4. Numerical simulations

#### 4.1 Geometrical model

We have used a geometrical model of this simulation experiment in a simplified form. The scheme encloses the PF-1000U stainless steel vacuum chamber, a set of

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

#### Figure 19.

A cut-view of the geometrical MCNP input: ZX plane. Numbered spheres are "detectors", the star with the letter "n" denotes neutron source (PF-6).

electrodes with insulators, a collector with cables, as well as all details of the hall interior. The vacuum cover of the PF-1000U chamber is detached and removed 2 meters away along Z-axis. Thus, an air is filled in the chamber. A frame of reference is originated in the center of the anode end. The Z-axis is on the axis of symmetry of the chamber, the X-axis is horizontal, whereas the Y-axis is vertical. A cross-section of the model in the X-Z plane is offered in Figure 14 and in Figure 19.

#### 4.2 Neutron source

The source of neutrons is point and lies on the Z-axis near the end of the opened chamber of the PF-1000U facility. The coordinates are (0, 0, 160) cm. The energy spectrum of neutrons is the Gaussian one. The peak of the most probable energy of the Gaussian spectrum of neutrons depends on the direction of neutron emission.

The group of neutrons that are emitted in-Z direction (in opposite direction in the respect to Z-axis) has energy around 2.7 MeV, neutrons that are emitted in XY plane (90° in respect to Z-axis) have their peak energy around 2.45 MeV, and the group of neutrons emanated in +Z direction (i.e., at 0° in respect to Z-axis) has their energy peak around 2.3 MeV. In all in-between directions, the neutron groups have corresponding intermediary energies. The widths of all Gaussian peaks are assumed to be 120 keV. The neutron emissions in all directions are unequal (i.e., anisotropic) as it is described above.

#### 4.3 The code

The MCNP code (X-5 Monte Carlo Team, MCNP—A general Monte Carlo Nparticle transport code, Version 5, Los Alamos National Laboratory, LA-UR-03-1987, 2003 [16]) was carried out for calculations. Cross-sections used in these computations have been derived from the ENDF/B-VI library [17].

#### 4.4 Tallies

Calculations of neutron flux density and spectra have been performed for seven spheres of air placed in positions with spherical coordinates that are described in Section 3.2.

Let us denote φ<sup>n</sup> (n = I, II, …, VII) as MCNP calculated neutron flux density in n-th SAC. Then anisotropy An(θn) equals:

$$A\_n(\theta\_n) = \frac{\rho\_n r\_n^2}{\rho\_1 r\_1^2} \tag{10}$$

Using the above procedure, we have provided test calculations both for neutron anisotropy and spectra for several configurations close to the experimental ones including those for the pulse shape as it is seen in the oscilloscope traces.
