Measuring fusion reactions: Neutron diagnostics

Fusion is the process of nuclei combining and creating new products, such as neutrons. Free neutrons are rare in nature and thus can serve as a unique signature of the fusion process. Neutrons are also neutral (uncharged) particles, whereas other fusion products are not. This property makes it highly probable that neutrons created during the fusion process travel outside the fusion reaction vessel. These neutrons can tell us if fusion is occurring, how much, what types of fuels are reacting, and give insights into the performance of fusion machines. At Helion, we employ a suite of diagnostics to detect neutrons and glean the valuable information they carry.

 

The key to measuring fusion power

 

Every neutron diagnostic relies on observing interactions between neutrons and matter and/or the subsequent effects. One of the oldest and most trustworthy ways to detect neutrons is to place a foil of material near the fusion machine, let neutrons hit it, and examine the foil afterwards. When neutrons hit the foil, they can interact with atoms and make them radioactive. By watching and counting how many of those unstable atoms eventually decay, we can count the total number of neutrons that hit the foil and work backwards to calculate how many neutrons were made by the fusion process during a pulse.

 

Selecting different foils serves different purposes. Sometimes we use foils that decay slowly (over the course of hours and days) so that we get a clear signal after all the fusion is done. Other times, we use different foils that decay quickly (on the order of microseconds to minutes) to get rapid feedback between series of pulses. The type of foil we use can also help us differentiate neutron energies, which in turn can inform what reactions created the neutron, and what the neutron did between birth and detection.

 

There are many neutron interactions that we can utilize. For example, neutrons can scatter in organic molecules that respond by emitting visible light or “scintillating,” and these are called organic scintillators. We build upon the fundamental principle of interactions with matter with modern electronics and data processing. Instead of human eyes watching for flashes of light in organic scintillators, we now use electronics (photomultiplier tubes) to convert visible light into electrons, multiply them, convert electrons into an electrical signal by collecting them on an electrode, and then transmitting the signal to a computer via a digitizer. Electronics improve the rigor of diagnostics, can increase the speed at which operators get information, and also enable access to new information.

 

It is by deploying a variety of complementary diagnostics, which each give us a different piece of the puzzle, that we can use neutrons to closely monitor and optimize the fusion power generated in our machines.

 

Inside Helion’s neutron diagnostic suite

 

At Helion, our neutron diagnostic suite comprises industry-standard tools, as well as diagnostics that we’ve created in-house to suit our needs. Our suite for Polaris was tested and calibrated with our partners including the Department of Energy, Los Alamos National Laboratory, and Lawrence Livermore National Laboratory. We have used machines like a dense plasma focus, a pulsed assembly (Godiva) that generates nearly as many neutrons as Polaris, and portable neutron generators in both the lab and positioned in and around Polaris. The Polaris diagnostic suite includes the following, and we have several additions that rotate into deployment, serve radiation safety purposes, and/or are undergoing development iterations.

 

Activation foils are pieces of different metals, for example indium, copper, or zirconium, depending on which fusion reaction we are measuring. We expose them to the neutrons, which “activate” the metals by changing or exciting the atoms in the metal. At the end of an experiment, we remove the foils with a pneumatic tube and spend hours counting decay signals in high purity germanium detectors. Activation foils are highly reliable and tell us the total number of neutrons generated by fusion over an entire pulse. Stacks of several foils are also used to discern the neutron energy spectrum throughout the shield vault, which provides valuable dosimetry information and enables us to confirm calculations made in Monte Carlo simulation codes.

 

Fast activation detectors tell us the relative neutron production between pulses (trading accuracy and precision for speed). For example, we deploy a silver detector for thermal neutrons, beryllium for deuterium-deuterium neutrons, a novel Cherenkov detector using SiO2, and LaBr3 with yttrium. These detectors provide feedback within seconds of a pulse so we can review fusion performance pulse-to-pulse.

 

Scintillators and fission chambers tell us when neutrons are created during a pulse, in addition to relative neutron production. Paired with plasma diagnostics, these detectors tell us about the lifetime of plasmas and how long fusion reactions occur.

 

Diamond detectors interact with neutrons in several ways. For example, neutrons from deuterium-deuterium fusion will elastically scatter, whereas neutrons from deuterium-tritium fusion can additionally induce (n,alpha) reactions that can be used to measure the energy of the original neutron. When fielded as a single detector, diamond informs relative production and the time-history of the fusion process. When fielded as an array of diamond detectors, signals can be combined to glean information regarding the (ion) temperature of the fuel that led to the creation of the neutrons

 

SRAM detectors are a form of high-speed computer memory that turns a “bug” (neutron upset) into a “feature” (a detectable signal). When neutrons strike SRAM circuitry, they can “flip” memory bits (zeros become ones and vice versa). By comparing the memory before and after a pulse, SRAM gives a quick estimate of neutron production. These detectors can be deployed for a low cost, and each unit can be easily replaced when damaged by too many neutrons.

 

A window into the fusion reaction

 

Neutrons carry an abundance of insight towards our fusion performance. Their ability to travel directly from the plasma to our diagnostics with limited interference makes them exceptional at providing clear signals about the fusion power we generate.

 

We have built a robust neutron diagnostic suite to take advantage of just that here at Helion. There is nothing else on Earth like fusion neutrons. Coming directly from fusion reactions, the precise paths of each of these particles tells a specific story that we can use to improve our machines, pulse by pulse.