A cold atom electron source

Abstract

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 mi3 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.