Neutron Emission Spectrometer To Measure Ion Temperature On The Fusion Demonstration Plant P. J. F. Carle1a F. Retière2 A. Sher2 R. Underwood2 K. Starosta3 M. Hildebrand 1 S. Barsky1 S.

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Neutron Emission Spectrometer To Measure Ion Temperature On The Fusion

Demonstration Plant

P. J. F. Carle,1,a) F. Retière,2 A. Sher,2 R. Underwood,2 K. Starosta,3 M. Hildebrand,1 S. Barsky1, S.

Howard1

1General Fusion Inc., 108 – 3680 Bonneville Place, Burnaby, British Columbia V3N 4T5, Canada

2TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada

3Department of Chemistry, Simon Fraser University, 8888 University Dr, Burnaby, British Columbia V5A 1S6, Canada

a)Author to whom correspondence should be addressed: pat.carle@generalfusion.com.

(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX)

General Fusion is building the Fusion Demonstration Plant to demonstrate a magnetized target fusion scheme

in which a deuterium plasma is heated from 200 eV to 10 keV by piston-driven compression of a liquid-lithium

liner. The multilayer coaxial time-of-flight (MCTOF) neutron emission spectrometer is designed to measure

the ion temperature near peak compression at which time the neutron yield will approach 1018 neutrons/s. The

neutron energy distribution is expected to be Gaussian since the machine uses no neutral beam or radio-

frequency heating. In this case, analysis shows that as few as 500 coincidence events should be sufficient to

accurately measure the ion temperature. This enables a fast time resolution of 10 μs, which is required to track

the rapid change in temperature approaching peak compression. We overcome the challenges of neutron pile-

up and event ambiguity with a compact design having two layers of segmented scintillators. The error in the

ion temperature measurement is computed as a function of the neutron spectrometer’s geometric parameters

and used to optimize the design for the case of reaching 10 keV at peak compression.

I. INTRODUCTION

General Fusion is working towards building the Fusion

Demonstration Plant (FDP) in Culham, U.K. with

operations scheduled to begin in 2025. The FDP will use a

Magnetized Target Fusion (MTF) [1] scheme to compress a

deuterium plasma to fusion conditions. A conceptual

drawing is shown in Fig. 1.

Inside the main vacuum vessel, the machine’s rotor spins up

a liquid lithium liner to create a 3 m diameter cylindrical

cavity. An array of pistons is fired with a relative delay

between rows to initiate the spherical collapse of the liner.

The FDP will use a magnetized Marshall gun [2] at the top

of the device to form a spherical tokamak plasma

configuration via fast coaxial helicity injection [3] with the

initial plasma having expected parameters of T ~ 200 - 400

eV and    m-3. Current running through a solid

metal shaft along the geometric axis of the machine

generates toroidal flux to control the q-profile of the plasma.

The plasma is heated first by internal Ohmic decay and then

reaches fusion conditions through rapid, near-adiabatic

compressional heating with corresponding density increase.

The compression is timed to begin once the plasma has

stabilized in the cavity. The plasma reaches peak

compression in approximately 3-5 ms at which point the

now spherically shaped liner cavity has a diameter of 30 cm.

At peak compression, the plasma temperature and density

are expected to increase to 10 keV and    m-3

respectively.

Fig. 1. FDP conceptual drawing.

The ion temperature, , is a key parameter needed to

evaluate the success of a compression. Three main

diagnostic systems are being designed to measure : ion

Doppler spectroscopy, neutron yield, and neutron emission

spectroscopy. Other  diagnostics have been considered

such as collective Thomson scattering and neutral beam

charge exchange spectroscopy, but access issues are

currently disqualifying them from further consideration.

Access to the plasma will be a challenge for many FDP

diagnostics. Obstruction-free sightlines for conventional

spectroscopic or laser-based diagnostics will only be

possible via the top and bottom of the machine since the

liquid lithium liner will block the view to the plasma from

the sides. As compression proceeds, the liner will occlude

an increasing number of sight lines. An example

compression trajectory is given in Fig. 2.

