A two species trap for chromium and rubidium atoms

We realize a combined trap for bosonic chromium 52Cr and rubidium 87Rb atoms. First experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross section of sigma_{inelRbCr}=(5.0+-4.0)*10^{-18}m^2 for light induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross section of the excited 5P_{3/2} state at an ionizing wavelength of 426nm is measured to be sigma_{p}=(1.1+-0.3)*10^{-21}m^2

Bose-Einstein condensation of chromium

We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which will make studies of the effects of anisotropic long-range interactions in degenerate quantum gases possible. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52Cr atoms within a crossed optical dipole trap. At a critical temperature of T~700nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. Released from an anisotropic trap, the condensate expands with an inversion of the aspect ratio. We observe critical behavior of the condensate fraction as a function of temperature and more than 50,000 condensed 52Cr atoms.Comment: 4 pages, 4 figure

Distribution Coefficients of Ionized and Un-ionized Halogenated Phenols in an Octanol-water System and their Relationship to Molecular Structure

Life supporting biological membranes are barriers to toxic chemicals. One of the factors determining the toxicity of chemical compounds is their distribution between membranes and their an environment. An octanol-water system is frequently used as a model for biological membranes to estimate the toxic potency of environmental pollutants. The distribution of a chemical between the octanol and the water phase is described by the octanol-water partition coefficient Kow. This study is concerned with the relationship between Kow and the molecular structure of the toxic chemical. In the study the following trihalophenols were included: 2,4,6-trifluorophenol (TriFP), 2,4,6-trichlorophenol (TriCP); 2,4,6-tribromophenol (TriBP) and 2,4,6-triiodophenol (TrilP). The distribution of halophenols between octanol and water was measured as a function of the pH. Experimental results were analyzed in terms of a two compartment distribution model which accounts for the pH dependent dissociation of the trihalophenol. We showed that, with the exception of TriIP, the pH dependence of the distribution coefficient of 2,4,6 trihalophenols can be understood with this model. From the fit of the distribution model to the experimental results, the following log(Kow) of the neutral molecules were determined: 1.96 (TriFP), 3.65 (TriCP}, 4.11 (TriBP) and approximately 4.42 (TrilP). For the ionized species the log(Kow) are 1.38 (TriFP), 0.15 (TriCP), 0.08 (TriBP) and 1.16 (TriIP). In relation with these distribution studies, the following values for the dissociation constants pKa were obtained: 7.12 (TriFP}, 6.15 (TriCP), 5.9 (TriBP) and 6.6 (TriIP). We also found that octanol dissolved in the water phase does not significantly affect the dissociation of TriIP. The relationship between Kaw and trihalophenol molecular structure was studied in two models. Linear relationships were found between log(Kow) and the surface area as well as between log(Kow) and molar volume for both the neutral and the ionized halophenols. It was not possible to discriminate between the two models. Distribution coefficients of the ionized and unionized 2,4,6-trihalophenols are proportional to the each other. The proportionality factor, (3.54±1.49)x104, is a measure of the effect of electric charge on the transfer across the water-octanol interface

