Radiation Characteristics of
the FN-II Plasma Focus
Device
Julio Herrera1 , Fermín Castillo1, Guillermo Espinosa2,
Isabel Gamboa1, José Ignacio Golzarri2, José Rangel Gutiérrez1
Instituto de Ciencias Nucleares
Instituto de Física
Universidad Nacional Autónoma de México
Outline
• Description of thel FN-II device and its diagnostics
• Soft X rays observed with pinhole camera and PIN
diode.
• Measurement of the hard X-rays angular distribution by
means of TLD-100 dosimeters.
• Use of needle point electrode for contrast radiography.
Dosimetry using TLD-200 dosimeters.
• Hard X-rays and neutron detection using scintillatorphotomultiplier arrays.
• Conclusions
Sketch of the FN-II Dense Plasma Focus Device
Main parameters
(F. Castillo et al., Brazilian J. Phys. 32 (2002) 3)
• Inner electrode:
(oxygen-free copper)
• Outer electrode:
(oxygen-free copper)
• Insulator (Pyrex®)
• Usual operating
voltage:
• Stored energy:
• Current
length
40 mm
diam.
50 mm
squirrel cage type (12 bars)
diam.
100 mm
length
19 mm
diam.
50 mm
36 kV
4.8 kJ
350 kA
Main diagnostics
• Current (Rogowski coil)
• Neutrons
2 Silver activation counters
CR 39 nuclear track detectors
5 BC-400 Scintillator-photomultiplier systems
• X-rays
Pinhole cameras
PIN diodes
TLD-100 and TLD-200 dosimeters
5 BC-400 Scintillator-photomultiplier systems
Cámara de agujero
Soft X-rays – Hollow electrode
Rogowski and PIN in Hydrogen
Bored electrode after several hundreds of
shots
Electrode with a 2 mm INOX-304 inserted
needle
Pin-hole image of a discharge with a needle
Worn out 2 mm needle point after a few
shots
Worn out 1 mm needle point after a few
shots
Erosion of the needle
TLD-100 dosimetry (1)
• 60 shots for the hollow electrode. Number of counts at 20o/90º for
the activation detectors is 68774/65312
→ 4.12  108/ 6.13  108 neutrons/steradian
54 TLD-100 dosimeters were placed 1 m away from
the focus, measurenments were made of four
groups of 9 dosimeters:
1. 9 Unfiltered
2. 18 Filtered by 0.5 mm Al
3. 18 Filtered by 1.0 mm Al
A dose estimate is made using the Unfiltered group, using their TL
response and the calibration factor previously obtained by means of
60Co irradiation.
TLD-100 dosimetry (2)
• 79 shots for the needle electrode. Number of counts at 20o/90º for
the activation detectors is 63448/58478
→ 3.81  108/5.49  108 neutrons per steradian.
63 TLD-100 dosimeters were placed 1 m away from
the focus, measurenments were made of four
groups of dosimeters:
1. 9 Unfiltered
2. 18 Filtered by 0.5 mm Al
3. 18 Filtered by 1.0 mm Al
4. 18 Filtered by 2.0 mm Al
A dose estimate is made using the Unfiltered group, using their TL
response and the calibration factor previously obtained by means of
60Co irradiation.
Attenuation of the thermoluminiscent (TL) response of
filtered TLD-100 dosimeters placed at 1m from the focus.
TLD-100 dosimetry (3)
Hollow electrode:
60 shots. 4.12  108/ 6.13  108 neutrons/steradian at silver
activation detectors
4.6 ± 0.5 mGy
→
0.077 ± 0.006 mGy
Needle electrode:
79 shots. 3.81  108/5.49  108 neutrons per steradian.
at silver activation detectors
8.5 ± 0.5 mGy
→
0.11 ± 0.01 mGy
(Environmental daily dose ~ 0.01 mGy)
Sketch of the FN-II Dense Plasma Focus Device
Angular distribution of X-rays measured with TLD-200
dosimeters
Sensibilty peaked at ~ 30keV
Tracks at upper detectors (400X)
Tracks at lower detectors (400X)
Angular distribution of protrons
Angular distribution of neutrons at
upper detectors
Angular distribution for neutrons at
lower detectors
Isotropic and Anisotropic Contributions

