Ultrafast Laser Laboratory
Laser-Triggered High Voltage Pulser
The inherent emittance and brightness of the electron beam from the RF injectors are limited by the temporal and spatial variation of the applied RF field at the cathode while the electrons are emitted and transported. The space charge forces experienced by the electrons in the vicinity of the cathode where the electrons are nonrelativistic also affect the brightness adversely. Nonlinear correction techniques that minimize these effects in RF injectors are being studied. Another technique to minimize the emittance growth due to varying field is to use a pulsed electric field at the cathode. For this scheme to be successful, the pulse duration of the high voltage must be significantly longer than both the emission time and transport time of the electrons in the nonrelativistic regime and the electrode geometry should be optimized for minimum field variation over the emitting area. In addition, very large field should be established at the cathode to overcome the space charge effects. The highest field that could be maintained at the cathode is limited by the occurrence of electrical breakdown. It has been shown that field gradients exceeding 1 GV/m for duration of 1-5 ns can be supported without breakdown by using carefully prepared electrodes. To satisfy these requirements, a high voltage pulser with pulse duration of ~ 1 ns, rise and fall times of ~150 ps and amplitude of ~ 1 MV has been constructed. With such a pulser, field gradients of 1.6 GV/m has been supported using copper cathode and stainless steel anode without significant dark current.
Typical spot at 1st BMP, sigma=0.58 mm
Typical spot at 2nd BMP, sigma=0.20 mm
The simulated behavior of the electron beam produced by the pulsed-power photo-injector is very promising. In order to test these predictions, a system to measure the beam emittance has been installed in the pulser beamline. This apparatus consists of a solenoid magnet to focus the electron beam, and a pair of beam profile monitors (BPM) to observe the beam size at two locations. The profile monitors have been used to obtain beam images for a variety of solenoid settings. By comparing the image size at the focus (on the second BPM) to the image size at the first BPM, the beam emittance can be obtained. These BPMs also act as Faraday cups, allowing the bunch charge to be measured. The normalized emittance for the spots shown spots shown is 1.3 mm-mrad, for a bunch charge of 5 pC and laser pulse duration of 300 fs. The smallest focal spots obtained have had a 1-s width of 100mm and a normalized emittance of 0.7 mm-mrad. These measurements have been made at a beam energy of 400 keV, with accelerating fields between 200-400 MV/m. The gradient can be controlled by adjusting the anode position in-situ. The system has been used to produce electron bunches of up to 60 pC with a Ti-Sapphire pulse duration of 300 fs, corresponding to a peak current of over 200A from a 1 mm diameter laser spot on the cathode. The system is capable of producing output energies of nearly 1 MeV, with accelerating gradients in excess of 1 GV/m. This should lead to a further improvement of the beam quality, and further increase in the peak current.
Already, a number of institutions such as the Lawrence Livermore National Laboratory, the Source Development Laboratory in BNL have shown interest in using this device as an electron source. This device can also be used to generate microwave radiation at frequencies > 30 GHz, and optical radiation at < 1000 Å. Proposals to explore these avenues are under preparation. This project has also proved to be a sound learning ground for graduate students since it encompasses a wide range of topics such as the lasers, high voltage systems, electron optics and electron diagnostics and yet small enough for one person to be responsible for all these systems.
An SBIR Phase I grant has been awarded for the construction of a similar electron gun. If this award continues into Phase II, a CRADA has been established to use our device as a working model.
Under this program, another high voltage pulse generator capable of delivering 5 MV on a 160 Ohm load in 1 ns with rise and fall times of 100 ps has been built and is being tested.
Picture of the 5 MV pulsed electron gun. hi-res
Voltage trace of the 5 MV pulser
For more information or preprint request contact Triveni Rao
Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST