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High densities of ultra cold positrons are required for applications such as positronium production, scattering processes with atoms, surface analysis, cooling of highly charged ions and antihydrogen production. At the University of Aarhus, Denmark, an accelerator based slow positron source delivers about 5 × 104 positrons within a 10 ns bunch at a repetition rate of 10 Hz. The energy spread is below 1 eV and the beam diameter is about 1 mm. The positron bunches shall be injected into a 6-T Penning trap at the temperature of liquid helium. The bunches can be captured at nearly 100% efficiency by a fast time variation of the trap potential. The cyclotron motion cools down by synchrotron radiation with a time constant of 80 ms. The axial motion can be cooled by coupling to the radial motion or by resistive cooling in a tuned circuit. By stacking of 100 pulses about 5 × 106 positrons can be accumulated within 10 s. After this time most of the positrons have cooled down sufficiently that the trapping cycle can be started again. At the anticipated accumulation rate a positron plasma at the space charge limit should be obtainable within 1 h.

The methods to produce positrons and positronium are shortly reviewed. Different sources for positrons are compared. Implantation, diffusion and emission of positrons and some typical moderators are discussed. Special consideration is given to accelerator based sources, to the design of conversion targets, to the conversion efficiency and to the problems of thermal power dissipation. Finally the possible accumulation of large numbers of positrons by using pulsed accelerators is outlined.

Thegj factor measurement of hydrogenic ions in the 1s ground state is with an expected accuracy of 10−7 a sensitive test of bound state QED. We expect to determine the deviations from the free electron value, caused by relativistic and radiative corrections, up to the orderα/4π(Zα)2 with an accuracy of 1%. As a first step, light ions like C5+ will be investigated. Later on, heavier hydrogenic ions up to U91+ will be examined using the accelerator facilities at GSI in Darmstadt.

Electric dipole transitions in the microwave range have been induced between the fine-structure levels of positronium in the excited staten=2. As an indication of the transitions, we used the increase in Lyman-α radiation when the metastable 23S1-level is depopulated. The results for the transitions 23S1→23P0,1,2 areΝ 0=18499.65±1.20±4.00 MHz,Ν 1=13012.42 ±0.65±1.54 MHz andΝ 2=8624.38±0.54±1.40 MHz. The first error is statistical and the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound state QED-calculations have been extended to the orderR t8 α 4lnα −1. The not yet completely calculated orderR t8 α 4 is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weak magnetic field, we were able to observe the transition 23S1→21P1 which is strictly forbidden byC-invariance in zero field. Our result, corrected for Zeeman- and motional Starkeffect, isΝ 3=11180.0±5.0±4.0 MHz. An upper limit for theC-violating matrix element of $$|\langle 2^1 P_1 |H_ \not\subset |2^3 P_1 \rangle |< 76$$ MHz could be deduced. Our experiment used moderated slow positrons from the bremsstrahlung and pair production of a pulsed electron linear accelerator (TEPOS facility at the university of Giessen).

The slow positron facility TEPOS at the Giessen electron LINAC (36 MeV, 120 µA) has been used to produce an intense beam of moderated positrons which is magnetically guided over a distance of 9 m. At a transportation energy of 100 eV about 106 slow e+/s could be extracted out of the magnetic field (0.01 T) and have been electrostatically focussed inside a microwave guide. A small fraction of the positrons form positronium in the excited staten=2. The spontaneous emission of Lyman-α photons (λ=243 nm) from the 2P-states is observed by a photomultiplier. Microwave induced fine-structure transitions 23S1©23P2,1,0 have been observed at 8617(2), 13010(3) and 18494(2) MHz by an increase of the Lyman-α counting rate. The present errors take into consideration only statistical contributions; systematic errors in the same order of magnitude may originate from frequency dependent variations of the microwave power. The observed linewidth exceeds the natural linewidth of 50 MHz by Doppler-effect and power broadening. Values around 100 MHz could be reached at the lowest applied power levels.

In view of a future antihydrogen programme at CERN, the options for producing MeV antiprotons are revisited. The current limitations, operational performances and foreseen improvements are detailed. An alternative scheme using a dedicated machine for production and deceleration is also discussed.

During the last 6 years it has been demonstrated that electro-produced intense beams of slow positrons are possible. High energy electrons from an accelerator generate bremsstrahlung in a thick conversion target of high element number Z. The photons produce electronpositron pairs and a small fraction of the positrons may be moderated to thermal energies. A review is given of the existing slow positron beam lines using this technique. At accelerator energies of 100 MeV total conversion efficiencies of several slow positrons per 106 primary electrons have been obtained, resulting in average intensities of several 109 slow positrons per second or more than 105 slow positrons in pulses having a duration of a few ns. A further increase in intensity by at least one order of magnitude seems possible at higher accelerator energies.