As a unique energy source, super lasers play a key role in many aspects such as scientific research, industry, and medicine. In order to obtain high-intensity laser pulses, the beams generally converge to a very small size in space, and after converging, they will quickly diverge due to diffraction effects. However, in areas such as laser wakefield acceleration, lasers are required to maintain high light intensity over a considerable distance. A study led by Marlene Turner, a scientist at Lawrence Berkeley National Laboratory (LBNL), extends to this field.
In the laser wakefield acceleration, the super-powered laser is used to excite the electrostatic waves in the plasma, and the charged particles can be accelerated in the electrostatic waves, similar to surfing on the sea. The most special feature of this type of accelerator is that the acceleration distance required for charged particles to obtain a certain amount of energy is thousands of times shorter than that of traditional acceleration methods. However, if the laser beam is not guided, it will disperse soon after the focus, greatly reducing the intensity of the laser pulse and the acceleration distance that can drive the high-intensity wake field. Therefore, the shortening of the acceleration distance will cause the particles to not obtain the best acceleration energy.
For low-intensity pulses, the solution to diffraction is optical fiber glass, which can guide the laser beam for thousands of kilometers, but high-intensity lasers can damage the optical fiber. In an article in the second issue of High Power Laser Science and Engineering 2021, Professor Marlene Turner and others studied the plasma fiber used for super-powered lasers. Plasma can reduce the diffraction effect and guide the laser beam to extend its high-intensity transmission distance. . The research team showed the longest 40 cm high-quality discharge capillary to date.
How does the plasma waveguide guide the laser? The lens or optical fiber can deflect the laser light through the strongest refractive index distribution in the center. For plasma, it is achieved by the lowest electron density distribution in the center. Gradually increasing the distribution of electron density in the radial direction leads to a gradual increase in the refractive index in the radial direction, which is like a super-powered lens or laser tube for high-power lasers.
How can such a plasma be generated? A number of technologies have been implemented so far. In this paper, the researchers used a gas-filled sapphire capillary tube with electrodes connected to both ends. The plasma is generated by high-voltage discharge. The discharge current heats the plasma and cools it near the tube wall, making the temperature closer to the tube wall lower. Because the air pressure is balanced, the electron density from the center to the ends gradually increases, which results in a super strong waveguide for guiding the laser beam.
Unlike a static glass lens or optical fiber, the plasmonic waveguide is re-established every pulse. Therefore, the researchers studied in detail the parameter changes of each discharge and demonstrated excellent stability and repeatability. This is very important for the accelerating beam with multi-parameter changes in the acceleration of the laser wake field. The researchers found that the change of waveguide parameters in different discharge processes is less than 1%, and the density distribution in each channel is very close. This means that each laser pulse will travel in the same way along the same path in the waveguide.
"This work shows that the capillary tube can generate a very stable plasma, which indicates that the fluctuations observed in the accelerator performance are mainly due to the fluctuations of the laser drive, and very instant laser feedback control is required to ensure stability." California LBNL accelerator technology Dr. Cameron Geddes, Director of the Applied Physics Department, gave the above comments on this work.
The precise control of the shape of the glass lens determines the optical performance, but it is a challenge to control the plasma to the same level. Ideally, the electron density distribution is parabolic, but in fact it is no longer a parabola far away from the axis of the channel. The researchers found that this is very important in the plasma as a telescope system to increase the focal spot of the beam. Through very precise control, the researchers in this paper use parabolic plasma distributed near the focal spot of the laser to guide the laser, so that the quality of the beam will not decrease during the propagation of the beam. The discharge capillary waveguide has obtained high-energy electrons in the laser wakefield accelerator. The 40 cm long waveguide developed by the research team is expected to push the cut-off energy to a higher level.