f    B  H exp   / 2
2
2


A  2  f  sin d  A1  A2
0
A1  4B
A2  2

C

K


K   sin  exp   2 / 2 2 d
0
Comparisson Between Both Samples I
B
H
σ
K
A1
A2
A
Iso.
Aniso.
Neutrons
up
4.684
8.855
32.42
0.2881
58.86
16.03
74.89
0.7860
0.2140
Neutrons
down
20.44
17.20
44.34
0.4925
256.9
53.22
310.1
0.8284
0.1716
Comparisson Between Both Samples II
<Scale
factor>
106
YIso
106
YAniso
106
YTotal
106
Iso
%
Aniso
%
Neutrons
up
0.370
0.178
21.78
10.48
5.93
2.85
27.71
78.6
21.4
Neutrons
down
0.1030.0
51
26.4613.
10
5.482.71
31.94
82.8
17.2
Scintillator-Photomultiplier systems
Rogowski and PIN in Deuterium
Shielding on scintillators
Shielding on scintillators
Shielding on scintillators
No. de detector
Shielding (Pb cm)
Channel
1(PMT only)
5
Osc. 1-Ch.2
2
23 (water)
Osc. 2-Ch.2
3
5
Osc. 2-Ch.3
4
10
Osc. 1-Ch.4
5
10
Osc. 1-Ch.3
Shielding on scintillators
Shot 7735-Low performance
Shielding on scintillators
Shot 7726-High performance
Response of the neutron detectors
Conclusions (1)
• The signals from the pin-hole camera show that the
plasma column is around 3 cm long and 3 mm diam.
• The PIN signals are peaked at the moment of the dip in
the B-dot signal, and then show a platform which lasts
about 1μm.
• By placing a needle at the tip of the elecrode, the hard Xrays can be well localised, and the dosimetry shows that
the yield is doubled. Further work needs to be done in
order to estimate the spectrum.
• The better concentration of the x-ray bright-spot allows
their use for high contrast radiography.
• Some of the hard X rays are highly energetic (gamma
rays?)
• In “high performance” shots the rate and production of
gamma rays is highly increased, masking the neutron
signal (scattering of gamma rays?).
Conclusions (2)
• The angular distribution of neutrons shows an isotropic
and an anisotropic component. The latter usually
contributes less than 30%.
• The angular distribution of protons is aligned to the axis
and falls rapidly for angles greater than  40º, implying
the existence of strong magnetic field.
• The X-rays show a bimodal angular distribution.
• While the needle slightly increases the X-ray emission,
the hollow electrode is best for neutron generation.
References
[1] F. Castillo, J.J.E. Herrera, J. Rangel, A. Alfaro, M.A. Maza y V. Sakaguchi
"Neutron Anisotropy and X-Ray Production of the FN-II Dense Plasma Focus"
Brazilian Journal of Physics 32, 3-12(2002).
[2] F. Castillo, J.J.E. Herrera, J. Rangel, J.I. Golzarri y G. Espinosa
"Neutron Angular Distribution in a Plasma Focus Obtained Through Nuclear Track
Detectors", Radiation Protection Dosimetry 101, 557-560(2002).
[3] F. Castillo, J.J.E. Herrera, J. Rangel, M. Milanese, R.Moroso, J. Pouzo,
J.I. Golzarri y G. Espinosa "Isotropic and Anisotropic Components of the Neutron
Emissions at the FN-II and PACO Dense Plasma Focus Devices” Plasma Physics
and Controlled Fusion 45, 289-300 (2003).
[4] F. Castillo, J.J.E. Herrera, J. Rangel, I. Gamboa, G. Espinosa y J.I. Golzarri “Angular
Distribution of fusion products and X-rays emitted by a small dense plasma focus
machine” Journal of Applied Physics
101 013303-1-7 (2007).