Workshop on Polarized Electron Sources and Polarimeters

Ultra Cold Photoelectron Beams for
Ion Storage Rings
D. A. Orlov a , C. Krantz a , A. Shornikov a , M. Lestinsky a , J. Hoffmann a ,
A.S. Jaroshevich b , S.N. Kosolobov b , A.S. Terekhov b and A. Wolf a
a Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany
b Institute of Semiconductor Physics, 630090, Novosibirsk, Russia
Abstract. An ultra cold electron target with a cryogenic GaAs photocathode source, developed
for the Heidelberg TSR, delivers electron currents up to a few mA with typical kinetic energies
of few keV and provides unprecedented energy resolution below 1 meV for electron-ion
recombination merged-beam experiments. For the new generation of low-energy electrostatic
storage rings, cold electron beams from a photocathode source can bring additional benefits,
improving the cooling efficiency of stored ions and making it possible to cool even heavy, slow
molecules by electron beams of energies of only a few eV or even below.
Keywords: photocathode, electron beams, electron cooling, electron target, storage ring.
PACS: 52.25.Xz , 29.25.Bx, 29.20.D, 85.60.Ha, 34.80.Lx, 37.10-x
INTRODUCTION
Semiconductor photoemitters with negative electron affinity (NEA) are widely used in
various scientific experiments for the generation of spin-polarized, short bunched
high-density electron beams. Photoelectron sources also offer the possibility to employ
extremely cold electrons with emission energy spread of about the cathode
temperature, which can be as low as 7 meV at 100 K [1]. One of the main difficulties
is to keep the beam cold as a space charge causes beam divergence at high currents.
To avoid this heating, high magnetic guiding field is used in the electron coolers of
storage rings, which decouples transverse and longitudinal electron motions. Electronion
merged beams at storage rings have also opened a new domain for electron-ion
recombination studies where ultra cold photoelectron sources can bring great benefits
in high-resolution measurements. A cryogenic photocathode target was developed
[2,3,4] for recombination measurements at the Heidelberg Test Storage Ring (TSR)
and is in operation since 2002 delivering dc electron beams of mA currents and with
ultra-cold temperatures below 1 meV [4,5]. For the upcoming electrostatic Cryogenic
Storage Ring (CSR), where the cooling of stored slow ions has to be performed at
typical electron energies of 10 eV down to the sub-eV range, the impact of the cold
photocathode source can be of high additional importance. Indeed, at low energies of
1-100 eV the electron current drops down to the A range due to a limited gun
perveance, which strongly decreases the cooling efficiency. To make cooling possible,
electron beams of low temperatures have to be used [6]. In this paper, we present the