Fig. 2. Example compression trajectory with liquid lithium liner

and poloidal flux contours shown as solid lines. Neutron emission

spectrometer line of sight is dashed. This shows a time sequence

during the compression from left to right of t = 0, 1.7, 2.95, 3.6,

and 3.8 ms, respectively.

II. Neutron Emission Spectroscopy

Fusion neutrons emerging from an MTF plasma will have a

Gaussian energy distribution that is a function [4] [5] of the

temperature of the reacting ions:





where  is the standard deviation of the neutron energy

distribution in keV and  is in keV. A neutron emission

spectrometer (NES) measures the spread in the neutron

energy distribution to estimate .

Neutrons that have scattered before reaching the NES will

contaminate the measurement with a low-energy population

of detection events. A neutron collimator, consisting of a

thick neutron absorbing material with a long central hole, is

needed to shield the NES from this background of lower

energy scattered neutrons; so what reaches the NES is

predominantly a beam of directed neutrons sourced from the

unobstructed line of sight passing through the plasma.

Design of neutron collimators that sufficiently reduce the

ambient background of scattered neutrons is well

understood in the field of radiography [6] and for fusion

diagnostic applications [7] [8].

In the case of a deuterium plasma, the collimated neutron

beam has an average energy of 2.45 MeV with a thermal

spread due to plasma ion temperature. An advantage of this

MTF scheme is that the neutron energy spectrum should be

a simple Gaussian since the FDP has no neutral beam or

radio-frequency heating.

There are several possible NES techniques [9] [10], some of

which are briefly highlighted below.

A compact detector (e.g. diamond) is a device in which an

incident neutron deposits some of its energy and generates

a pulse of some height and shape that can be measured. The

properties of a given pulse depend on the incident neutron

energy, the deposited energy, and type of interaction

between the neutron and detector material. The pulse height

is related to the incident neutron energy but, to be useful as

a spectrometer, the detector’s complex response must be

characterized over the range of possible incident neutron

energies and deposited energies.

In a magnetic proton recoil (MPR) neutron spectrometer, a

hydrogen-rich thin foil is placed in the path of a collimated

neutron beam originating from the plasma. Neutron

collisions with the foil generate recoil protons that can be

deflected by a magnetic field to an array of detectors to

analyze their momentum.

A time-of-flight (TOF) neutron emission spectrometer [7]

consists of a neutron collimator and two groups of

scintillators, Layer 1 and Layer 2 as shown in Fig. 4. The

beam of fusion neutrons is first incident on scintillator(s) in

Layer 1, and neutrons either pass through the material

undetected, or collide with a scintillator proton creating a

burst of light that is detected. A deflected neutron heading

toward the ring of detectors in Layer 2 has some chance of

scattering again within Layer 2. The time between

correlated scattering events in Layers 1 and 2 is related to

incident neutron energy. After some integration time, a

distribution of neutron energies will emerge.

In several previous TOF spectrometer designs, Layer 2

scintillators are positioned tangent to the surface of the

sphere of constant TOF [7] [8]. When a neutron collides

with a proton in Layer 1, the neutron will exit with an energy

that depends on its angle of deflection. Large-angle

deflections result in low exit-energies making it so that a

trajectory in any direction will intersect the surface of this

special sphere after a fixed period of time. The transit time

to pass across the sphere of constant TOF will only depend

on the incident energy of the neutron before the first

scattering. Positioning moderately large detector plates

tangent to this sphere will result in very small errors in the

estimate of the original neutron energy because scintillation

hits anywhere in that large detector plate are nearly

equivalent to each other in terms of the transit time across

the sphere. With this simplifying principle it is possible to

construct a high-resolution spectrometer with only a modest

number of detectors.

Diagnostic complications arise in a successful MTF

compression scenario, where the ion temperature and

neutron yield will rapidly increase many orders of

magnitude during compression (Fig. 3). This poses a very

different diagnostic challenge compared to the nearly steady

state fusion rate in a tokamak. To understand the conditions

achieved near peak compression, it is required to

accumulate neutron energy spectra on a timescale of 10 μs

and have good enough statistics in each spectrum to

estimate  with a 10% uncertainty or less. The expected

high count rate at peak neutron yield requires a strategy that

(1)

avoids pileup in any one detector, while making a choice of

overall size that optimizes efficient coincidence detection.

