A cold atom electron source

Pulsed bright electron sources offer the possibility to study the structure of matter in great spatial and temporal detail. An example of an indirect method is to generate hard X-ray °ashes with high brilliance, a new Free Electron Laser facility is under construction. It requires an electron source with a very high quality. Electron beams may also be used directly to study matter with, e.g., ultrafast electron diffraction. This also requires a pulsed electron source with high brightness. An overview of experiments that require a bright electron source is presented in Chapter 1. Also the pulsed electron sources used at this moment, i.e., photo-emitters and field-emitters, are described in Chapter 1. Brightness is an important figure of merit for electron source quality. It is expressed in its most general form as the current density per unit solid angle and unit energy spread. Recent brightness improvements are based on increasing the current density at the source, but this is not sufficient for all types of experiments. A new type of source, based on ultracold plasma, is described in Chapter 2. Contrary to the usual approach to increase the current density at the source to obtain a higher brightness, the new method tries to increase the angular intensity for moderate values of the emission area. For the field-emitters and photo-emitters the effective electron temperature of the source is typically 10 3 – 10 4 K. If one is able to lower these temperatures at the source, then a gain in brightness proportional to the reduction of the temperature can be achieved for the same current density. The new source concept based on this idea proposes pulsed extraction of electrons from an ultracold plasma, that is created from a laser-cooled cloud of neutral atoms by photoionization just above threshold. These plasmas are characterized by electron temperatures of 10 K. A simple estimate serves to illustrate the possible performance of such a source. Laser-cooled atomic clouds can have central densities up to n = 1018 m¡3 and contain up to 1010 atoms, requiring a cloud with rms (root-mean-square) size ¾ = 1 mm. If all these atoms could be ionized to form a UCP (ultracold plasma) with an electron temperature T = 10 K, then an electron bunch with a charge Q = 1 nC and an emittance " = 0:04 mm mrad could be extracted. If, in addition, all of these electrons can be packed into a temporal bunch length on the order of ¾t = 100 fs, the transverse brightness of the resulting electron bunch would be B? = 6£1016 A/(m2 sr). This is a few orders of magnitude higher than what has been achieved so far in the regime of (sub)-ps electron bunches. A four-step procedure is used to realize a UCP-based electron source in practice. First, atoms are cooled and trapped in a magneto-optical trap. Second, part of the cold atom cloud is excited to an intermediate state with a quasi-continuous laser pulse. Third, a pulsed laser beam propagating at right angles to the excitation laser ionizes the excited atoms only within the volume irradiated by both lasers. Subsequently, a UCP is formed. Finally, the electrons of the UCP are extracted by an externally applied electric field pulse. Each step toward the ultracold plasma is explained in detail in Chapter 2. Subsequently, with the help of simulations with a particle tracking program, the expectations from a more realistic situation are investigated. Two geometries are discussed. First, an initial charge distribution called "pancake" (bunch length much smaller than its transversal size) with a half-circle radial charge density distribution offers for a beam transverse size of 2 mm an emittance of 0:1 mm mrad and a temporal bunch length of 150 fs. This results in a transverse brightness of 6 £ 1013 A/(m2 sr). Second, a "cigar" geometry (transverse size much smaller than bunch length) with a parabolic longitudinal charge density distribution offers for a beam transverse size of 1 mm an emittance of 0:07 mm mrad and a temporal bunch length of 20 fs. This results in a transverse brightness of 1 £ 1014 A/(m2 sr). In this Thesis the first practical steps are reported towards this new concept. In Chap- ter 3, a specially designed accelerator structure and a pulsed power supply are described. They are essential parts of a high brightness cold atoms-based electron source. The acce- lerator structure allows a magneto-optical atom trap to be operated inside of it, and also transmits sub-nanosecond electric field pulses. The power supply produces high voltage pulses up to 30 kV, with a rise time of up to 30 ns. The resulting electric field inside the structure is characterized with an electro-optic measurement and with an ion time-of-fight experiment. In Chapter 4 measurements of the transverse momentum spread of pulsed electron beams are presented. Rubidium atoms are cooled and trapped in a magneto-optical trap. A small cylinder of these atoms is photoionized, resulting in free electrons. The electrons are extracted by a DC electric field. Images of the cylinder-like electron beam are obtained on the detector. On the path that they travel to a phosphor screen, they interact with an electromagnetic beam transport system, composed of an electrostatic lens (the accelerator itself) and a magnetic lens (the trapping coils). Due to the magnetic lens, this optical system is energy dependent. A dependence between the size of the small axis of the cylin- der at the detector and the beam kinetic energy is obtained. With the help of an optical matrix that describes this electromagnetic system, the size of the cylinder is related to the initial electron temperature, which is the parameter that we are actually interested in. Transverse electron temperatures ranging from 200 K down to 15 K are demonstrated, ea- sily controllable with the wavelength of the ionization laser. The temperature is influenced due to the Stark effect by the presence of the accelerating electric field. In this experiment the temporal length of the bunch is limited by the length of the ionization laser pulse to 4:7 ns. A typical bunch contains a charge of 10 fC. To lower the bunch length, another experiment was carried out. The results are presen- ted in Chapter 5. This time, excited Rydberg states of rubidium atoms are field ionized. The atoms are first magneto-optically trapped at the center of the accelerator structure. Subsequently they are excited to a Rydberg state (here between 26 and 35) and then field ionized by a pulsed electric field with a slew rate of 58 (V/cm)/ns. Electron temperatures at the source on the order of 10 K are measured. In the same way as in Chapter 4, the temperature is deduced from images of the electron pulses on the phosphor screen, using a model of the beam transport system. An advantage of this method is that sub-ns temporal bunch lengths might be reached. Here, the length is measured to be 2 ns FWHM, which is limited by instrumental resolution; particle tracking simulations show that it might be on the order of tens of ps. As a continuation of the experiments presented in Chapter 4 and 5, a method to pro- duce electron bunches with high energy and low temperature at the source is presented in Chapter 6. Rydberg atoms with the principal quantum number n between 15 and 25 are employed. It is shown that energies up to 14 keV can be produced. An Einzel lens is employed to focus the beam on the detector. An optical model including the Einzel lens is built, this time with the transverse beam size at the detector being dependent on the voltage applied on the Einzel lens. It does not fit very well the expectations due to the work of the Einzel lens, but this is a new model that can be further used to describe the behavior of a bunch in an optical system. Source temperatures of about 15 K are expected, but an upper limit of 1000 K is estimated using the optical model. In conclusion, in this Thesis an electron source with a 30 ¹m rms size, temperature of 10 K, and normalized transverse emittance of 0:001 mm mrad has been produced. Electron bunches with charges up to 10 fC and kinetic energies up to 14 keV have been produced. An upper limit for the FWHM length of 2 ns has been established. On the basis of these numbers, a transverse brightness B? = 8 £ 1010 A/(m2 sr) can be calculated. To further improve the brightness of this source, the source parameters as the charge column density Q=(¾x¾y), the bunch length ¾t, and the source temperature T should be improved. Together they may improve the brightness a few orders of magnitude. This project provides a solid basis for the next generation of cold electron sources that combines the present source based on cold atoms with radio-frequency technology. In addition, with this technique new research directions have been opened, as illustrated by an experiment with cold ions

