* Corresponding author. Tel.: #49-40-8998-3862; fax: #49- 49-8998-4305. E-mail address: klaus.balewski@desy.de (K. Balewski) Nuclear Instruments and Methods in Physics Research A 441 (2000) 274}280 Studies of electron cooling at DESY K. Balewski *, R. Brinkmann, Y. Derbenev, K. Floettmann, P. Wesolowski , M. Gentner, D. Husmann, C. Steier DESY, D-22607 Hamburg, Germany Universita ( t Bonn, Physikalisches Institut, D-53115 Bonn, Germany Abstract A possibility to improve the luminosity in hadron colliders is to apply electron cooling. For the e}p and possibly in the future for e-ion a cooling scheme is proposed for the HERA collider which improves the brightness of the hadron beam in two steps. In a "rst step the hadron beam is cooled at an energy of 15}20 GeV in the pre-accelerator PETRA. In a second step the hadrons are cooled at top energy in HERA, mainly to maintain the beam quality. The "rst part of the cooler in PETRA consists of a thermionic gun which delivers electron bunches of 120 keV. These bunches are then compressed so that they "t into a 208 MHz travelling wave linac where they are accelerated to an energy of 10 MeV. Gun, compressor, and linac are surrounded by a solenoid so that emittance growth is avoided especially at the low energy end. In order to reduce the energy spread of the electron beam and to lengthen it, the electrons run through a decompressor before they are brought together with the hadron beam in a 40 m long cooling section which is again surrounded by a solenoid. In this paper a schematic layout of the system will be presented and simulation results of the "rst part of the cooler will be shown. The cooler in HERA consists of a small electron storage ring with two long straight sections. It is shown that all requirements of such a ring running at an energy between 180 and 450 MeV can be met to "ght emittance growth in case of light hadron beams (protons) and to cool heavy ions su$ciently fast.  2000 Elsevier Science B.V. All rights reserved. PACS: 41.75Lx; 29.20 Dh Keywords: PETRA; HERA; Electron cooling; Particle accelerators; Ions; Luminosity 1. Introduction The HERA electron proton collider [1] has been running with colliding beam experiments since 1992. Electrons of 27.5 GeV and protons of 820 or 920 GeV are brought into collisions at two straight sections. The data collected by the two experiments H1 and ZEUS show interesting and indications of unexpected physical results. Detailed investigation of these results requires a substantial increase of the luminosity [2]. A "rst step towards an increase in luminosity is now under way [3,4]. This "rst step requires sub- stantial modi"cations of the interaction regions and will lead to a reduction of the beta function. A second step to increase the luminosity could be the reduction of the emittances of the proton beam by electron cooling. This cooling technique was proposed by Budker in the early 1960s and has 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 1 4 5 - 6