Abstract. An ultra cold electron source was developed for the storage ring TSR in Heidelberg to study electron-ion interactions with high energy resolution. The heart of the source is a GaAs photocathode which emits electrons with energy spreads below 10 meV. Photoemitted electrons are extracted in the space charge mode and then undergo adiabatic magnetic expansion and adiabatic acceleration to obtain an ultra cold electron beam which is overlapped with the stored ion beam in a straight section of the ring. In the first recombination measurements on HD + unprecedented energy resolution for low-energy resonances was found, demonstrating a transverse and longitudinal temperature of the electron beam of about 0.5 meV and 0.02 meV, respectively.
IntroductionMerged beam experiments at storage rings were used successfully to study electron-ion inelastic collisions with high energy resolution (see, e.g., [1]). While the typical energies of the stored ions and electrons are about a few MeV/u and keV, respectively, collision energies below a milli-eV can be realized by small longitudinal velocity detuning between the ion beam and the magnetically guided electron beam. In such experiments the ion velocity is well defined and the collision energy spread is mainly given by the electron energy distribution, being strongly anisotropic in the co-moving reference frame [2]. At zero detuning energy and for typical electron densities n e ∼ 10 5 -10 7 cm −3 , the longitudinal temperature kT || of the electrons is mainly defined by potential energy relaxation: kT || ∼ Cn 1/3 e 2 /4πε 0 . Here the parameter C describes the quality of the acceleration and can be suppressed by slow acceleration [2]. The longitudinal temperature is typically below 0.1 meV and always much smaller than the transverse temperature kT ⊥ which so far has been in the range of 2-30 meV. In recombination experiments, the electron temperatures lead to an asymmetric broadening δ E of the observed cross-section resonances. The low-energy side of the resonance is broadened by kT ⊥ , while kT || broadens the resonance symmetrically by 4(E r kT || ln 2) 1/2 , with E r being the resonance energy [3]. For E r below 0.1 eV, δ E is limited by kT ⊥ . Hence, to improve δ E in this energy range, kT ⊥ has to be decreased. Initially kT ⊥ is given by the cathode temperature kT c and can be reduced by an adiabatic magnetic expansion: kT ⊥ = kT c /α, where α is the expansion coefficient [4]. While for thermocathodes kT c is above 100 meV, semiconductor photocathodes operated at room temperature or below were considered as promising candidates for cold electron sources.We have developed an electron gun with a cryogenic GaAs photocathode which is suitable for obtaining mA currents of emitted electrons with energy spreads of 5-7 meV [5,6]. This electron source has been installed into the TSR electron target section which provides adiabatic magnetic expansion