The proposed solution [11] discards the sphere of constant

TOF concept in favor of many small, segmented

scintillators in a compact arrangement to maximize the

number of useful events while maintaining a high energy

resolution.

Fig. 3. Example of compression scenario for FDP, a) Wall motion

begins at t =0, plasma is injected at t = 7 ms, peak compression

occurs at t = 12.5 ms. Exact timing values depend on details of

compression parameters. b) Dynamic neutron yield increases by

13 orders of magnitude during compression as temperature and

density rapidly increase. Dashed curves after peak compression

represent a conservative estimate of decreased fusion due to

cooling during rebound. FWHM of the neutron pulse is 12



Direct neutron travel time from source to detector is 300 ns.

III. MCTOF DESIGN

The neutron spectrometer estimates  from the distribution

of N neutron energy measurements. The relative uncertainty

in  is given by





 





  

where  measures the broadening of the distribution due

to the ion temperature and  is the standard deviation of

the distribution due to the system’s finite energy resolution

(measured  for =0 case).

A neutron scatters elastically at an angle θ from Layer 1 to

Layer 2 and travels a radial distance    , and an

axial distance Z, in a time . The energy of the incident

neutron is

 





where  is the neutron mass. The energy resolution of the

system can be estimated from propagation of uncertainty.

A measurement of  is needed at peak compression, when

the expected peak D-D yield is 1018 neutrons/s, and the TOF

energy spectrum will be accumulated from neutrons passing

along the unobstructed line of sight through the collimated

hole in the shaft as shown in Fig. 5. The distance from Layer

1 scintillators to the plasma is 6 meters, and the hole through

the shaft has a diameter of 2 cm. This gives an instantaneous

neutron flux of 109 neutron/s at Layer 1. Neutron and

gamma shielding directly surround the MCTOF

spectrometer.

During the 10 μs at peak compression there will be ~10,000

D-D fusion neutrons that will pass into Layer 1 at the exit

of the collimator. The overall efficiency of the system is

estimated to be around 5%, giving a count of 500 TOF

coincidence events out of which to compose an energy

spectrum and an average count rate of 50 MHz. One clear

challenge with such a high neutron count rate is the

increased chance of neutron pulse “pile-up” as well as an

issue of ambiguity in matching events in Layer 1 to events

in Layer 2. The pile-up problem can be addressed by

segmenting the scintillators so there is a lower chance of

multiple neutron events producing overlapping signals in

the same detector channel.

For event ambiguity, while it is possible to exclude some

events in post-data analysis according to pulse height [12],

an intrinsic improvement can be found by correctly sizing

the overall TOF distance between the layers to match the

expected peak count rate. To avoid the ambiguity of

multiple coincidence pairs overlapping in time with each

other, there is advantage in minimizing the TOF between

the two layers, so that the crossing time of one neutron is

complete before the next neutron is likely to arrive at the

spectrometer. However, a shorter overall TOF increases the

relative uncertainty in each  measurement, which will

Fig. 4. Multilayer Coaxial Time-Of-Flight (MCTOF) conceptual

3D model. A neutron trajectory which scattered in both scintillator

layers is shown in red.

(2)

(3)

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NeutronEmissionSpectrometerToMeasureIonTemperatureOnTheFusionDemonstrationPlantP.J.F.Carle,1,a)F.Retière,2A.Sher,2R.Underwood,2K.Starosta,3M.Hildebrand,1S.Barsky1,S.Howard11GeneralFusionInc.,108–3680BonnevillePlace,Burnaby,BritishColumbiaV3N4T5,Canada2TRIUMF,4004WesbrookMall,Vancouver,BritishColumbi...

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Neutron Emission Spectrometer To Measure Ion Temperature On The Fusion Demonstration Plant P. J. F. Carle1a F. Retière2 A. Sher2 R. Underwood2 K. Starosta3 M. Hildebrand 1 S. Barsky1 S.

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