1. Introduction

Contemporary and future nuclear fusion reactors are of rather sophisticated assemblies positioned in intricate surroundings. Elements of their environment and constructions may absorb and scatter the basic fusion energy carriers—neutrons [1].

The contemporary main-stream nuclear fusion installations using magnetic and inertial plasma confinement, namely: Joint European Torus (JET, U.K.) [2], Wendelstein 7X Stellarator (W7X, Germany) [3] (in the nearest future—the International Thermonuclear Experimental Reactor (ITER) [4]), the powerful laser devices Iskra-5 (in future Iskra-6, R.F.) and National Ignition Facility (NIF, U.S.A.) [5] as well as the Z-Machine [6] (Jupiter expected in future [7], U.S.A.)—generate around their chambers 3-D neutron fields that are distorted at their irradiation outside. The elements of the sheds, numerous structures of the Nuclear Fusion Chambers (NFC), power sources as well as specific apparatus belonging to these parts of the facilities exemplify scatterers and absorbers irradiated by neutrons. So produced by the fluctuations ("voids" and "hot spots") in neutron intensity and in spectra around the NFC must be taken into account at the interpretation of the operational results. The neutron intensity changes and spectra imperfections observed out of NFC because of elastic and inelastic neutron scattering may happen even at an absolutely isotropic initial expansion of neutrons into space from a source with symmetric nuclear fusion neutron spectral content (e.g., from a laser target in a laser fusion facility or from an element of the toroidal plasma ring in a tokamak).

But there is an opportunity to describe a 3-D neutron field formed around a nuclear fusion chamber before its full-scale operation with a help of a foreign powerful point neutron source that has pulse duration in a nanosecond (ns) range. Indeed the intense short neutron flash will allow attributing and describing all elements of a NFC that absorb and/or scatter neutrons separately by using measurements of neutron fluxes (with neutron activation methods) and spectra (with photomultiplier tube plus plastic scintillator—PMT + S—by means of time-of-flight (TOF) method) in all directions. These two procedures will also be important from the point of view of the radiation material science: they will give information where one may expect increased or diminished values of dpa in the plasma-facing and construction materials of a NFC.

A very intense ns neutron pulse irradiated from a tiny volume (about 1 cm<sup>3</sup> ) can be generated by a nuclear fusion device named dense plasma focus (DPF) [8]. Moreover, its neutron emission is quasi-mono-energetic one. So by means of this device, one may have an opportunity to distinguish elastic scatterings produced by different parts of a chamber or by dissimilar chemical elements of their content. DPF may also be used in time-of-flight technique for spectra measurements with a moderate path length.

Nuclear fusion reactions in a DPF are produced at the interaction of selfgenerated and magnetized fast deuterons with pinched plasma [8]. It is similar to the process taking place in tokamaks with an external neutral beam heating of its plasma. Accelerated fast deuterons have spectrum spreading to MeV range and peaked at hundreds keV. The DPF device may be exploited with D or D-T mixture as working gases. In these cases, it will produce neutrons with mean energy at around 2.5- or 14-MeV energy peaks correspondingly as it is so in the contemporary main-stream NFCs. With these ns neutron pulses, a majority of materials used in activation technique will have the activation time much shorter compared with the time of their radioactive decay.

Nanosecond neutron pulses are irradiated from the DPF chamber into space as a neutron "shell" (Figure 1a) of a finite thickness. It has almost a spherical shape. The thickness of the shell (i.e., a space between the surfaces A and B of the sheath filled

#### Figure 1.

A neutron "sheath" (a) irradiated from a compact neutron source (shown by a star) of an ns pulse duration having a thickness in space Δl given by formula (1); its possible use (b) in a large main-stream NFC (a sketch of a cross-section of the ITER chamber is here).

Taxonomy of Big Nuclear Fusion Chambers Provided by Means of Nanosecond Neutron Pulses DOI: http://dx.doi.org/10.5772/intechopen.89364

with neutrons) has a value Δl equal to pulse duration of neutron radiation Δt multiplied by neutron speed v:

$$
\Delta l = \Delta t \times v \tag{1}
$$

This sheath during its propagation from the compact source outwards will be distorted because of absorption and scattering on elements and systems belonging to a NFC. Thus, such a source can be able to uncover each element of a NFC producing the above-mentioned distortions during neutron radiation expansion through the chamber components (Figure 1b).

These alterations may be found in data on the absolute neutron flux measured in certain 3-D points in the exterior of the chamber. It will also be discovered as confident modifications in a neutron temporal evolution in time and, consequently, in neutron spectral composition after their transit through elements and systems of the nuclear reactor.

The spatial thickness of the above neutron "shell" will have a value of about 10 of cm being much less compared with the main construction elements of a NFC of a main-stream fusion facility. Thus, for the taxonomy of objects by such a bright

#### Figure 2.

Dense plasma focus device PF-6 (a) and a big chamber of the PF-1000U facility (b), simulating a section of a main-stream fusion chamber of a present-day tokamak (JET here) (c).

short-pulse neutron radiation, one may use elastic and inelastic scattering of neutrons upon nuclei of unknown elements. It is evident that this short powerful neutron flash allows using TOF technique with short flight bases for modern NFCs.

This type of measurements can be provided by positioning of a DPF-based compact neutron source in the center of the spherical chamber used in laser fusion facility or at the movement of this neutron source along the circumference of the toroidal chamber of a modern fusion device with magnetic plasma confinement. Such characterization procedures should preferably be repeated after each important stage of assembling of a new main-stream fusion facility to describe its novel elements and their influence on neutron field.

Here in the very beginning, we shall observe the activation methods applied by using ns neutron pulses generated in a DPF device PF-6 (Figure 2a) due to D-D reactions. Thus, the generated in the device 2.5-MeV neutrons are subjected mainly to elastic scattering on parts and structures of a simulator of a NFC [1]. Then later, a neutron spectroscopic technique will be talked over.

In this case, we exploit in the capacity of the simulator of a big NFC a large chamber that belongs to the PF-1000U facility [9] available at the Institute of Plasma Physics and Laser Microfusion (Figure 2b), Poland. This chamber looks quite similar to the section of JET tokamak (U.K.)—Figure 2c. The vacuum chamber of the PF-1000U device has a shape of a large cylinder with walls made of stainless steel. The discharge circuit of the chamber consists of a set of capacitors, cables, and spark-gaps connecting the battery with cylindrical concentric electrodes playing the role of a plasma accelerator.
