Design study of an antiproton trap for the GBAR experiment
Kyoung-Hun Yoo, Seok-Ho Moon, М. Chung, Dong Hwan Won, Kwan Hyung Park, Byungchan Lee, S.K. Kim, Eunhoon Lim, Eun-San Kim, Bong Ho Kim, Dirk Peter van der Werf, N. Kuroda, P. Pérez
Abstract The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN has been proposed to measure the gravitational acceleration of the ultracold antihydrogen atoms. This experiment produces antihydrogen ions through interactions between antiprotons and positronium atoms. Then, antihydrogen atoms are produced for the free-fall experiment after the photo-detachment of an excess positron from the cold antihydrogen ions. The energy of the antiproton beam before the positronium target chamber will be in the range of 1–10 keV. The cross-section for the reaction between the antiprotons and positroniums depends mainly on the energy of the antiprotons. Hence, to maximize the productivity of antihydrogen ions, a sufficient number of antiprotons should be provided with well-controlled energy. In this regard, an antiproton trap is considered to accumulate and slow down antiproton beams, and cool them utilizing the electron cooling technique. This trap is designed based on the Penning-Malmberg trap, which consists of a superconducting solenoid magnet and a series of ring electrodes including high-voltage electrodes to trap antiprotons. In addition, a set of extraction electrodes and optics for beam transport are used. Each electrode has been designed and optimized using the WARP PIC simulations. In this study, the design and simulation results of each trap component are presented.
Abstract The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN has been proposed to measure the gravitational acceleration of the ultracold antihydrogen atoms. This experiment produces antihydrogen ions through interactions between antiprotons and positronium atoms. Then, antihydrogen atoms are produced for the free-fall experiment after the photo-detachment of an excess positron from the cold antihydrogen ions. The energy of the antiproton beam before the positronium target chamber will be in the range of 1–10 keV. The cross-section for the reaction between the antiprotons and positroniums depends mainly on the energy of the antiprotons. Hence, to maximize the productivity of antihydrogen ions, a sufficient number of antiprotons should be provided with well-controlled energy. In this regard, an antiproton trap is considered to accumulate and slow down antiproton beams, and cool them utilizing the electron cooling technique. This trap is designed based on the Penning-Malmberg trap, which consists of a superconducting solenoid magnet and a series of ring electrodes including high-voltage electrodes to trap antiprotons. In addition, a set of extraction electrodes and optics for beam transport are used. Each electrode has been designed and optimized using the WARP PIC simulations. In this study, the design and simulation results of each trap component are presented.
The GBAR experiment (Gravitational Behaviour of Anti hydrogen at Rest) at CERN, aims to measure the free fall acceleration of ultracold neutral anti hydrogen atoms in the gravity field of the Earth. Anti hydrogen ions (one anti proton and two positrons) are produced in the interaction of antiprotons and positronium in two subsequent charge exchange reactions. These ions will be sympathetically cooled with Be + ions to less than 10 µK. The ultracold ions will then be photo-ionized just above threshold, and the free fall time over a known distance measured.
The formation of antihydrogen atoms from three-body collisions between positronium atoms and antiprotons plays a crucial role in the gravitational behavior of antihydrogen at rest (GBAR) experiment devoted to the measure of the action of gravity on antimatter. One of the main challenges facing this experiment is to find the means for increasing the production of antihydrogens which are produced by the reaction $\overline{p}+\text{Ps}\ensuremath{\rightarrow}\overline{\text{H}}+{e}^{\ensuremath{-}}$ with an extremely low rate. Along these lines, we investigate here the possibility to influence the collision process by using a laser field. A perturbative approach combining the Coulomb-Born approximation for modeling the charged-particle collision and a first-order perturbation theory for describing laser-atom interactions is employed to estimate laser-mediated charge-exchange cross sections. By carefully considering the laser specifications compatible with the experimental constraints, we present an extensive study of the influence of the laser parameters (laser wavelength and intensity) on the cross sections in the energy range of interest for GBAR. We show that under special irradiation conditions the rate of antihydrogen production may be significantly increased by the presence of the laser field.
