Green Light for Cool Electrons

A new high-power green-light laser generates beam-cooling electrons at the Relativistic Heavy Ion Collider.

Image courtesy of Brookhaven National Laboratory

View of light from a high-power green laser developed for low-energy electron cooling at RHIC, shown during a test after it has been transported into a vacuum chamber and illuminating  the photocathode (bright spot about 1/3rd from the bottom of the picture) and other laser light scattered and reflected from the vacuum chamber.

The Science

Scientists at Brookhaven National Laboratory have produced a powerful green laser that will be crucial to experiments in nuclear physics at the Lab’s Relativistic Heavy Ion Collider (RHIC), a particle accelerator where nuclear physicists collide two beams of gold ions to explore the fundamental building blocks of matter and the forces through which they interact. The light—the highest average power green laser ever generated by a single fiber-based laser—will be used to create pulses of electrons needed to cool the beams of gold ions circulating inside RHIC.

The Impact

The goal of heavy ion collisions at RHIC is to recreate the conditions of matter just after the universe began. To study how this matter evolved to form the protons, neutrons, and atoms of the universe today, scientists need lots of collisions between gold ions, gold atoms stripped of their electrons, in two counter rotating beams stored in the accelerator. Cool beams, with tightly packed ions, collide more often than hot, spread-out beams. So physicists will use the laser to generate a beam of relatively cool electrons to extract some of the ions’ heat, keeping collision rates high. Beyond nuclear physics, this type of laser could also have applications in materials processing, laser machining, and generating other lasers.

Summary

Keeping ion beams cool keeps the particles tightly packed, so more ions collide to generate data. RHIC physicists will use relatively cool electrons to cool the ion beams, much like a liquid refrigerant injected periodically into the circulating stream of ions to extract some of their heat. To create the electrons, they’ll use a photocathode electron gun triggered by a powerful laser with a wavelength in the green portion of the visible spectrum. When this wavelength of light strikes the photocathode inside the electron gun, the material that makes up the cathode emits electrons. In order for the pulses of electrons generated by the gun to be accelerated to precisely match the pulses of ions circulating in RHIC, the laser must be precisely synchronized, aligned, and have the appropriate power.

The laser specialists and accelerator physicists at RHIC achieved the required specifications starting with a relatively low power infrared (IR) laser, amplified through a series of optical fibers. After amplification, the infrared laser strikes a “frequency-doubling” lithium triborate crystal. When two photons of infrared light strike the crystal, it emits one photon of a shorter wavelength, changing the IR input to green visible light.

Throughout the amplification and frequency-doubling steps, the laser must stay precisely aligned as it zigzags and bounces off mirrors attached with micrometer-scale precision to a tabletop anchored for stability to a 50-ton steel block buried deep underground. Additional processing prepares the green laser to create on-off pulses that trigger the emission of electrons that can be accelerated to match the pace of the ions circulating in RHIC. After the electrons absorb heat from the ions, they are dumped and replaced by another cool batch.

Fiber lasers are especially well suited for generating high-brightness electron bunches in photocathode electron injectors. The high surface-to-volume ratio of the fiber supports the generation and delivery of laser pulses at high repetition rate and high average laser power. Also, the dynamics of the laser light propagating through the fiber leads to excellent laser profiles, low variations in the laser’s position, and maintenance-free operation. Taken together these properties result in long-term operation of a highly stable laser, which is essential for the RHIC physics program.

Contact

Michiko Minty
Collider Accelerator Department, Brookhaven National Laboratory
minty@bnl.gov

Funding

DOE Office of Science (NP)

Publications

Z. Zhao, B. Sheehy, and M. Minty, “Generation of 180 W average green power from a frequency-doubled picosecond rod fiber amplifier.” Optics Express Vol. 25, Issue 7, pp. 8138-8143 (2017). DOI: 10.1364/OE.25.008138

Related Links

Low-Energy RHIC Electron Cooling Gets Green Light, Literally

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Program: NP

Performer: DOE Laboratory , SC User Facilities , NP User Facilities , RHIC