Dispersion and Beam Optic Parameter Measurements in the Transport Line (E-Weg) from DESY II To PETRA III

The transport line E Weg extends from the extraction septum in DESY II to the injection septum in PETRA III, and transports electrons at a beam energy of 6.0 GeV. It consists of 3 parts. The first part is in the DESY tunnel, the second part is a long drift space in a slanted tube and the third part is in the PETRA III tunnel. The vertical plane difference between the accelerators is 1.28 m. The optics was derived from initial values at Übergabepunkt (UGP) from a previous optics. The total length of the transfer line is about 203 m. Ten screen monitors are used to estimate the profiles of the beam spot for the optics measurements, while 8 BPMs, mostly adjacent to the screens, are used to compare and control the orbits. Two scrapers are installed on either side of the long drift space to trim the beam dimensions in transverse plane. Two FCTs are used to measure the beam current and the transfer efficiency. The transverse dispersion and beta functions are measured by extracting the beam from DESY at different energies and analysing the beam profiles at the screen as well as positions at BPMs. The details of such measurements are reported in this paper. The E Weg is designed to deliver an electron beam from DESY II to PETRA III[1 3], a 6 GeV synchrotron light source with a horizontal beam emittance of 1.3 nm.rad with 1% emittance coupling. It consists of 3 parts such as, a part of it is in DESY tunnel, the long drift space is in a slanting tube and the third part is in PETRA III tunnel. The vertical plane difference between the tunnels are 1.28m, the DESY tunnel is in lower respect to PETRA III. The E Weg optics was derived from the initial values at UGP (transfer point) from previous optics. The UGP is at the entrance of the first quadrupole magnet in the transfer line. Most of the references were available from this point. The total length of the E Weg is 203.37m. It consists of 12 quadrupoles, 7 horizontal bending magnets, 5 horizontal correctors, 7 vertical correctors, and one vertical bending magnet. There are 10 screen monitors to provide the beam profiles of the intercepted beam spot. In addition there are 8 BPMs mostly adjacent to the screen monitors to compare and control the transverse orbits. For trimming of the beam profiles in transverse plane two scrapers are installed on either side of the long drift space. Two current transformers (FCT) are in use for measuring the beam current in the transport line. The extracted beam from DESY II with a theoretical horizontal emittance of 350nm.rad and vertical emittance of 35nm.rad considering coupling of 10%, passes through the magnetic fringe fields of quadrupole QD43, sextupole SD43, dipoles B43 and B44 of DESY II booster ring in operation. So the extracted electron beam gets kicks from these magnets before reaching at UGP. From an old optics printouts[4] information about the effect of these magnets on E Weg was implemented to match the optics at UGP and then whole E Weg was further modified to take the requirements of PETRA III injection point ( x = 19 m; x = 0.4m, y=9.31 m) with E Weg parameters at point of injection ( x = 12.981 m; y=10 m). The present optics used is shown in the Fig.1. Figure 1: The theoretical transverse betatron and dispersion functions of the E Weg with tune values x = 1.7167, y = 1.5620. Transverse Dispersion Measurements The dispersion function can be calculated from the centroid shift of the beam spot at screens (Beam Profile Monitors) by extracting the beam from DESY II at different energies. The centroid shift will be z = z P/P, where z = (x, horizontal or y, vertical) and is the dispersion function of the respective plane at that location with the relative momentum spread of P/P. It is possible to change the energy of the extracted beam from DESY II with P/P ~0.001 or 0.002 i.e. 6 to 12MeV at on energy of 6GeV. With P/P ~ 0.001, this will give a centroid shift (0.099, 0.59)mm at 72m screen and (1.59, 1.12)mm at 119m screen location as per the theoretical expectations. It is seen that the quadrupoles also change the focussing strength due to energy change that affects the dispersion function too. But the change of dispersion function due to quadrupole strength change is neglected. A change of 1000 turns at DESY extraction makes an ___________________________ gajendra.kumar. sahoo@desy.de energy change of 90.8MeV for the measurements. In practice 20 turns from the working point is considered for dispersion measurements. Beam Optics Parameters Measurements Consider a beam in the two dimensional phase space (x; x ) and assume the distribution is Gaussian with the density function given by [5], 2 2 exp ) , ( 2 11 12 2 22 x x x x N x x , with symmetric beam matrix, 22 12 12 11 . The task is to find the emittance . When a profile monitor intercepts the entire beam, only the spatial width at that point, 11 is determined. However, the other matrix elements may be inferred from beam profiles taken under various transport conditions downstream of the given point if the transformation of the beam matrix between those points is understood. If a beam has matrix at some point, s , and matrix at some other point, s , downstream, the transformation of the beam between s and s may be characterized by a transfer matrix R