The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN, situated on the antiproton decelerator ring (AD), is aimed at investigating the free fall of antihydrogen atoms prepared at rest, as suggested by J. Walz and Th. Hänsch [1]. This experiment employs two trapped ions: one is a Be+ ion cooled via laser, while the other, an Hbar+ ion, undergoes cooling through interactions with the Be+ ion (sympathetic cooling). We are developing a test-bed experiment designed to explore sympathetic cooling of a light ion using a cloud of laser-cooled heavy ions, mimicking the conditions anticipated in the GBAR project [2]. The experimental setup involves the pairing of 88Sr+ (laser-cooled ion) and 9Be+ (sympathetically cooled ion). The choice of these ions offers two advantages: the ability to optically address the 9Be+ ion for thermometry measurements and their mass ratio (88/9 ≈ 9.8) closely resembling that in the GBAR project (9/1). Preliminary experimental results [3] demonstrate sympathetic cooling of 9Be+ ions by laser-cooled 88Sr+ ions, forming a Coulomb crystal. Detection of sympathetically cooled ions relies on analyzing Coulomb crystal images, revealing dark areas where non-laser-cooled Be+ ions reside. Molecular dynamics simulations showed strong spatial segregation for both species, due to their high mass ratios [4]. However, the initial experiment did not include laser addressing of the 9Be+ ions for measuring cooling dynamics and control over the initial energy of Be+ ions. The next stage of the project therefore involves adding the 313nm laser for addressing Be+ ions and utilizing a 2-zone trap to control the initial energy of a single Be+ ion. This will enable us to measure for the first time the capture dynamics of a light ion by a Coulomb crystal and to follow its cooling over several decades (typically from 10000K to mK). We will compare these measurements with numerical simulations [4]. Initially, only Sr+ ions were employed to validate trapping and cooling conditions, characterize photon collection optics, and test ion transport protocols between trapping zones. A method utilizing Doppler recooling was developed to characterize the initial energy of Sr+ ions upon arrival in the target trap. Subsequently, using the Be+ ion cooling laser at 313 nm, it will be possible to cool a single Be+ ion and transport it with controlled kinetic energy to the second trapping zone, already loaded with a Sr+ Coulomb crystal. The kinetic energy loss of the Be+ ion is quantified by measuring laser-induced fluorescence rate at resonance, which produces no laser cooling or heating. Thermalization of the light ion via coulomb interactions will then be studied for different heavy-ion crystal temperatures, shapes and ion numbers.
L'expérience GBAR vise à mesurer l'accélération gravitationnelle de l'antimatière via la chute libre d'un atome d'antihydrogène. Il s'agit d'un test du principe d'équivalence faible qui, bien que bien vérifié à un niveau de précision de 10⁻¹⁵ pour la matière, n'a jamais fait l'objet de test expérimental concluant avec l'antimatière. La première étape consiste à produire un ion d'antihydrogène qui peut être refroidi à des températures de l'ordre du μK. Ensuite, la charge supplémentaire est photo-détachée et on peut observer la chute libre d'un atome d'antihydrogène neutre - "au repos" - sous l'effet seul de la gravité.Le principal défi de GBAR est la production de l'anti-ion à partir d'une double réaction d'échange de charges, d'antiprotons sur le positronium Ps (un état lié d'un positon et d'un électron). Dans la première réaction, un antiproton se lie à un positon du positronium pour former un antihydrogène de quelques keV. Lors de la seconde réaction, le nouvel atome d'antihydrogène réagit à nouveau avec un atome de positronium pour former un ion antihydrogène positif, composé de 2 positons et d'un antiproton.La première réaction, qui produit de l'antihydrogène à des états d'excitation faibles à partir de l'impact d'un faisceau d'antiprotons de 6 keV sur un nuage de positronium, est démontrée dans cette thèse. Le schéma spécifique de production « en vol » et le dispositif expérimental utilisé pour la prise de données sont décrits.D'après les calculs théoriques, l'énergie optimale pour la production d'anti-ions, compte tenu de la conjugaison des sections efficaces des deux réactions, est d'environ 6 keV. Pour décélérer davantage les antiprotons de 100 keV délivrés par l'anneau ELENA au CERN, GBAR a développé un décélérateur linéaire utilisant un « tube à dérive » pulsé. Cela permet d'obtenir une efficacité très élevée par rapport aux « dégradeurs » couramment utilisées. Ce dispositif a été largement retravaillé au cours de cette thèse. Les différents problèmes rencontrés et les solutions développées sont décrits.Une simulation complète de la ligne antiproton GBAR a été développée dans le logiciel SIMION et est présentée ici. L'optimisation de la ligne pour transporter le faisceau d'antiprotons décélérés jusqu'à la chambre de réaction où ils se mélangent au positronium est également détaillée. Enfin, la question de la section efficace pour la réaction de synthèse de l'antihydrogène est discutée.