Interactions in ultra-cold dipolar gases

Seit der ersten experimentellen Realisierung eines Bose-Einstein Kondensats (BEC), erfuhr das Feld der atomaren Quantengase eine rasante Entwicklung und stellt heute eines der spannendsten und interdisziplinärsten Feldern in der Atomphysik dar. Die Eigenschaften dieser Gase werden hauptsächlich durch die Wechselwirkung zwischen den Atomen bzw. Molekülen bestimmt, die in den bisher realisierten Kondensaten durch die Kontaktwechselwirkung dominiert wird. Theoretisches und experimentelles Interesse richtet sich seit kurzem auf weitere Wechselwirkungen in diesen entarteten Gasen. Insbesondere wurde die anisotrope und langreichweitige Dipol-Dipol-Wechselwirkung in einem Quantengas, das aus in einem externen Feld ausgerichteten Dipolen besteht, theoretisch untersucht. Im Hinblick auf die Realisierung eines dipolaren atomaren Quantengases stellt Chrom dabei ein äußerst Erfolg versprechendes Element dar. Im Vergleich zu bisherigen BEC-Experimenten, in denen meist Alkali-Atome verwendet wurden, ist die Dipol-Dipol-Wechselwirkung um einen Faktor 36 größer und in seiner Stärke mit der Kontakt-Wechselwirkung vergleichbar. Ein wesentlich stärkeres Dipolmoment wird bei einem im elektrischen Feld ausgerichteten Cr-Rb-Molekül erwartet. In einem solchen Molekülgas wird die Wechselwirkung durch die Dipol-Dipol-Wechselwirkung dominiert. Zur Erzeugung solcher Quantengase wurde in dieser Arbeit ein neuer Aufbau konzipiert und realisiert. Der Schwerpunkt dieser Arbeit liegt auf der Untersuchung ultrakalter, klassischer, bosonischer Cr-Gase, die zur Erzeugung eines dipolaren BECs dienen sollen. Ausgehend von einem lasergekühlten, magnetisch gespeicherten Cr-Ensemble wird in dieser Arbeit die Dipol-Dipol-Wechselwirkung in diesem ultrakalten, dipolaren Gas experimentell studiert. Die theoretische Beschreibung durch die Streuung zweier Dipole führt dabei zu einem sehr allgemeinen Verständnis der Streuprozesse in dipolaren Gasen. In dieser Arbeit wird experimentell und theoretisch gezeigt, dass bereits bei einem magnetischen Moment von sechs Bohrschen Magnetonen aufgrund von dipolaren Relaxationsstößen die Kondensation durch Evaporation der Cr-Atome in einer Magnetfalle nicht möglich ist. Die aus den Streuexperimenten gewonnenen Erkenntnisse bilden nun die Grundlage zur Entwicklung einer sehr aussichtsreichen Strategie. Dabei soll die Kondensation durch Verdampfungskühlen in einer optischen Falle im energetisch tiefsten Zustand der Dipole erfolgen, in dem keine Spinrelaxationsprozesse mehr möglich sind. Durch die Implementation dieses neuen Fallentyps für Cr wird somit eine entscheidende Hürde auf dem Weg zu einem BEC genommen. Obwohl es in dieser Arbeit noch nicht gelingt, die Atome in dieser Falle im energetische tiefsten Zustand zu polarisieren, können zwei Konzepte demonstriert werden, mit denen die Kondensation in diesem Fallentyp erreicht werden kann. Mit ersten Experimente an kombinierten Cr-Rb-Fallen wird in einem weiteren Teil dieser Arbeit ein neues Forschungsprojekt begonnen mit dem Ziel, ein entartetes heteronukleares Molekülgas aus einem zweikomponentigen entarteten Quantengas zu erzeugen. Es werden erste Resultate zum simultanen Betrieb zweier magneto-optischer Fallen (MOT) und dem überlagerten Betrieb von Rb-MOT und Cr-Magnetfalle vorgestellt.Since the first realization of a Bose-Einstein condensate (BEC) in an atomic gas in the year 1995, the physics of ultra-cold atomic gases has rapidly developed and has become one of the most competitive and interdisciplinary fields in atomic physics. Among the related topics, the research on dipolar gases constitutes one of the most promising future directions. The interaction in up to now realized BECs is dominated by the isotropic and short-range contact interaction. In contrast to this interaction, the dipole-dipole interaction is long-range and anisotropic. Theoretical investigations show that if the dipole moment is high enough, the resulting dipole-dipole force can influence or even completely change the properties of a BEC. However, a BEC with a significant dipole-dipole interaction has not been experimentally achieved yet. A promising candidate for observing the influence of the dipole-dipole interaction on the dynamics of a BEC is atomic chromium. As a result of its comparably large magnetic dipole moment of 6 Bohr magnetons, the dipole-dipole interaction is of the same order of magnitude as the contact interaction. A much stronger electric dipole moment is expected from a Cr-Rb molecule. In such a molecular gas the interaction will be dominated by the dipole-dipole interaction. For the creation of these quantum gases, a new experimental setup was designed and assembled. This thesis mainly focuses on the creation of a BEC with atomic chromium (^{52}Cr). Chromium is accumulated in a magnetic trap using a non-standard continuous loading scheme. Applying Doppler-cooling in the compressed magnetic trap, we achieve densities of 10^{11} atoms cm^{-3} and a phase-space density of about 10^{-7} in the magnetically trapped gas. Though this density is 3-4 orders of magnitude lower than in typical BEC experiments, it is already sufficient to observe atom loss and heating due to inelastic dipolar relaxation processes induced by the dipole-dipole interaction. In this process the atomic spin flips and the Zeeman energy is released. Dipolar relaxation in a gas of magnetically trapped chromium atoms is investigated, both theoretically and experimentally. We find that the large magnetic moment of 6 Bohr magnetons results in an event rate coefficient for dipolar relaxation processes of up to 3.2x10^-11 cm^3 s^{-1} in a magnetic field of 44 G. We present a theoretical model based on pure dipolar coupling, which predicts dipolar relaxation rates that agree with our experimental observations. This very general approach can be applied to a large variety of dipolar gases. It will be shown experimentally and theoretically that for chromium heating and density dependent atom loss due to dipolar relaxation are so significant that it is impossible to reach quantum degeneracy by the standard procedure of rf-evaporation in a magnetic trap. To overcome this limitation a new strategy including an optical dipole trap is developed. First promising measurements using this crucial experimental approach are presented. Intending to prepare a Cr-Rb molecule BEC, combined traps for bosonic chromium (^{52}Cr) and rubidium (^{87}Rb) atoms are studied. First experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the ^{87}Rb magneto-optical trap (MOT) in absence and presence of magnetically trapped ground state ^{52}Cr atoms we determine the scattering cross section of 5 10^{-18} m^2 for light induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross section of the excited 5P_{3/2} state at an ionizing wavelength of 426 nm is measured to be 10^{-21} m^2

