PPPL Stories

The Neutron Express
December, 2009

PPPL technician Sly Vinson installs the tracks inside NSTX.
Photo credit: Elle Starkman, PPPL

A ride around toy train tracks inside a fusion experimental machine can reveal much about a hot gas called plasma. PPPL research staff recently mounted a neutron source on a toy train inside the National Spherical Torus Experiment (NSTX), placed it on hobby train tracks that had been installed and sent it on a three-day continuous zip around the rails. The purpose: to calibrate the neutron rate during fusion experiments.

Plasma, the fourth state of matter, is a hot gas that makes up the stars and is used as the fuel to produce fusion energy. Energy from plasma is released in the form of neutrons.

"Since NSTX plasmas emit copious amounts of fusion neutrons, scientists need an accurate measurement of the neutron rate. They use this to compare, for example, whether the simulation code has faithfully modeled the various phenomena occurring in the plasma," said PPPL physicist Doug Darrow. Darrow, along with PPPL engineer Lane Roquemore, led the project. PPPL technician Sylvester "Sly" Vinson led the assembly and installation portion. About two dozen PPPL scientific and technical, health physics, computing, management, and review staff supported the effort.

Besides helping scientists understand the physics of energetic particles, the measurement helps them develop a better predictive capability for future fusion reactors.

The train tracks for the NSTX calibration.
Photo credit: Elle Starkman, PPPL

To perform the calibration at NSTX in early December, researchers used a Californium-252 neutron source. This particular source emits neutrons with energy similar to deuterium-deuterium fusion neutrons. Deuterium is a form of hydrogen used to fuel fusion reactions. Two deuterium atoms collide — or fuse — at hot temperatures to create energy.

To obtain the needed calibration data, the team mounted the small source on the model train and remotely ran it around five different circular tracks inside the NSTX vacuum vessel for three days. Two neutron detectors picked up the data from the exterior of the vessel.

"The tracks were set up in five different positions, which span the volume of the plasma that generates the neutrons," Darrow noted. "Physically it works because plasma is a big circle. Running the neutron source on a set of circular tracks is a good approximation of plasma from the point of view of the neutron detectors."

The absolute calibration measured the production of neutrons per second and provided a realistic simulation of plasma-created neutrons. "It mimicked a ring of neutron emission that is most like the shape of the neutron emitting region of NSTX plasmas," Darrow said. And since the source was securely encapsulated, it made any possibility of a radiation release very remote.

PPPL developed the idea to use toy trains for neutron calibrations several years ago. Researchers have since used the method on fusion machines at the Lab and at other fusion research institutions.

"A model train may sound low tech, but it provides us with exactly the right arrangement we need for this calibration," said Darrow. "Besides, thousands of hobbyists have tested it before us so it has high reliability for this job." Choo-Choo!

by Patti Wieser
pwieser@pppl.gov

The Neutron Express [video]