It is a basic and unavoidable fact in the antimatter business that in order to produce antihydrogen, antiprotons and positrons must be mixed. So, ALPHA must have the ability to confine and manipulate charged plasmas with reasonable efficiency and at cryogenic temperatures to boot!
This is accomplished in ALPHA through the use of Penning traps, a type of trap commonly used in plasma physics experiments to confine charged plasmas. Charge is in fact the difference, and indeed the dilemma faced when attempting to trap antihydrogen. Because antihydrogen is neutral, it cannot be held in a traditional Penning trap. This is where ALPHA’s unique magnetic trap comes in.
Plasmas in a Penning trap are axially confined by quadratic electric potentials. In a perfect Penning trap, the potentials are produced by the application of voltages to equipotential surfaces shaped like hyperboloids of rotation. Radial, or transverse, confinement of particles in the trap is provided by a solenoidal magnetic field applied along the trap axis. Charged particles in the trap travel in the direction of the magnetic field lines and hence cannot escape outwards, or perpendicular to the field.
ALPHA uses a Penning trap variation called a Penning-Malmberg trap. The difference is that the electric potentials in a Penning-Malmberg trap are not perfectly quadratic. The potentials in ALPHA are produced by the application of DC voltages to a stack of hollow cylindrical electrodes, as opposed to hyperboidal. It’s true that quadratic potentials would allow for a more straightforward analytical interpretation of particle motion in the trap. However, the Penning-Malmberg scheme in ALPHA is actually extremely effective in confining particles in sufficiently well shaped (if not perfectly quadratic) and is much more technically feasible.
Figure 1. An illustration of a typical Penning Malmberg trap. Voltages are applied to the two outer electrodes, forming a potential well.
(Figure from Charlton et. al, "Antihydrogen for precision tests in Physics" 2008.)
In the trap, particles are subjected to the Lorenz force. The Lorenz force deflects particles perpendicular to the magnetic field, causing them to spiral around magnetic field lines. The spiraling particles are also accelerating, and hence emitting radiation. This effect is called cyclotron radiation. Cyclotron radiation is important in ALPHA because the emission of radiation corresponds to the cooling of the particles themselves. For positrons and electrons, this effect works fantastically: cyclotron radiation effectively cools the particles in a matter of seconds. Antiprotons, however, are far more massive. It would take antiprotons over 300 years of sitting in a 1 Tesla magnetic field to cool through cyclotron radiation alone! Needless to say, waiting so long is out of the question. Instead, ALPHA uses several additional tricks to bring antiprotons down to temperatures suitable for antihydrogen formation.
The ALPHA electrode stack is comprised of 35 electrodes. Made from aluminum and plated with gold, the electrodes are 36.6mm to 44.6mm in diameter and have varying lengths. The trap electrodes are mounted inside of a vacuum chamber surrounded by a liquid helium cryostat. Because they are individually wired and insulated from one another, the voltage applied to each electrode can be controlled. This allows the shape of the potentials inside of the trap to be known precisely.
Figure 2. A schematic of the ALPHA trap electrodes.
Three of these electrodes are outfitted for high voltage and are used in the capture of antiprotons and positrons. Additionally, two electrodes are azimuthally segmented. A sinusoidal voltage can be applied to each segment, where each voltage is phase shifted from the voltage applied to the segment before it. This generates an electric field, which applies a torque on the confined plasma and causes it to compress. In ALPHA, this technique is nicknamed the ‘rotating wall,’ and is used to compress both positron and antiproton plasmas. For antihydrogen formation, the colder, denser and smaller a plasma is, the better.
The trap electrodes are divided into three groups, each playing a different role in the process of producing and trapping antihydrogen: the catching trap, the positron electrodes, and the mixing trap.
Figure 3. A Photo of the edge of the electrode stack
The catching trap is the region in which antiprotons are extracted from CERN’s Antiproton Decelerator (AD). During the time that ALPHA has access to the AD beam, one ‘shot’ of approximately 3x10^7 antiprotons at an energy of 530 keV is delivered to the apparatus every 100 seconds. The particles pass from the AD through a thin foil degrader and into the ALPHA catching trap at energies distributed from 0 to around 500 keV. At this point, a voltage is applied to one of the electrodes, creating a potential barrier. Particles at higher energies escape and annihilate, but the lower energy particles cannot overcome this barrier. When second high voltage is quickly applied to a nearby electrode, the lower energy antiprotons find themselves caught in a Penning-Mamlberg trap at around 4 keV.
Even though they can now be trapped, the antiprotons are still too energetic for antihydrogen formation. The antiprotons are held in the catching trap region and are subjected to electron cooling (link) and evaporative cooling, as well as compression by the rotating wall, before they are finally cold and dense enough for antihydrogen formation.
On the positron side of the electrode stack, positrons are transferred out of the accumulator as a plasma and are held in yet another Penning-Malmberg trap. Any remnant ions are eliminated by quickly lowering of one of the potential walls of the trap. The higher energy positrons escape faster than the ions, and are recaptured in an adjacent potential well. Here, the positron plasma is prepared for mixing: the particles cool radiatively and are compressed with the rotating wall.
The central region of the electrode stack is where the ‘magic’ happens. These electrodes comprise the mixing trap: it is here that the antiprotons and positrons finally meet. The plasmas are transferred into this region and are held in potential wells until the positrons are injected into the antiproton plasma via a technique called autoresonant injection. The mixing trap electrodes are surrounded by the octopole magnet and the mirror coil. These electrodes are especially thin, so that antihydrogen atoms formed in the trap can get as close as possible to the octopole magnet with out annilihating on the electrode surface. Once the magnets are energized, the penning trap is superimposed with the magnetic minimum atom trap and neutral antihydrogen can be successfully trapped.
Figure 4. (Left) a. An illustration of the inner mixing trap electrodes surrounded by the octopole and mirror coil magnets as well as the Silicon detector. b. A plot of the on axis electric potential in the mixing trap region. Figure from: Andresen et al., "Trapped Antihydrogen" Nature, 2010.