---
title: General Fusion Plasma Injector PI3
url: https://www.emergentmind.com/topics/general-fusion-s-plasma-injector-3-pi3
type: topic
---

# General Fusion Plasma Injector PI3

General Fusion’s Plasma Injector 3 (PI3) is a spherical tokamak specifically engineered as a purely Ohmically-heated, low-density experimental platform to advance diagnostic techniques for fusion plasmas. Operating from 2017 to 2024 at General Fusion’s Richmond facility, PI3 was designed both as a test-bed for magnetized target fusion configurations and as a source of well-characterized deuterium plasmas for time-resolved neutron-based diagnostics, with core applications including the inference of fuel ion temperature from neutron counting [2601.01566].

## 1. Device Architecture and Operational Parameters

PI3 consists of a 1 m-diameter, 1.5 m-long vacuum chamber, into which a magnetized deuterium plasma slug is injected and accelerated via a coaxial "Marshall" plasma gun. The plasma is confined within a stationary, water-cooled aluminum flux conserver. Upon entry, plasma completes its own poloidal magnetic circuit by generating a toroidal plasma current $I_p$ (up to 50 kA), inducing a poloidal field $B_θ$ of approximately 0.05 T. An auxiliary static toroidal field $B_φ$ up to 0.1 T is maintained by an axial current passing down the central shaft for MHD stabilization.

Typical plasma shots achieve a line-averaged electron density $\langle n_e \rangle ≈ 3 \times 10^{19}$ m$^{-3}$, with core electron temperatures $T_e > 400$ eV and lifetimes up to 30 ms under favorable "catch" conditions. Deuterium fueling is achieved by a prefill gas puff at a few Pascal, supporting the formation and acceleration of a $\sim$1 m long, $\sim$0.1 m wide plasma slug via a 5–10 kV discharge. Plasma propagation into the flux conserver is driven by $J \times B$ thrust. Martialing these parameters, PI3 routinely produces up to $10^8$ D–D fusion neutrons per shot.

## 2. Neutron Detection Array and Absolute Efficiency Calibration

Neutron yield measurements in PI3 utilize a surrounding array of four uncollimated organic liquid scintillator detectors—two 0.83 L EJ-309 and two 3.49 L EJ-301 units—each shielded within a 25 mm-thick lead-lined steel enclosure and coupled to dedicated PMTs. Both high- and low-gain anode outputs are digitized at 1 GS/s for time-resolved analysis.

Detection thresholds for proton-recoil energies are 1.0 MeV (for SC9, SC10) or 0.5 MeV (for SC12, SC13), which are selected in software to suppress low-energy backgrounds. The absolute, position-dependent detection efficiency $\epsilon_i(E_\text{thr})$ for each detector is established via MCNP6.2 neutron transport simulations. These simulations model 2.45 MeV neutrons emitted isotropically from a 2D $n_e^2$-weighted source distribution reflecting the reconstructed plasma. PTRAC tallies provide event-level proton recoil data in the scintillator volumes, which are post-processed using a nonlinear light response model (with effective electron-equivalent energy $E_\text{ee}$ mapped from proton energy $E_p$ as $E_{ee} = A E_p - B [1-e^{-C E_p^D}]$). The efficiency is defined as the ratio of detected events above threshold to total simulated source neutrons, with statistical uncertainties of 1–3 %.

The time-dependent total neutron yield $Y(t)$ is derived as a weighted combination of pile-up–corrected count rates from each detector:

$$
Y(t) = \frac{\sum_i \dot{N}_i(t)}{\sum_i \epsilon_i}
$$

where $\dot{N}_i$ is the local detector rate and $\epsilon_i$ its efficiency [2601.01566].

## 3. Signal Discrimination and Event Corrections

Discrimination between neutron-induced and gamma-induced pulses in each detector is achieved through pulse-shape discrimination (PSD). PMT pulses exceeding digitization threshold are processed by integrating "tail" and "total" light regions, comparing their ratio to a power-law bifurcation curve established using calibrated Co-60 gamma-ray sources.

Observable pulse pile-up events—characterized by temporal overlap—are excluded from PSD and binned separately. In each time bin $\Delta t$, let $N_{\text{PSD},n}$ and $N_{\text{PSD},\gamma}$ denote the neutron and gamma counts identified by PSD. The neutron fraction is defined as

$$
P_n = \frac{N_{\text{PSD},n}}{N_{\text{PSD},n} + N_{\text{PSD},\gamma}}
$$

The excluded pile-up count, $N_\text{pu}$, is multiplied by $P_n$ to estimate the number of neutrons lost to pile-up, $N_{\text{pu},n}$, which, divided by $\Delta t$, gives the pile-up neutron rate:

$$
\dot{N}_{\text{n}}(t) = \dot{N}_{\text{PSD},n}(t) + \dot{N}_{\text{pu},n}(t)
$$

This aggregate, pile-up–corrected neutron signal enhances the accuracy of subsequent neutron yield and plasma parameter inference.

