Positrons from a radioactive source are slowed to electron-volt energies and accumulated and stored in a trap which uses a magnetic Geld for radial confinement and an electrostatic well for axial confinement. The positrons lose energy and become trapped through inelastic collisions with nitrogen molecules introduced into the trap for this purpose. The trap has three stages with progressively lower nitrogen pressure. It is found that the trapping in each stage is due primarily to electronic excitation of the nitrogen molecules, with the energy loss being approximately 9 eV per collision. Positronium formation is believed to be the dominant loss mechanism during the trapping process. Trapping efficiencies of greater than 25% have been achieved. Using a 150-mCi ' Na source, a maximum stored number of -1 X 10' positrons have been stored with a lifetime of 40 s, limited by the annihilation on the nitrogen gas at a pressure of 2X 10 Torr. The positrons cool to room temperature in a few seconds via rotational and momentum transfer collisions with the nitrogen.PACS number(s): 34.90.+ q, 34.50. Gb, 52.55.Mg, 52.20.Hv

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3  ×  1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.

Advances in positron trapping techniques have led to room-temperature plasmas of 10' positrons with lifetimes of lo3 s. Improvements in plasma manipulation and diagnostic methods make possible a variety of new experiments, including studies just being initiated of electronpositron plasmas. The large numbers of confined positrons have also opened up a new area of positron annihilation research, in which the annihilation cross sections for positrons with a variety of molecules have been measured, as well as the energy spread of the resulting gamma rays. Such measurements are of interest for fundamental physics and for the modeling of astrophysical plasmas.

Positron annihilation on many molecular species occurs via capture into vibrational Feshbach resonances. The study of the downshifts in the energy of these resonances from the vibrational modes in the molecule using a tunable, high-resolution positron beam provides a measure of the positron-molecule binding energy. Regression analysis on data for 30 molecules is used to identify the molecular properties that affect these binding energies. One parameterization that fits the data well involves a linear combination of the molecular dipole polarizability, the permanent dipole moment and the number of π bonds in aromatic molecules. The predictions of this empirical model are compared with those from positron-molecule binding energy calculations. They are also tested in cases where other experimental evidence indicates that molecules do and do not bind positrons. Promising candidate molecules for further experimental and theoretical investigation are discussed.

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