A bright ultracold atoms-based electron source

An important application of pulsed electron sources is Ultrafast Electron Diffraction [1]. In this technique, used e.g. in chemistry, biology and condensed matter physics, one can observe processes that take place at the microscopic level with sub-ps resolution. To reach the holy grail of UED, single-shot diffraction images of biologically relevant molecules, electron bunches of 1pC charge, 100fs length and 10nm coherence length are required. Conventional pulsed electron sources cannot fulfil these requirements, but according to the simulations reported in [2] and [3] a new type of source can.The new source combines the use of magneto-optical atom trapping with fast high voltage technology. We start by cooling and trapping rubidium atoms, followed by ionisation just above threshold, leading to an ultracold plasma. Another possibility is to excite the atoms into a high Rydberg level, from which they spontaneously evolve into an ultracold plasma. Applying a fast high voltage pulse, electron bunches can be extracted. In an initial study [2] it has been shown that this type of source can provide a very high brightness. Depending on the initial particle distribution, the reduced brightness can be in the order of 1x109 A/(rad2m2V), which is orders of magnitude higher than established technology such as an electron photogun can provide.Here we report the first experiments toward realisation of the source. Here, a simple accelerator structure consists of four bars surrounding a MOT, on which an 800V pulsed voltage with a rise time of 1ƒÝs is applied. An MCP together with a phosphor screen and a CCD camera are used as detection system. The bunch size obtained from the phosphor screen is fitted with a Gaussian distribution, from which the electron temperature is extracted. For small extracted charges, the electron temperature is found to have an upper limit of 500K, the measurement being limited by stray magnetic fields due to the low electron energy (10eV). We have also extracted a pulsed ion beam by reversing the sign of the accelerating voltage. Since ions are heavier, they obtain higher energy and are less influenced by the magnetic fields. The temperature in this case is found to b

Laser-cooled ion source

Focused Ion Beams (FIB) are widely used in the semiconductor industry for milling, sputtering and imaging applications. In particular it is used for quality control of wafers, by using a combination of a FIB and an electron microscope to make cross-sectional inspections of wafers. In addition, FIB's are used for mask repair through gas-assisted etching

Breast cancer management and outcome according to surgeon's affiliation: a population-based comparison adjusted for patient's selection bias

