Ultra cold electron bunches:towards higher brightness

Pulsed high brightness electron bunches are used in many applications in science and technology. Novel concepts and techniques stimulate developments towards even higher brightness. The brightness of an electron bunch is a parameter that describes its quality and is expressed in the most general form as the current density per unit solid angle and unit energy spread. At present the most widely used pulsed high brightness electron sources are photo and field emission sources like the radio frequency (rf) photogun. The drawback of such a source is that the electrons have properties as if they were coming from a source with a temperature of around 5000 K. Therefore, their emittance is relatively high, since emittance is the product of the source size and bunch divergence and is proportional to the square root of the source temperature. The bunch brightness is inversely proportional to the square of the emittance. Most efforts on improving electron bunch brightness focussed on reduction of the source size while keeping the same amount of charge, which results in a decrease of the emittance. The approach to improve the bunch brightness that is investigated in this project is the reduction of the source temperature of the electrons and with that reduction of the bunch emittance. Such an ultra cold source is created by laser cooling and trapping of rubidium atoms. The ultra cold atoms are photoionized by a laser pulse that is tuned on or slightly below the ionization wavelength, leaving little excess energy per photon to be transformed into kinetic energy. The electrons created in this way are accelerated to an energy in the order of kilo electronvolts. This thesis presents a characterization of the ultra cold electron bunches. The bunch length, charge, source size and expected low source temperature are determined. The source temperature was not measurable directly in the experimental setup, but was determined indirectly by linking it to the bunch emittance. The bunch emittance and thus the source temperature was obtained by use of a model that is fitted to the experimental data, which is thoroughly explained in this work. The results are an electron bunch length of 4.7 ns fwhm, total charge around 10 fC, source size of 50 μm in the transverse direction and an upper limit for the source temperature of 15 K. The source temperature is extremely low, which is the most important message of this thesis. Together with the source size this corresponds to an extremely low bunch emittance of 2.5 · 10−3 mm mrad. This value for the bunch emittance is about three orders of magnitude lower than in currently used sources. All values together result in a normalized peak brightness of the bunch of 4 · 108 A m2 rad2 . This value is still four orders of magnitude lower than the current state of the art. Strategies to further increase the bunch brightness are to decrease the ultra cold electron bunch length by orders of magnitude. This will be achieved by a different acceleration scheme employing field ionization of highly excited Rydberg atoms. Furthermore, efforts will focus on increasing the total charge in a bunch while keeping a similar source size.