Subnanosecond pulsed-DC ultra-high gardient photogun for bright relativistic electron bunches

This work is devoted to a novel method for the production of bright, relativistic electron bunches for Laser Wakefield Acceleration. The production of high brightness, ultrarelativistic electron bunches to be used, for example, in X-ray Free- Electron-Lasers is a big challenge for accelerator physicists. Utilization of classical schemes of acceleration leads to significant size of the bunch forming system (injector) and requires large accelerators. This makes it impossible to have such a system available in the laboratory. The only option is to use large facilities shared between many users, and even then, the price of experiments will be high. Therefore novel methods of acceleration and bunch compression have been explored during the last few decades. One of these methods is Laser Wakefield Acceleration, where the acceleration process occurs in a plasma channel. For LWA an injector is needed which produces ultrashort (less than 100 fs) electron bunches. The method investigated in this thesis is based on acceleration of an initially short (25 fs) photo-emitted electron bunch in an extremely high DC field (1 GV/m). To avoid breakdown the acceleration field should be applied for a time shorter than the typical formation time of vacuum breakdown, which is in the range of one nanosecond. For this project it was proposed to use an electrical pulse with a duration of 1 ns and an amplitude of up to 2.5 MV. For production of such a short and high voltage pulse conventional pulsed high voltage generators are not suitable due to many problems related to synchronization, electrical insulation and output pulse reproducibility. Therefore sub nanosecond pulse forming techniques were used in order to create the required output pulse. The main element of this technique is a pulse forming line (PFL), where pulse sharpening and shortening occurs. A Tesla type pulse transformer produces 2.5 MV pulses with (sub-) microsecond duration. The output of the pulsed transformer is switched in a liquid filled spark gap. The PFL is then charged in 5 ns to the 2.5 MV level. The PFL consists of a short storage line and a sharpener discharger to produce pulses with a risetime of around 200 ps. After the sharpener discharger, a cut off discharger reduces the duration to around 1 ns. The pulser was designed and built specially for TU/e at the Efremov institute, St. Petersburg, Russian Federation. During first start up of operation after the system was installed in Eindhoven, we identified several critical points in the design of the pulser. Together these points seriously affected the life time of the entire pulser system. During the first two years of this work the pulser setup was considerably improved. Many parts of the pulser were redesigned and remanufactured. Finally the pulser could be operated for several thousand shots without replacement of the components. In addition, the reproducibility of the output pulses from the transformer was improved from around 20% to about 1%. Variations of the output of the transformer cause instabilities in the breakdown formation in the main liquid spark gap between the transformer and the pulse forming line (PFL). The overall voltage reproducibility achieved at the end of the PFL, i.e. the output of the system, was better than 10%. An acceleration section consisting of a vacuum diode and a beam line setup was designed and built, including several diagnostics. Consideration of the vacuum diode as a part of the PFL showed that the voltage across the acceleration gap can be practically two times higher than the amplitude of the incident pulse (the output pulse of the PFL). This was confirmed later by dark current energy spectra measurements. The maximum registered energy of the electrons was 3.6 MV at 75% of the practical maximum output of the PFL (100% is 2.5 MV), this is the highest registered electron energy for this type of accelerators. For the successful realization of the pulsed DC acceleration concept, a femtosecond laser pulse should arrive on the cathode surface within the applied voltage pulse duration. This imposes strong requirements on the jitter of the whole system, which must be less than 1 ns. For synchronization of the pulser to the laser system we used a laser pulse for the triggering of the main liquid spark gap. The experiments performed showed that even with laser triggering of the liquid spark gap it is impossible to achieve jitter less than the requirements. This is due to the fact that random processes which occur after triggering determine the final breakdown jitter. The best synchronization that was achieved with laser triggering is 29 ns. Photoemission was attempted in a series of several hundred shots. In this series, only two shots with possible photoemission result were detected. The registered charge of these (two) bunches was around 350 pC, close to the expected value. On the basis of this work we identified several critical points of the existing device which must be improved in order to get a suitable setup. Most of them are related to the pulser. As was motioned before, the main issue for stable operation is synchronization, which is mainly determined by the liquid spark gap operation.. To improve the existing situation we propose to use a gas filled spark gap, shorter applied voltage pulses (higher operation frequencies of the pulse transformer), and cylindrical optics for the triggering laser pulse to ionize a larger part of the gap directly by the laser. Another problem is related to the lifetime of the cathode. In our setup the output pulse from the PFL has a long after-pulse with an amplitude up to 1 MV. During this after-pulse the probability of vacuum breakdown in the diode is significant. This significantly decreases the lifetime of the cathode and the reproducibility of the charge of the bunches. To improve this, an active (ohmic) load should be installed in the final part of the PFL. Another possible way to stabilize the current is to use a selfrecovering cathode, such as a liquid metal cathode. In conclusion we can state that the concept of pulsed DC acceleration can be used for the production of ultrashort relativistic electron bunches, but it requires considerable additional engineering research.