[en] The HERA electron ring was in operation again between the beginning of August until the end of September 1989 following a long break during which installation work for the superconducting proton ring continued in the tunnel. The new machine had already stored the first electron beam on August 20th 1988 (see DESY Journal 2-88). At the time, however, the ring had not been fully equipped with technical components and could not be ramped up to its design energy. But this time the complete rf-system was available (6 klystron transmitters of about 1 MW each) and the magnets could be operated at full current. As a result, a maximum energy of 27.5 GeV was reached, close to the theoretical limit where the synchrotron radiation losses are just compensated by the energy fed back to the beam by the rf-cavities. No problems occurred during the acceleration process (the injection energy was 13 GeV), mainly due to the high precision with which the large number of magnet power supplies synchronously tracked. The beam lifetime at high energy was up to 5 hours, limited by the vacuum of about 10^-8 mbar. (orig.)

[en] Using the GEANT computer code, the proton-induced background for the HERA experiments is investigated. A realistic geometrical setup including all magnetic elements and fields of the p-beam line is used. It is shown that the flux of particles from the tunnel can be drastically reduced with a relatively small amount of additional screening material surrounding the beam pipe at selected positions. (orig.)

[en] Achromatic optical solutions for the HERA e^--ring are presented and discussed. The dynamic aperture for the three different optics is studied by particle tracking and perturbation theory. We found a significant advantage of quasi-superperiodic solutions with respect to the non-periodic solutions and an advantage of a 6-sextupole family solution with respect to a 2-family solution. The sextupole acceptance for the preferred optical solution is as large as 27 standard deviations at a beam energy of 35 GeV. (orig.)

[en] Sextupole distributions for various values of phase advance per FODO cell (φ_c = 60⁰ ... 90⁰) in the HERA electron ring are investigated. The chromatic corrections reduce the changes of the beta-functions to less than 5% and the changes of tunes to less than 0.01 in an energy range between -1% and 1%. The nonlinear acceptance accomodates at least 13 σ for a 35 GeV electron beam in the whole range of φ_c. The scaling of the dynamic aperture with φ_c is found to be in good agreement with a simple analytical estimate. (orig.)

[en] Recent successful lasing of the Linac Coherent Light Source (LCLS) in the hard x-ray regime and the experimental demonstration of a possibility to produce low-charge bunches with ultra-small normalized emittance have lead to the discussions on optimistic scenarios of operation of the European XFEL. In this paper we consider new options that make use of low-emittance beams, a relatively high beam energy, tunable-gap undulators, and a multibunch capability of this facility. We study the possibility of operation of a spontaneous radiator (combining two of them, U1 and U2, in one beamline) in the SASE mode in the designed photon energy range 20-90 keV and show that it becomes possible with ultra-low emittance electron beams similar to those generated in LCLS. As an additional attractive option we consider the generation of powerful soft X-ray and VUV radiation by the same electron bunch for pump-probe experiments, making use of recently invented compact afterburner scheme. We also propose a betatron switcher as a simple, cheap, and robust solution for multi-color operation of SASE1 and SASE2 undulators, allowing to generate 2 to 5 X-ray beams of different independent colors from each of these undulators for simultaneous multi-user operation. We describe a scheme for pump-probe experiments, based on a production of two different colors by two closely spaced electron bunches (produced in photoinjector) with the help of a very fast betatron switcher. Finally, we discuss how without significant modifications of the layout the European XFEL can become a unique facility that continuously covers with powerful, coherent radiation a part of the electromagnetic spectrum from far infrared to gamma-rays. (orig.)

[en] The optics for the 350 m long proton beam transport system from PETRA to HERA is presented. The special geometry of the beam line, involving both horizontal and vertical deflections, leads to a coupling of horizontal and vertical emittances. A simple compensation scheme using skew quadrupoles is proposed. The requirements for the magnets with respect to apertures, tolerances and flexibility (for matching various optics of PETRA and HERA) are discussed. An estimate for the emittance growth due to focussing errors in the beam line is given

[en] The European XFEL will operate nominally at 17.5 GeV in SP (short pulse) mode with 0.65 ms long bunch train and 10 Hz repetition rate. A possible upgrade of the linac to CW (continuous wave) or LP (long pulse) modes with a corresponding reduction of electron beam energy is under discussion since many years. Recent successes in the dedicated R and D program allow to forecast a technical feasibility of such an upgrade in the foreseeable future. One of the challenges is to provide sub-Aangstrom FEL operation in CW and LP modes. In this paper we perform a preliminary analysis of a possible operation of the European XFEL in the hard X-ray regime in CW and LP modes with the energies of 7 GeV and 10 GeV, respectively. We consider lasing in the baseline XFEL undulator as well as in a new undulator with a reduced period. We show that, with reasonable requirements on electron beam quality, lasing on the fundamental will be possible in sub-Aangstrom regime. As an option for generation of brilliant photon beams at short wavelengths we also consider harmonic lasing that has recently attracted a significant attention.

[en] The optics for the 350 m long proton beam transport system from PETRA to HERA is presented. The special geometry of the beam line, involving both horizontal and vertical deflections, leads to a coupling of horizontal and vertical emittances. A simple compensation scheme using skew quadrupoles is proposed

[en] The authors report on first experience with colliding electron and proton beams in HERA. In 1992, the first year of operation, HERA has delivered some 60(nb)^-1 of e-p Luminosity to the experiments H1 and ZEUS. The beam energies amounted to 26 GeV and 820 GeV. A maximum luminosity of 2.2 · 10²⁹cm^-2s^-1 has been achieved by colliding a train of nine electron bunches against nine proton bunches. The beams could be brought into and held in collision without problems. The lifetime of the proton beam in collision is as long as 50 h. This requires both careful matching of the electron and proton beam cross sections and also that the two orbits coincide within ≅ 0.2σ beam size at the interaction point. The proton beam suffered a beam-beam tune shift of up to ΔQ_x ≅ 0.0018. This is close to the limit which was assumed in the design of HERA. Nonetheless, there is only little degradation in the proton beam quality in collision. Under these conditions, the proton beam fills could be stored and be made available for collision for some 24h in HERA

[en] The maximum degree of electron spin polarization in a real storage ring is mainly limited by the tilt of the equilibrium polarization direction (n-bar)₀ with respect to the direction of the main bending fields. The tilt is mainly caused by random vertical closed orbit kicks introduced by nonzero vertical offsets inside the quadrupoles. A tilt correction algorithm is presented which makes use of the known correlations between transverse offsets of quadrupoles and adjacent beam position monitors. These offsets can be determined by the application of a beam-based alignment technique. An automatic alignment procedure has been successfully applied to the vertically focussing quadrupoles in the arcs of HERA which should give the main contribution to the (n-bar)₀ axis tilt. The results of measurements made in spring 1995 are presented followed by a discussion on the possible influences on the degree of spin polarization. (author)