, the presence of antimatter in our universe was considered to be only theoretical. In 1928, British physicist Paul A.M. Dirac revised Einstein's famous equation E=mc2. Dirac said that Einstein didn't consider that the "m" in the equation -- mass -- could have negative properties as well as positive. Dirac's equation (E = + or - mc2) allowed for the existence of anti-particles in our universe. Scientists have since proven that several anti-particles exist.
These anti-particles are, literally, mirror images of normal matter. Each anti-particle has the same mass as its corresponding particle, but the electrical charges are reversed
When antimatter comes into contact with normal matter, these equal but opposite particles collide to produce an explosion emitting pure radiation, which travels out of the point of the explosion at the speed of light. Both particles that created the explosion are completely annihilated, leaving behind other subatomic particles. The explosion that occurs when antimatter and matter interact transfers the entire mass of both objects into energy. Scientists believe that this energy is more powerful than any that can be generated by other propulsion methods.
The problem with developing antimatter propulsion is that there is a lack of antimatter existing in the universe. If there were equal amounts of matter and antimatter, we would likely see these reactions around us. Since antimatter doesn't exist around us, we don't see the light that would result from it colliding with matter.
It is possible that particles outnumbered anti-particles at the time of the Big Bang. As stated above, the collision of particles and anti-particles destroys both. And because there may have been more particles in the universe to start with, those are all that's left. There may be no naturally-existing anti-particles in our universe today. However, scientists discovered a possible deposit of antimatter near the center of the galaxy in 1977. If that does exist, it would mean that antimatter exists naturally, and the need to make our own antimatter would be eliminated.
The Antiproton Decelerator is a very special machine compared to what already exists at CERN and other laboratories around the world. So far, an "antiparticle factory" consisted of a chain of several accelerators, each one performing one of the steps needed to produce antiparticles. The CERN antiproton complex is a very good example of this.
At the end of the 70's CERN built an antiproton source called the Antiproton Accumulator (AA). Its task was to produce and accumulate high-energy antiprotons to feed into the SPS in order to transform it into a "proton-antiproton collider". As soon as antiprotons became available, physicists realized how much could be learned by using them at low energy, so CERN decided to build a new machine: LEAR, the Low Energy Antiproton Ring. Antiprotons accumulated in the AA were extracted, decelerated in the PS and then injected into LEAR for further deceleration. In 1986 a second ring, the Antiproton Collector (AC), was built around the existing AA in order to improve the antiproton production rate by a factor of 10.
The AC is now being transformed into the AD, which will perform all the tasks that the AC, AA, PS and LEAR used to do with antiprotons, i.e. produce, collect, cool, decelerate and eventually extract them to the experiments.
Antiparticles have to be created from energy (remember: E = mc2). This energy is obtained with protons that have been previously accelerated in the PS. These protons are smashed into a block of metal, called a target. We use Copper or Iridium targets mainly because they are easy to cool. Then, the abrupt stopping of such energetic particles releases a huge amount of energy into a small volume, heating it up to such temperatures that matter-antimatter particles are spontaneously created. In about one collision out of a million, an antiproton-proton pair is formed. But given the fact that about 10 trillion protons hit the target (about once per minute), this still makes a good 10 million antiprotons heading towards the AD.
The newly created antiprotons behave like a bunch of wild kids; they are produced almost at the speed of light, but not all of them have exactly the same energy (this is called "energy spread"). Moreover, they run randomly in all directions, also trying to break out 'sideways' ("transverse oscillations"). Bending and focusing magnets make sure they stay on the right track, in the middle of the vacuum pipe, while they begin to race around in the ring.
At each turn, the strong electric fields inside the radio-frequency cavities begin to decelerate the antiprotons. Unfortunately, this deceleration increases the size of their transverse oscillations: if nothing is done to cure that, all antiprotons are lost when they eventually collide with the vacuum pipe. To avoid that, two methods have been invented: 'stochastic' and 'electron cooling'. Stochastic (or 'random') cooling works best at high speeds (around the speed of light, c), and electron cooling works better at low speed (still fast, but only 10-30 % of c). Their goal is to decrease energy spread and transverse oscillations of the antiproton beam.
Finally, when the antiparticles speed is down to about 10% of the speed of light, the antiprotons squeezed group (called a "bunch") is ready to be ejected. One "deceleration cycle" is over: it has lasted about one minute.
A strong 'kicker' magnet is fired in less than a millionth of a second, and at the next turn, all antiprotons are following a new path, which leads them into the beam pipes of the extraction line. There, additional dipole and quadrupole magnets steer the beam into one of the three experiments.
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