Precise spectroscopic measurements of anti-hydrogen at the ALPHA experiment are hindered by small numbers of cold anti-atoms. This thesis describes a cooling technique for positron plasmas which can be used to increase the number of trappable anti-hydrogen atoms. The technique builds on previous work which allows control of spontaneous emission via the Purcell Effect. Our implementation incorporates a novel microwave resonator into an existing Penning trap to enhance spontaneous emission. Preliminary data suggests that temperatures and cooling rates for these plasmas can be improved by at least a factor of 10. Eventually this work could result in an order of magnitude increase in anti-hydrogen production at ALPHA.
Precise spectroscopic measurements of anti-hydrogen at the ALPHA experiment are hindered by small numbers of cold anti-atoms. This thesis describes a cooling technique for positron plasmas which can be used to increase the number of trappable anti-hydrogen atoms. The technique builds on previous work which allows control of spontaneous emission via the Purcell Effect. Our implementation incorporates a novel microwave resonator into an existing Penning trap to enhance spontaneous emission. Preliminary data suggests that temperatures and cooling rates for these plasmas can be improved by at least a factor of 10. Eventually this work could result in an order of magnitude increase in anti-hydrogen production at ALPHA.
Evaporative cooling has proven to be an invaluable technique in atomic physics, allowing for the study of effects such as Bose-Einstein condensation. One main topic of this thesis is the first application of evaporative cooling to cold non-neutral plasmas stored in an ion trap. We (the ALPHA collaboration) have achieved cooling of a cloud of antiprotons to a temperature as low as 9 K, two orders of magnitude lowerthan ever directly measured previously. The measurements are well-described by appropriate rate equations for the temperature and number of particles. The technique has direct application to the ongoing attempts to produce trapped samples of antihydrogen. In these experiments the maximum trap depths are ex tremely shallow (~0.6 K for ground state atoms), and careful control of the trapped antiprotons and positrons used to form the (anti)atoms is essential to succes. Since 2006 powerful tools to diagnose and manipulate the antiproton and positron plasmas in the ALPHA apparatus have been developed and used in attempts to trap antihydrogen. These efforts are the second main topic of this thesis.
Answering the question of why we live in a matter-dominated universe is of great interest to contemporary physicists, as the Standard Model of Particle Physics predicts that matter and an-timatter should only ever be produced in equal parts. Antihydrogen is a good candidate for searches for asymmetries between matter and antimatter as it is the simplest antimatter bound state, and it has an extremely well-understood matter counterpart: the hydrogen atom. The AL-PHA collaboration at CERN can now routinely trap several hundred antihydrogen atoms in a magnetic trap, allowing precise measurements of the fundamental properties of antihydrogen.ALPHA currently traps around 20 antihydrogen atoms every few minutes, so accumulat-ing enough antiatoms to perform precision measurements can take many hours. Increasing this trapping rate would allow for faster or more precise measurements. Simulations and experimen-tal data show that there is a strong correlation between the temperature of the positron plasma used in antihydrogen formation and the trapping rate that can be achieved. This thesis describes work towards using laser-cooled beryllium ions to obtain colder positron plasmas in the ALPHA trapping apparatus. Singly-charged beryllium ions are liberated from the surface of a solid beryl-lium target inside the apparatus via laser ablation, where the ions can be trapped, laser-cooled and mixed with positron plasmas. By mixing the ions and positrons together, they can exchange energy, and the ions can provide cooling to the positrons. Careful tuning of laser and trap pa-rameters, as well as use of other techniques, has allowed for the successful sympathetic cooling of positron plasmas in ALPHA.
