Hula Hoop

Hula Hoop (フラフープ) is the largest particle accelerator in the world which is situated inside Academy City.

Etymology
A is a toy hoop which is spun around the waist, limbs or neck.

Principles
The giant ring-shaped particle accelerator follows the shape of the city's walls and is built 200m underground. Its control center is the same, situated near the outer edges of District 23, which borders the outer walls. To prevent the Hula Hoop from releasing gamma rays in case of accidents, there are protection walls on the level of nuclear shelters built around it. According to the different stages in speed, Hula Hoop is split into first, second and third rings, and accelerates particles separately starting from the smallest one. From the configuration, the third ring cannot work at a low speed of 30% of the speed of light - it is something that can only be used in experiments that force particles to go over 70% of the speed of light. It is capable of accelerating protons to 99.22% of the speed of light, and maintain that speed for up to 300 seconds. Surpassing the speed it was designed to hold, or running it longer than it was supposed to would both lead to Hula Hoop’s explosion; that meant enough gamma radiation to cover a third of Academy City would be released.

The accelerator's operation involves billions of eVs, with a single electrical short involving enough energy to vaporize a sports stadium. As its operations involves atoms or protons colliding at more than 99% of the speed of light, if tunnel shielding were to be compromised while it is in use, there would be a major release of neutrons and radiation. As such, the Hula Hoop has thick lead shielding, capable of containing radiation from the accelerator's operation, as well as shielding its equipment from external radiation. The accelerator has strict safety features and takes the highest priority within the hierarchy of precedence for the wall facility systems. If its internal stability is damaged, all resources will be used to help its recovery, leading to a drop in wall security in the meantime.

As well as requiring a large amount of electricity for its operation, the accelerator also requires a large amount of power for an emergency stop. As such, it has a back-up power generator in case the main generators fail.

DRAGON Arc
Spark Signal, having revolted against Academy City, carefully made a plan that included the seizing of Hula Hoop on October 17th and put into it the majority of their efforts and resources; the demand that they made to Academy City was "disclose all of the information you possess about DRAGON". Before Spark Signal's attack on Hula Hoop, they kidnapped around 30 primary school students on an astronomy field trip, along with their teacher and driver as a ‘bargaining chip’; while threatening to detonate Hula Hoop, which would have released enough gamma radiation to cover a third of the city.

The higher ups wanted to eliminate them and end the threat, dispatching GROUP to do so. GROUP used an elevator in District 23 to gain entry to it where they then waited ten minutes to strengthen the signal from the Misaka Network to Accelerator's collar. He then effortlessly eliminated Spark Signal in 300 seconds while also saving the first hostage that set to be executed.

Aleister Crowley Arc
On December 11th, the Hula Hoop was still operational, having being protected by its shielding from the recent heat wave. During his attempt to escape the city with his sister and Karasuma Fran, Tsuchimikado Motoharu attempted to take advantage of the particle accelerator's precedence in the wall facility's hierarchy which prioritized its recovery over maintaining the wall's security. He damaged the power cable connecting the wall and the underground using a drill and, intending to use the five minute reduction in security to climb over the wall, however Tsuchimikado Maika was struck by Aleister's curse magic sword at the wall, forcing them to change their plans.

Trivia

 * Given that Academy City is around 1/3rd the size of Tokyo, the city's circumference, and thus the length of the wall and the accelerator by extension, can be calculated as around 95km. This is three and a half times longer than the, although its proposed successor, the would be just as long.