Fast Automatic Ramping of High Average Power Guns

The electron guns at PITZ, FLASH and European XFEL are standing wave structures which operate at high average power (>40 kW) to produce long trains of high quality beams. This amount of power heats the cavity surface enough to change signicantly the gun resonance frequency. As consequence, to keep the reection low, the RF power ramp must be enough slow to permit the water cooling system to keep the gun temperature close to the set-point. Also, as the temperature probe sits close to the surface of the iris, the required gun temperature set-point to maintain the gun on resonance is a function of the average power. The RF power ramping is a difficult process in which temperature and reection must be monitored to adjust accordingly the temperature set-point and the ramping speed of the RF power. An automatic software to adjust the RF frequency and the temperature set-point of the PITZ gun in parallel to the RF power ramping has been developed. The use of this software has signicantly reduced the time spent to start up the gun or to recover from interlocks, increasing the time spent at nominal parameters which would also be very important for user facilities

Low Level RF Control Implementation and Simultaneous Operation of Two FEL Undulator Beamlines at FLASH

The Free-Electron Laser in Hamburg (FLASH) is a user facility delivering femtosecond short radiation pulses in the wavelength rangebetween 4.2 and 45 nm using the SASE principle. The tests performed in the last few years have shown that two FLASH undulator beamlinescan deliver FEL radiation simultaneously to users with a large variety of parameters such as radiation wavelength, pulse duration,intra-bunch spacing etc. FLASH has two injector lasers on the cathode of the gun to deliver different bunch trains with different charges,needed for different bunch lengths. Because the compression settings depend on the charge of bunches the low level RF system needsto be able to supply different compression for both beamlines. The functionality of the controller has been extended to provide intra-pulse amplitude and phase changes while maintaining the RF field amplitude and the phase stability requirements. The RF parameteradjustment and tuning for RF gun and accelerating modules can be done independently for both laser systems. Having different amplitudesand phases within the RF pulse in several RF stations simultaneous lasing of both systems has been demonstrated

Low Level RF Control Implementation and Simultaneous Operation of Two FEL Undulator Beamlines at FLASH

The Free-Electron Laser in Hamburg (FLASH) is a user facility delivering femtosecond short radiation pulses in the wavelength range between 4.2 and 45 nm using the SASE principle. The tests performed in the last few years have shown that two FLASH undulator beamlines can deliver FEL radiation simultaneously to users with a large variety of parameters such as radiation wavelength, pulse duration, intra-bunch spacing etc. FLASH has two injector lasers on the cathode of the gun to deliver different bunch trains with different charges, needed for different bunch lengths. Because the compression settings depend on the charge of bunches the low level RF system needs to be able to supply different compression for both beamlines. The functionality of the controller has been extended to provide intra-pulse amplitude and phase changes while maintaining the RF field amplitude and the phase stability requirements. The RF parameter adjustment and tuning for RF gun and accelerating modules can be done independently for both laser systems. Having different amplitudes and phases within the RF pulse in several RF stations simultaneous lasing of both systems has been demonstrated