Antihydrogen, a positron bound to an antiproton, is the simplest anti-atom. Its structure and properties are expected to mirror those of the hydrogen atom. Prospects for precision comparisons of the two, as tests of fundamental symmetries, are driving a vibrant programme of research. In this regard, a limiting factor in most experiments is the availability of large numbers of cold ground state antihydrogen atoms. Here, we describe how an improved synthesis process results in a maximum rate of 10.5 ± 0.6 atoms trapped and detected per cycle, corresponding to more than an order of magnitude improvement over previous work. Additionally, we demonstrate how detailed control of electron, positron and antiproton plasmas enables repeated formation and trapping of antihydrogen atoms, with the simultaneous retention of atoms produced in previous cycles. We report a record of 54 detected annihilation events from a single release of the trapped anti-atoms accumulated from five consecutive cycles.Antihydrogen studies are important in testing the fundamental principles of physics but producing antihydrogen in large amounts is challenging. Here the authors demonstrate an efficient and high-precision method for trapping and stacking antihydrogen by using controlled plasma.
One of the best ways to study antimatter is to investigate antihydrogen, the bound state of an antiproton and a positron. Antihydrogen atoms do not exist naturally and must be synthesized in the lab by merging carefully-prepared plasmas of positrons and antiprotons. If the atoms are created in a magnetic trap like the one used by the ALPHA experiment at CERN, then a fraction of the coldest atoms remain trapped, while the rest escape and annihilate on the trap walls. The trapped atoms may then be probed using microwaves or lasers to make high-precision comparisons with hydrogen. Increasing the trapping rate would allow us to perform precision measurements on antihydrogen in a shorter period of time and with better systematics. Particle simulations indicate that by sympathetically cooling positrons using laser-cooled beryllium ions, we have the ability to improve the antihydrogen trapping rate by up to two orders of magnitude. This thesis describes the effort to design and qualify a beryllium ion source that is compatible with the extreme environment of the ALPHA trapping apparatus. To produce the ions, pulsed laser ablation of a beryllium target is investigated and the ion plume is characterized. By carefully choosing the laser parameters, a plume with a suitable number of ions and kinetic energy distribution can be created for subsequent trapping and laser-cooling in a Penning trap. This thesis reports the successful demonstration of the ion source, in particular its compatibility with the requirements of the ALPHA experiment. The ion source has been installed into the main apparatus at ALPHA and is expected to be commissioned early next year.
Antihydrogen, the bound state of a positron and an antiproton, is a uniquely well-suited system for testing fundamental symmetries between matter and antimatter. The Antihydrogen Laser Physics Apparatus (ALPHA collaboration) synthesises antihydrogen atoms by slowly merging cold non-neutral positron and antiproton plasmas and traps the antiatoms in a magnetic minimum trap. While trapping efficiency has improved since its first demonstration in 2010, it plateaued after 2017.In ALPHA’s experimental conditions, antihydrogen is predominantly formed via three-body recombination, which has been theoretically and experimentally shown to strongly depend on the temperature of the positron plasma: lower positron temperatures yield higher antihydrogen trapping rates. In the ALPHA-2 trap, positrons alone reach a lower temperature limit of 15−20 K. The implementation of sympathetic cooling of positrons through collisions with laser-cooled Be+ions enabled stable lowering of these temperatures to below 10 K. This thesis describes the work towards antihydrogen synthesis with positrons prepared using this cooling method. A novel ion plasma preparation technique was developed to improve reproducibility, and a sophisticated control system with automated beam steering was implemented to ensure long-term operational stability. Moreover, the first demonstration of sympathetic cooling of positrons to < 10 K in a radially asymmetric magnetic field was carried out. Culminating in the first Be+-assisted antihydrogen synthesis and trapping. Careful optimisation of this technique resulted in a near eightfold increase in the antihydrogen trapping rate, and allowed for systematic studies of three-body recombination that were not possible before its implementation. Overall, this technique represents a paradigm shift for the ALPHA physics programme and provides a deeper understanding of antihydrogen synthesis, opening new avenues for precision studies of antimatter.
We propose a usage of microwave radiation in a magnetic trap for improving the cooling and trapping of cold antihydrogen atoms which are initially produced in high magnetic moment states. Inducing transitions toward lower magnetic moments near the turning points of the atom in the trap, followed by spontaneous emission, should enhance the number of trappable atoms. We present results of simulations based on a typical experimental condition of the antihydrogen experiments at CERN. This technique should also be applicable to other trapped high magnetic moment Rydberg atoms.