Lifetime Studies at Metrology Light Source and ANKA

Abstract The Metrology Light Source (MLS), situated in Berlin (Germany) is an electron storage ring operating from 105 MeV to 630 MeV and is serving as the national primary radiation source standard from the near infrared to the extrem ultraviolet spectral region INTRODUCTION To provide users of synchrotron radiation with temporally stable experimental conditions, the lifetime τ of the stored beam with current I is a parameter of concern. This is valid for machines with a decaying beam such as ANKA and MLS, but as well for machines operated in top up mode such as BESSY II in Berlin. In 2012, the standard user operation at MLS yielded a lifetime of 3.5 hours at 150 mA beam current. Although reasonable due to the energy, this is a low value compared to 16 hours at ANKA and it would benefit the users of synchrotron radiation if it could be improved. THEORY There are two major loss mechanisms determining the lifetime of the electrons in an accelerator: The scattering of the electrons with residual gas atoms and the scattering of the electrons with other electrons within the bunch. The latter is known as the "Touschek effect", named after Bruno Touschek who first observed the effect at the small AdA electron-positron collider. The two contributions are called gas lifetime and Touschek lifetime respectively. The gas lifetime depends on the pressure P and a scattering cross section σ gas for particle losses, for which the interested reader is referred to The cross section itself is a function of the acceptance of the accelerator δ acc = Δp max /p 0 , while the pressure P depends in some respect on the beam current. The electrons in a bunch perform transverse betatron oscillations. Being an incoherent motion, this leads to * [email protected] Coulomb scattering. During the scattering process, transverse momentum gets transferred to longitudinal momentum. If the particles momentum deviation exceeds the momentum acceptance it will be lost. The resulting Touschek lifetime depends on the rate of scattering processes and therefore on the density within the bunch, i.e. on the bunch volume and the bunch current. Furthermore, it depends on the momentum acceptance with the power of three with σ x,y,s being the rms-bunch sizes and length and D(ξ) being a slowly varying function with respect to the acceptance δ acc . D also depends on the optical functions around the ring through ξ. The loss rates from Touschek effect and gas scattering add to the total loss rate 1/τ which can be measured. Multiplying the number of particles N (or the stored current I) to the Touschek lifetime τ T results in a constant: N · τ T = const. Therefore, when plotting I · τ for a Touschek dominated lifetime a constant can be expected with respect to current. Acceptance Touschek lifetime and gas lifetime depend on the acceptance of the accelerator. Two acceptances are important here, and whichever is the smallest is the limiting one: • RF-acceptance • Geometrical acceptance. The RF-acceptance δ acc,RF approximately depends on the applied cavity voltage V as [4] The geometrical acceptance depends on the minimal aperture of the vacuum chamber a(s) and the dimensions of the beam. For MLS a first order approximation considering only the horizontal plane is: with D x being the horizontal dispersion function. In the upper plot of Predictions In the lower part of EXPERIMENT In LIFETIME IMPROVEMENT The lifetime at MLS can be improved if the acceptance is only RF-limited even to larger voltages than 300 kV. To do so, the geometrical acceptance has to be improved. In order to find optics with an increased Touschek lifetime, brute force optics scans using a Fortran code were performed In Eq. 4, the horizontal dispersion function D x is in the denominator. By decreasing the dispersion function at the place with minimum aperture, the geometrical acceptance δ acc,geom can be improved. At MLS each quadrupole is powered independently. By tuning some quadrupoles of one family against the remaining ones of that family, the dispersion function was tuned to be zero at the septum. In The peak lifetime with respect to cavity voltage is now located at 500 kV, being the maximum applicable cavity voltage at the moment. In The total lifetime increase is as high as 80 %. By solely increasing the acceptance, and the change in optical functions, the lifetime would have been expected to increase by about 30 %. An explanation for the additional increase could be a strong halo around the beam due to intra-beam scattering. With this, the effect of leading the beam through the centre of the vacuum chamber at the septum could be explained as well. CONCLUSION AND OUTLOOK The theory of the Touschek effect describes the dependencies on energy and acceptance well. By understanding the different loss mechanisms and the methods to manipulate the different acceptances, it was possible to generate a new user optics with an by 80 % improved lifetime. To completely explain the total lifetime increase, further measurements are needed. To further increase the lifetime, alternate optics determined by optics scans will be tested ACKNOWLEDGMENT The authors like to thank Andreas Jankowiak (HZB) and Gerhard Ulm (PTB) for supporting this work. REFERENCES [1] R. Klein et al., Phys. Rev. ST-AB 11, 110701, 2008. [2] A.-S. Müller et al., "Energy Calibration Of The ANKA Storage Ring", Proceedings of EPAC 2004. [3] T. Goetsch, "Lifetime Studies at Metrology Light Source and Angströmquelle Karlsruhe" -Diploma Thesis, Karlsruhe Institut für Technologie, May 2013. [4] M. Sands, "The Physics Of Electron Storage Rings -An Introduction", National Technical Information Service, Springfield, Virginia, 1970. [5] J. le Duff, "Current And Current Density Limitations In Existing Electron Storage Rings", In: Nucl. Instr. and Meth. in Physics Research, 1985

Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.Comment: 6 pages, 4 color figures. Accepted for publication in A&A Letters. Fig. 4 does not appair in the A&A version due to space restrictions. Includes aa.cls and txfonts.st

SN 2009E: a faint clone of SN 1987A

In this paper we investigate the properties of SN 2009E, which exploded in a relatively nearby spiral galaxy (NGC 4141) and that is probably the faintest 1987A-like supernova discovered so far. Spectroscopic observations which started about 2 months after the supernova explosion, highlight significant differences between SN 2009E and the prototypical SN 1987A. Modelling the data of SN 2009E allows us to constrain the explosion parameters and the properties of the progenitor star, and compare the inferred estimates with those available for the similar SNe 1987A and 1998A. The light curve of SN 2009E is less luminous than that of SN 1987A and the other members of this class, and the maximum light curve peak is reached at a slightly later epoch than in SN 1987A. Late-time photometric observations suggest that SN 2009E ejected about 0.04 solar masses of 56Ni, which is the smallest 56Ni mass in our sample of 1987A-like events. Modelling the observations with a radiation hydrodynamics code, we infer for SN 2009E a kinetic plus thermal energy of about 0.6 foe, an initial radius of ~7 x 10^12 cm and an ejected mass of ~19 solar masses. The photospheric spectra show a number of narrow (v~1800 km/s) metal lines, with unusually strong Ba II lines. The nebular spectrum displays narrow emission lines of H, Na I, [Ca II] and [O I], with the [O I] feature being relatively strong compared to the [Ca II] doublet. The overall spectroscopic evolution is reminiscent of that of the faint 56Ni-poor type II-plateau supernovae. This suggests that SN 2009E belongs to the low-luminosity, low 56Ni mass, low-energy tail in the distribution of the 1987A-like objects in the same manner as SN 1997D and similar events represent the faint tail in the distribution of physical properties for normal type II-plateau supernovae.Comment: 19 pages, 9 figures (+7 in appendix); accepted for publication in A&A on 3 November 201

The Host Galaxy and Optical Light Curve of the Gamma-Ray Burst GRB 980703

We present deep HST/STIS and ground-based photometry of the host galaxy of the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the burst. We find that the host is a blue, slightly over-luminous galaxy with V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host. We find that the galaxy is best fit by a Sersic R^(1/n) profile with n ~= 1.0 and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This corresponds to an exponential disk with a scale radius of 0.22 arcsec (= 1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves large residuals, which suggests that the host galaxy has a somewhat irregular morphology, but we are unable to connect the location of GRB 980703 with any special features in the host. The host galaxy appears to be a typical example of a compact star forming galaxy similar to those found in the Hubble Deep Field North. The R-band light curve of the optical afterglow associated with this gamma-ray burst is consistent with a single power-law decay having a slope of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late time properties of the light-curve are very poorly constrained. The decay of the optical light curve is consistent with a contribution from an underlying Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot be securely established.Comment: 9 pages, 5 figures, LaTeX using A&A Document Class v4.05, to appear in A&

Hypernova Nucleosynthesis and Galactic Chemical Evolution

We study nucleosynthesis in 'hypernovae', i.e., supernovae with very large explosion energies ( \gsim 10^{52} ergs) for both spherical and aspherical explosions. The hypernova yields compared to those of ordinary core-collapse supernovae show the following characteristics: 1) Complete Si-burning takes place in more extended region, so that the mass ratio between the complete and incomplete Si burning regions is generally larger in hypernovae than normal supernovae. As a result, higher energy explosions tend to produce larger [(Zn, Co)/Fe], small [(Mn, Cr)/Fe], and larger [Fe/O], which could explain the trend observed in very metal-poor stars. 2) Si-burning takes place in lower density regions, so that the effects of $\alpha$-rich freezeout is enhanced. Thus $^{44}$Ca, $^{48}$Ti, and $^{64}$Zn are produced more abundantly than in normal supernovae. The large [(Ti, Zn)/Fe] ratios observed in very metal poor stars strongly suggest a significant contribution of hypernovae. 3) Oxygen burning also takes place in more extended regions for the larger explosion energy. Then a larger amount of Si, S, Ar, and Ca ("Si") are synthesized, which makes the "Si"/O ratio larger. The abundance pattern of the starburst galaxy M82 may be attributed to hypernova explosions. Asphericity in the explosions strengthens the nucleosynthesis properties of hypernovae except for "Si"/O. We thus suggest that hypernovae make important contribution to the early Galactic (and cosmic) chemical evolution.Comment: To be published in "The Influence of Binaries on Stellar Population Studies", ed. D. Vanbeveren (Kluwer), 200