Synthèse de l'antihydrogène par échange de charge en vol d'antiprotons décélérés avec le positronium pour l'expérience GBAR L'expérience GBAR vise à mesurer l'accélération gravitationnelle de l'antimatière via la chute libre d'un atome d'antihydrogène. Il s'agit d'un test du principe d'équivalence faible qui, bien que bien vérifié à un niveau de précision de 10⁻¹⁵ pour la matière, n'a jamais fait l'objet de test expérimental concluant avec l'antimatière. La première étape consiste à produire un ion d'antihydrogène qui peut être refroidi à des températures de l'ordre du μK. Ensuite, la charge supplémentaire est photo-détachée et on peut observer la chute libre d'un atome d'antihydrogène neutre - "au repos" - sous l'effet seul de la gravité.Le principal défi de GBAR est la production de l'anti-ion à partir d'une double réaction d'échange de charges, d'antiprotons sur le positronium Ps (un état lié d'un positon et d'un électron). Dans la première réaction, un antiproton se lie à un positon du positronium pour former un antihydrogène de quelques keV. Lors de la seconde réaction, le nouvel atome d'antihydrogène réagit à nouveau avec un atome de positronium pour former un ion antihydrogène positif, composé de 2 positons et d'un antiproton.La première réaction, qui produit de l'antihydrogène à des états d'excitation faibles à partir de l'impact d'un faisceau d'antiprotons de 6 keV sur un nuage de positronium, est démontrée dans cette thèse. Le schéma spécifique de production « en vol » et le dispositif expérimental utilisé pour la prise de données sont décrits.D'après les calculs théoriques, l'énergie optimale pour la production d'anti-ions, compte tenu de la conjugaison des sections efficaces des deux réactions, est d'environ 6 keV. Pour décélérer davantage les antiprotons de 100 keV délivrés par l'anneau ELENA au CERN, GBAR a développé un décélérateur linéaire utilisant un « tube à dérive » pulsé. Cela permet d'obtenir une efficacité très élevée par rapport aux « dégradeurs » couramment utilisées. Ce dispositif a été largement retravaillé au cours de cette thèse. Les différents problèmes rencontrés et les solutions développées sont décrits.Une simulation complète de la ligne antiproton GBAR a été développée dans le logiciel SIMION et est présentée ici. L'optimisation de la ligne pour transporter le faisceau d'antiprotons décélérés jusqu'à la chambre de réaction où ils se mélangent au positronium est également détaillée. Enfin, la question de la section efficace pour la réaction de synthèse de l'antihydrogène est discutée.
This work reports on experiments in which antihydrogen atoms have been produced in cryogenic Penning traps from antiproton and positron plasmas by two different methods and on experiments that have been carried out subsequently in order to investigate the antihydrogen atoms. By the first method antihydrogen atoms have been formed during the process of positron cooling of antiprotons in so called nested Penning traps and detected via a field ionization method. A linear dependence of the number of detected antihydrogen atoms on the number of positrons has been found. A measurement of the state distribution has revealed that the antihydrogen atoms are formed in highly excited states. This suggests along with the high production rate that the antihydrogen atoms are formed by three-body recombination processes and subsequent collisional deexcitations. However current theory cannot yet account for the measured state distribution. Typical radii of the detected antihydrogen atoms lie in the range between 0.4 µm and 0.15 µm. The deepest bound antihydrogen atoms have radii below 0.1 µm. Antihydrogen atoms with that size have chaotic positron orbits so that for the first time antihydrogen atoms have been detected that cannot be described by the GCA-model. The kinetic energy of the weakest bound antihydrogen atoms has been measured to about 200 meV, which corresponds to an antihydrogen velocity of approximately 6200 m/s. A simple model suggests that these atoms are formed from only one deexcitation collision and methods that might lead to a decrease of the antihydrogen velocity are presented. By the second method antihydrogen atoms have been synthesized in charge-exchange processes. Lasers are used to produce a Rydberg cesium beam within the cryogenic Penning trap that collides with trapped positrons so that Rydberg positronium atoms are formed via charge-exchange reactions. Due to their charge neutrality the Rydberg positronium atoms are free to leave the positron trapping region. The Rydberg positronium atoms that collide with nearby stored antiprotons form antihydrogen atoms in charge-exchange reactions. So far, 14 +/- 4 antihydrogen atoms have been detected background-free via a field-ionization method. The antihydrogen atoms produced via the two-step charge-exchange mechanism are expected to have a temperature of 4.2 K, the temperature of the antiprotons from which they are formed. A method is proposed by which the antihydrogen temperature can be determined with an accuracy of better than 1 K from a measurement of the time delay between antihydrogen annihilation events and the laser pulse that initiates the antihydrogen production via the production of Rydberg cesium atoms. First experiments have been carried out during the last days of the 2004 beam time, but the number of detected antihydrogen annihilations has been too low for a determination of the antihydrogen temperature. Trapped antiprotons have been directly exposed to laser light delivered by a Titanium:Sapphire laser in order to investigate if the laser light causes any loss on the trapped antiprotons. Experiments have shown that no extra loss occurs for laser powers of less than 590 mW. This is an important result against the background of the future plan to confine antihydrogen atoms in a combined Penning-Ioffe trap and then to carry out laser spectroscopy on these atoms, since it reveals that laser light does not cause an increase of the pressure in the trapping region to the extend that annihilations with the background gas become noticeable. The ATRAP Collaboration plans to precisely investigate antihydrogen atoms. The ultimate goal is to test the CPT-theorem by a high precision measurement of the 1S-2S transition of antihydrogen and a comparison with the precisely known value of the corresponding transition in hydrogen. This thesis presents the achievement of the first step towards this challenging goal: the production of cold antihydrogen itself.