5.3 Experimental results on measurements of spectra of neutron emission using PF-6 device with an object simulating a section of a toroidal chamber of a mainstream fusion facility (the PF-1000U chamber)

First, we made the measurements in the "clean room" condition. We preserved the PMT + S-2 position from one side of the DPF chamber (at 105 cm) but moved PMT + S-4 along the steps shown in Figure 14. In the direction perpendicular to Z-axis (at the angle 90<sup>0</sup> ) of the chamber as it was mentioned above the neutrons' energy is 2.5 MeV [1]. Thus to tie the neutron pulse to the X-ray pulse in the center of the PF-6 chamber for the detectors Nos 1 and 2 placed at 1.05 m, one have to shift forward in time the hard X-ray pulse by 3.5 ns (vhxr = 3 <sup>10</sup><sup>8</sup> m/s) and the neutron pulse by 48.5 ns (vn = 2.1667 <sup>10</sup><sup>7</sup> m/s) in a way that is presented in Figure 18a and b. As a result of this process (averaged over 33 shots), the delay time of neutron pulse maximum inside the chamber in relation to the hard X-ray pulse front has been found as Δt = 25 ns for both stands. In this set of experiments and later we checked this figure in each shot by using fixed stand No. 1. After these experiments, we obtained the basic data for the subsequent measurements and corrections that have to be done for all other neutron pulses registered at different angles and at dissimilar distances from the neutron source based on the PF-6 device.

Figure 22.

Angular tracking of energy distribution of neutrons at the PF-6 device in "clean" conditions: (a)—distances from the source (PF-6) to the detector (PMT + S No. 2); (b)—polar diagram of the angular tracking of neutron energy distribution; (c)—data table of energies measured in the specific points.

Each time we begin from the front of the hard X-ray pulse, moving neutron pulse to the point delayed to the front by Δt inside the chamber. Then the TOF of this neutron pulse from the chamber to the detector No. 2 in each specific location is calculated. This time-of-flight can easily be recalculated into the energy of this neutron group by formula (6). Results of calculations gave us the angle tracking of the neutron spectral distribution in the space around our PF-6 device in a "clean" room that is presented in Figure 22.

In the next step, we have compared these results obtained by PMT + Ss with the data attained with the PF-1000U discharge chamber (Figure 14). The procedure looks as it was before for the "clean-room" experiment.

Again in the beginning, we made measurements of the delay of the maximum of a neutron pulse in relation to the front of the pulse of hard X-rays inside the DPF chamber. For the fixed stand No. 1 placed now at a distance of 0.9 m from the source the delay time of hard X-ray pulse here was 3 ns. The neutron pulse maximum inside the chamber in relation to the hard X-ray pulse front was found now to be in the interval of 9…18 ns in different sets of shots—see e.g., Figure 23.

#### Figure 23.

OTs obtained at 1.1-m distance from the source by a movable stand No. 2 (a) at the location perpendicular to Z-axis (position I of 14) and by a fixed stand No.1 placed at the distance of 0.9 m (b)—both with vertical lines showing shifts of the fronts of the hard X-ray pulses (first, blue) and of the maxima of the neutron pulses (second, red) by their time-of-flights for 2.5-MeV neutrons.

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


#### Table 8.

Data on the angular tracking of energy distribution of neutrons at the PF-6 device in the hall with a simulator of a tokamak section (the PF-1000U discharge chamber) with angles and distances in a horizontal plane with Z-axis of both devices and PMT + Ss.

The values calculated from the OTs for the simulation experiment are shown in Table 8 and in a polar diagram (Figure 24) where the "clean-room" conditions are depicted by blue color (a lower half-plane) and the simulator experimental data are presented by the red one (the upper half-plane).

The circles in this polar diagram have the same meanings as above: 2.0, 2.5, 3.0, and 3.5 MeV outwards.

From this diagram and the table, one may see that the difference in energy values between "clean-room" conditions and simulation experiments observed almost at all angles is not very much. The real change may be seen in forward direction along Z-axis. It is not surprising—namely in this zone, we have the most serious obstacles in the PF-6 and in the PF-1000U chambers (the stainless steel supplement of the PF-6 device, electrodes of the PF-1000U facility, and several metallic disks for vacuum preservation) that can lead to multiple scattering of even high-energy neutrons. But these features are not the only ones. Some other OTs demonstrate facts connected with the movable stand No. 2 when it is placed in large distances from the PF-6 device and at angles below 90°. In these positions (IV, V, VI, and VII), PMT + S-4 registered hard X-ray and neutron pulses passed through the PF-1000U chamber and interacting with its material. Among them:


The first detail cannot be explained by the larger distance only: the quadratic law results in only an order of magnitude lower value [e.g., (lV = 4.5 m/lI = 1.1 m)<sup>2</sup> ], that is, it gives merely a coefficient about 16. It means that we have in reality a strong absorption and scattering of hard X-rays and neutrons by our simulator.

The second characteristic gives a certain difficulty in interpretation. It appears that the first set of pulses following the main hard X-ray pulse cannot be attributed to the neutron ones because their energy calculated on the TOF bases gives a value much higher compared with the initial ones (above 10 MeV). Examination with high magnification of hard X-ray pulse shapes of the low intensity obtained at small distances by the probe No. 1 has shown that they have the same multiple peaks as in the probe No. 2 at large distances. Moreover, it appears that these subsequent pulses contain higher energy X-ray photons compared with the first pulse. We observe already in our earlier experiments such a phenomenon (see e.g., [23, 24]). Because

#### Figure 24.

A polar diagram showing angular energy distribution around a simulator (upper hemisphere, red) versus the angular tracking of neutron energy distribution in the "clean-room" conditions (lower hemisphere, blue).

of the higher energy of photons, a penetrability of each subsequent pulse appeared to be greater than the previous ones. That is why these pulses are sounder in these OTs. Their origin is the multiple current disruptions after the main one taking place during the DPF operation.

Multiple peaks and a very long "tail" of the neutron pulse is explained by neutron reflections and scattering on various elements of the PF-1000U chamber and its auxiliary equipment. We were able to attribute one neutron pulse (a quite high peak) to a real object-scatterer in the OT of the probe No. 2 at its VI position. It appears that the scatterer is a high-pressure cylinder with deuterium (10 liters, 150 atm) placed close to the PF-6 device.