## 4. Ion Temperature Inference via Maxwellian-Plasma Formalism

Assuming a Maxwellian velocity distribution for deuterium ions, the local fusion reactivity $\langle \sigma v \rangle(T_i)$ for the D(d,n)$^3$He reaction is evaluated using the Bosch–Hale fitting formula. The plasma is partitioned into 20 flux-surface-indexed shells, with reconstructed $n_{d,\text{shell}}$ and $V_\text{shell}$ (from Bayesian equilibrium and density reconstructions). The neutron yield from each shell is

$$
Y_{\text{shell}} = \frac{1}{2}\langle \sigma v \rangle_{\text{shell}} n_{d,\text{shell}}^2 V_\text{shell}
$$

The total measured yield thus satisfies

$$
Y(t) \approx \sum_{\text{shell}} Y_{\text{shell}}(T_i) = \frac{1}{2}\int \langle \sigma v \rangle(T_i(r))n_d^2(r)dV
$$

For the analysis, parameterized ion-temperature profiles of the form $T_i(\bar{\psi}) = T_i(0)(1-\bar{\psi}^2)^\alpha$ are considered, with $\alpha=0$ (flat) and $\alpha=1$ (peaked). A forward table of volume-averaged reactivity is precomputed across a range of trial core temperatures. From the experimentally measured $Y(t)$, the experimental average reactivity is computed as:

$$
\overline{\langle \sigma v \rangle}_\text{exp}(t) = \frac{2Y(t)}{\sum n_{d,\text{shell}}^2 V_\text{shell}}
$$

Interpolation within the forward table yields the core ion temperature $T_i(t)$ at each instant.

## 5. Uncertainty Quantification by Monte Carlo Sampling

Uncertainty in inferred ion temperature arises from several sources:

- Poisson statistical noise in the discrete neutron counts ($\sigma_\text{rate} \approx \sqrt{N}$)
- 1–3 % propagation of MCNP sampling uncertainty in $\epsilon_i$
- Uncertainties in reconstructed densities $n_{d,\text{shell}}$ and volumes $V_\text{shell}$
- Ambiguity in the assumed $T_i$ profile shape ($\alpha$)

A Monte Carlo procedure (typically $N_\text{MC} \approx 10,000$ trials) independently perturbs each above input using their respective $1\sigma$ errors, reconstructs the neutron yield and reactivity, and inverts for $T_i$ as described. The final estimate adopts the median over samplings, with the 16th and 84th percentiles reported as asymmetric error bounds.

## 6. Experimental Outcomes and Comparison to Spectroscopy

Evaluation of PI3 shots via this neutron-counting diagnostic demonstrates time-resolved core ion temperature measurement. In a representative example (shot 22714), total neutron yields reached $2.5 \times 10^7$ within the first 6 ms, and neutron-inferred core $T_i$ rose from $\sim 300$ eV at 1 ms to 500 eV at 4 ms for flat ($\alpha=0$) profiles, with systematically 10–20 % higher values for peaked ($\alpha=1$) profiles. Reported uncertainties (±20–50 eV) are dominated by density, efficiency, and profile-shape effects.

Comparative analysis with simultaneous chord-integrated ion Doppler spectroscopy (IDS) using the C V line at 227 nm (acquired across 33 shots at $t=2~\text{ms}$ and $3~\text{ms}$) yields consistently lower temperatures (200–400 eV) than the neutron-based method (400–800 eV). This observation supports an interpretation that IDS, with its off-axis, edge-sensitive viewing geometry, underestimates the hotter core region as captured by neutron emission, which is centrally weighted.

A summary of these experimental comparisons is presented in the following table:

| Diagnostic Method           | Typical $T_i$ Range (eV) | Spatial Weighting             |
|-----------------------------|--------------------------|-------------------------------|
| Neutron counting + inversion| 400–800                  | Weighted toward plasma core   |
| Ion Doppler Spectroscopy    | 200–400                  | Chord-integrated, edge-biased |

This suggests a peaked-core temperature profile and validates the neutron counting technique as a robust, time-resolved, remote ion temperature diagnostic—particularly valuable for configurations where line-of-sight optical access is obstructed by hardware, as is expected in future magnetized target fusion systems [2601.01566].

Source: https://www.emergentmind.com/topics/general-fusion-s-plasma-injector-3-pi3