High-intensity lasers provide a unique source of energy and are the key components in a variety of applications, ranging from scientific research to industry and medicine. To reach very high pulse intensities, laser light is typically focused to a small point in space. After focus, the pulse expands and its intensity decreases due to pulse diffraction. However, some applications such as laser wakefield acceleration require laser pulses to remain intense over an extended length. A new development led by Berkeley Lab scientist Marlene Turner expands the reach of this field. The related research results are published on High Power Laser Science and Engineering, Vol.9, Issue 2, 2021 (M. Turner, A. J. Gonsalves, S. S. Bulanov, et al. Radial density profile and stability of capillary discharge plasma waveguides of lengths up to 40 cm[J]. High Power Laser Science and Engineering, 2021, 9(2): 02000e17).
For laser wakefield acceleration an intense laser pulse is used to excite an electrostatic wave in a plasma. Charged particles can be accelerated by this wave, similar to a surfer on an ocean wave. What is special about this kind of accelerator is that charged particles can reach a given energy in a distance that is thousands times less than is possible with conventional technologies. However, without laser pulse guiding the pulse expands quickly after focus, reducing the laser pulse intensity and the distance over which the laser pulse can drive high amplitude waves. The acceleration distance is then too short, leading to suboptimal final particle energy.
For low laser intensity, the solution to diffraction is glass fibers, which can transport (guide) laser light over thousands of kilometers. However, high-power laser pulses destroy these fibers. In this article we studied fibers made out of plasma (plasma waveguides) that work for high-intensity pulses. Plasma waveguides counteract diffraction, guiding the pulses and extending the length of high laser intensity. We demonstrated high-quality capillary discharge waveguides with lengths up to 40 centimeters, the longest to date.
A 20cm-long capillary discharge waveguide, used to guide high-intensity laser pulses and accelerate electrons to energy up to 8GeV.
How can a plasma waveguide guide a laser pulse? A lens or optical fiber can bend light using a refractive index profile that is peaked in its center. For a plasma, this is achieved with a plasma electron density that is minimum in its center. This radially increasing density profile provides a radially decreasing refractive index and acts like a strong focusing lens or light pipe for the laser pulse.
How can such a plasma waveguide be produced? Several technologies exist. In our case, we attached electrodes to each end of a gas-filled sapphire capillary, then applied a high voltage. The voltage induced a discharge (basically a controlled and contained lightning strike) and created the plasma. The discharge current heats the plasma, which is cooled at the capillary wall. This results in a temperature that gets cooler the closer you get to the capillary wall. Since pressure is equilibrated, this leads to a higher electron density as you increase distance from the center, which is exactly what you need for guiding the laser pulses.
Unlike glass lenses or fibers which are static objects, our plasma waveguides are recreated for each laser pulse. Therefore, we investigated how much parameters change with each discharge and demonstrated outstanding stability and reproducibility. This is important since charged particle beams accelerated in laser plasma wakefield accelerators tend to show large parameter variations. We found that waveguide parameters varied by less than one percent from discharge to discharge. Each channel had a very similar density profile. This is important because it means that every laser pulse will be transported in the same way, along the same path.
'This work shows that capillaries can produce extremely stable plasma targets for acceleration and indicates that observed variations in accelerator performance are primarily laser fluctuation driven, which indicates the need for active laser feedback control.', says Dr. Cameron R. Geddes Director of the Accelerator Technology and Applied Physics Division at Lawrence Berkeley National Laboratory in California.
The shape of glass lenses can be controlled very precisely to optimize optical performance. Controlling a plasma in the same way is usually quite a challenge. Ideally, the plasma density should have a parabolic radial profile. Far enough from the axis the channels are no longer parabolic, and we showed that this is important when the plasma is used as a telescope to increase a focused beam's size. However, with unprecedented precision we showed that the profiles are indeed very parabolic over the laser pulse spot size they are designed to guide, which allows for pulse propagation in the waveguide without quality degradation.
Capillary discharge waveguides have been used to accelerate electrons to the highest energies from a laser wakefield accelerator. The precision 40cm-long waveguides we have now developed could push those energies even higher.