Atomic hydrogen beam

Purpose
The reliable operation of the antihydrogen hyperfine spectroscopy apparatus is a basic prerequisite for the proposed CPT test [internal link] and it requires a careful characterization of every component of the experiment. The use of antihydrogen for this task, however, has to be restricted to those parts, where other means are not applicable (e.g. the annihilation detector [internal link to hodoscope]…). For instance, the spin–flip cavity and the superconducting sextupole magnet can be tested just as well with conventional atomic hydrogen. Therefore a source of cold and polarized atomic hydrogen had been constructed and tested at SMI. It was shipped to CERN late 2013 and successfully operated during large parts of 2014. This project resulted in a detailed characterization of the spin–flip cavity and the superconducting sextupole magnet and a high precision verification (to a few parts per billion) of the apparatus and the measurement principle.
 
Principle
The hyperfine transition frequency of antihydrogen (hydrogen) will be determined by a so-called magnetic resonance measurement. This technique was invented by I. I. Rabi and improved by N. Ramsey (both Nobel prize winners) and requires the following four main components: (i) a beam of spin polarized particles, (ii) an oscillating magnetic field to induce spin-flips, (iii) a magnetic field gradient for spatial separation of spin states, and (iv) a detector for monitoring the amount of beam in the selected spin state. If the frequency of the driving magnetic is close to the transition frequency of a spin flip the amount of beam arriving at the detector is changed. By variation of the driving frequency a well-understood resonance curve can be recorded from which the transition frequency can be extracted with high precision.
 
A noteworthy property of the antihydrogen beam line, distinguishing it from conventional magnetic resonance experiments, is that it accepts a much stronger diverging beam in order to make use of the largest possible fraction of produced antihydrogen atoms.

The setup
The atomic hydrogen setup uses a source of polarized atomic hydrogen and a hydrogen detector in conjunction with the spin flip driving cavity and the spin state selecting magnetic field gradient of the antihydrogen experiment to form a Rabi-type beam experiment as described above.
Firstly, molecular hydrogen is generated via electrolysis from water, then it gets dissociated in a microwave driven discharge plasma to form atomic hydrogen. From this plasma atoms effuse into the vacuum of the beam line through a PTFE tubing kept at cryogenic temperature. This cooling mechanism results in a beam temperature of 50K to 100K, which is the anticipated temperature of the antihydrogen beam emerging from the CUSP trap. At this point the beam is a mixture of all spin states. A polarization is achieved by passage through a doublet of permanent sextupole magnets, where a certain spin component is selected. Furthermore, a chopper modulates the beam (tuning fork chopper: 180Hz, 50% duty cycle). This allows for a lock-in amplification of the detector signal. Next, the beam passes the microwave cavity which is optimized for ~1.42GHz, the hyperfine transition frequency of hydrogen. Then, the magnetic field gradients of the superconducting sextupole magnet follows and defocuses hydrogen atoms with a changed spin state while it refocuses those with unchanged spin states onto the detector. There, electrons from a filament are used to ionize the hydrogen atoms. The resulting protons (= ions of hydrogen) are then electrostatically extracted and guided through a quadrupole mass spectrometer, where all other ion species produced from residual gas are removed since the posses a different charge-to-mass ratio. The protons impinge on a channeltron for amplification and single-event counting.
 
Current Status
The atomic hydrogen source and the detector had been constructed, assembled, and tested at the SMI in Vienna. In October 2013 the setup was transported to CERN and upgraded with the permanent sextupole magnets to become a source of polarized atomic hydrogen.
Early 2014 the spin flip driving cavity and superconducting sextupole magnet of the antihydrogen experiment have been inserted between hydrogen source and detector. Detailed characterization of all components followed and in April 2014 the first hyperfine transitions could be observed with this setup using the Earth’s magnetic field as a guiding field. Afterwards a magnetic shielding was placed around the cavity and a pair of Helmholtz coils produced a homogeneous magnetic guiding field. An extensive measurement program has been completed in this configuration by September 2014. Afterwards the setup has been disassembled and the spin flip cavity and superconducting sextupole magnet have been installed at ASACUSA’s antihydrogen experiment at the antiproton decelerator facility.
The hydrogen beam setup currently undergoes upgrades and shall be operational again mid 2015.