Background Studies have reported that breast cancer (BC) units could increase the quality of care but none has evaluated the efficacy of alternative options such as private BC networks, which is our study objective. Patients and methods We included all 1404 BC patients operated in the public unit or the private network and recorded at the Geneva Cancer Registry between 2000 and 2005. We compared quality indicators of care between the public BC unit and the private BC network by logistic regression and evaluated the effect of surgeon's affiliation on BC-specific mortality by the Cox model adjusting for the propensity score. Results Both the groups had high care quality scores. For invasive cancer, histological assessment before surgery and axillary lymph node dissection when indicated were less frequent in the public sector (adjusted odds ratio (OR): 0.4, 95% confidence interval (CI) 0.3-0.7, and OR: 0.4, 95% CI 0.2-0.8, respectively), while radiation therapy after breast-conserving surgery was more frequent (OR: 2.5, 95% CI 1.4-4.8). Surgeon affiliation had no substantial effect on BC-specific mortality (adjusted hazard ratio (HR): 0.8, 95% CI 0.5-1.4). Conclusions This study suggests that private BC networks could be an alternative to public BC units with both structures presenting high quality indicators of BC care and similar BC-specific mortalit

Ultracold Electron Source

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Ultracold Electron Source

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Adiabatic Formation of Rydberg Crystals with Chirped Laser Pulses

Ultracold atomic gases have been used extensively in recent years to realize textbook examples of condensed matter phenomena. Recently, phase transitions to ordered structures have been predicted for gases of highly excited, 'frozen' Rydberg atoms. Such Rydberg crystals are a model for dilute metallic solids with tunable lattice parameters, and provide access to a wide variety of fundamental phenomena. We investigate theoretically how such structures can be created in four distinct cold atomic systems, by using tailored laser-excitation in the presence of strong Rydberg-Rydberg interactions. We study in detail the experimental requirements and limitations for these systems, and characterize the basic properties of small crystalline Rydberg structures in one, two and three dimensions.Comment: 23 pages, 10 figures, MPIPKS-ITAMP Tandem Workshop, Cold Rydberg Gases and Ultracold Plasmas (CRYP10), Sept. 6-17, 201

The circumstellar disk, envelope, and bi-polar outflow of the Massive Young Stellar Object W33A

The Young Stellar Object (YSO) W33A is one of the best known examples of a massive star still in the process of forming. Here we present Gemini North ALTAIR/NIFS laser-guide star adaptive-optics assisted K-band integral-field spectroscopy of W33A and its inner reflection nebula. In our data we make the first detections of a rotationally-flattened outer envelope and fast bi-polar jet of a massive YSO at near-infrared wavelengths. The predominant spectral features observed are Br-gamma, H_2, and a combination of emission and absorption from CO gas. We perform a 3-D spectro-astrometric analysis of the line emission, the first study of its kind. We find that the object's Br-gamma emission reveals evidence for a fast bi-polar jet on sub-milliarcsecond scales, which is aligned with the larger-scale outflow. The hybrid CO features can be explained as a combination of hot CO emission arising in a disk close to the central star, while cold CO absorption originates in the cooler outer envelope. Kinematic analysis of these features reveals that both structures are rotating, and consistent with being aligned perpendicularly to both the ionised jet and the large-scale outflow. Assuming Keplerian rotation, we find that the circumstellar disk orbits a central mass of >10Msun, while the outer envelope encloses a mass of ~15Msun. Our results suggest a scenario of a central star accreting material from a circumstellar disk at the centre of a cool extended rotating torus, while driving a fast bi-polar wind. These results therefore provide strong supporting evidence for the hypothesis that the formation mechanism for high-mass stars is qualitatively similar to that of low-mass stars.Comment: 13 pages, 18 figs. Accepted for publication in MNRA

Employment of the Master of Arts in Mathematics Education Graduates of a University in Northern Philippines

This study aimed to trace the 2015 to 2019 MAME graduates in the College of Teacher Education for Graduate Studies of the University of Northern Philippines in terms of their personal profile, their work-related profile before and after taking their master’s degree, reasons of taking up the program, competency level before and after taking the program, appraisal of the most useful courses offered in the program, evaluation on the contribution of the program to their personal and professional growth and assessment on the features of the program and suggestions of how to improve the program. The researcher utilized mixed method of research. The subjects were the MAME graduates, and a questionnaire and open-ended questions were sent to them thru various online flat forms. Based on the findings of the study, the following conclusions are derived: pursuing graduate studies chooses no sex, civil status or age as long as there is a will to improve one's self; students have math- related educational background and finished the program within the prescribed number of years; having master’s degree could contribute greatly in landing a permanent teaching job in public schools; professional and personal growth and development is the strongest drive to pursue advance education; the program offers subjects that are useful which enhanced the teaching competencies of the graduates and consequently contribute to the holistic development of the graduates; and that there are areas needing improvement in the MAME program