- Email: [email protected]
Measurement of energetic protons generated by a plasma focus device
Volume 83A, number 1
PHYSICS LETTERS
4 May 1981
MEASUREMENT OF ENERGETIC PROTONS GENERATED BY A PLASMA FOCUS DEVICE Y. YAMADA, Y. KITAGAWA, M. YOKOYAMA and C. YAMANAKA Institute of Laser Engineering, Osaka University, Suita, Osaka 565, Japan Received 2 September 1980
The energy spectrum and the ejected cone of protons generated by a plasma focus device were measured by track observations in a cellulose nitrate film. The protons below 2 MeV had two energy components. The component above 750 keV was ejected in a cone with a half angle of 10°.
Plasma focus devices can produce strong electric fields within 10 ns at the end of the dense-pinch phase, which can accelerate the ions and electrons to energies many times the product of their charge and the applied voltage of a capacitor bank. Due to this field, the ions are accelerated away from the anode and the electrons are accelerated towards it. These high-energy ions and electrons have been observed at various laboratories [1—7].In this letter, measurements of the energy spectrum and angular distribution of protons by using a simple method of track observations in a cellulose nitrate (CN) film are reported. Previously [8], we measured protons with energies greater than 1 MeV by the time of flight method using silicon PIN detectors as Gullickson et al. [9]. A silicon PIN detector is, however, affected by electrons or other ions emitted from electrodes. So a CN film was used as a proton detector in the present experiment. The Mather-type plasma focus device was operated at 30 kV, 18 kJ and a hydrogen fill-gas pressure of 4.2 Torr. The copper inner electrode (anode) with a hemispherical end face had a diameter of 50 mm and a length of 250 mm, and a tantalum rod was inserted at the hemispherical end. The outer electrode (cathode) consisted of 12 copper rods (8 mm diameter and 250 mm long) which were distributed uniformly over a diameter of 100 mm. The experimental arrangement for the measurement of the proton energy spectrum is shown in fig. la. A 5 cm diameter CN film with aluminum filters was located at 0.8 m from the end of the anode with
0.8 m
~
Al
CN
40 ~F (ci)
6.5
4
10
2~im no to 20
30
( b) Fig. I. (a) Experimental set up for the measurement of the proton energy spectrum by using a CN film. (b) Arrangement of Al foils. The proton energy ranges which ~n form the ~ Mm), key (13 Mm), 1.2—1.4 MeV (20 Mm), and 1.6—1.8 MeV (30 Mm), respectively.
the surface perpendicular to the plasma axis (z-axis). The CN films used here were Kodak CA8O-1 5 and CN85 films. After the irradiation of these fIlms by the proton beam, they were chemically etched in a 10%
0 031—9163/81/0000—0000/s 02.50 © North-Holland Publishing Company
9
Volume 83A, number 1
PHYSICS LETTERS
NaOH (2.5 N) at 60°C for 15 mm, and the number of tracks was counted by using a light microscope. Under these conditions, the CN films were able to record the tracks of protons with an energy lower than 200 keV, although the track density saturatesin the film at about 108 protons/cm2. Fig. lb shows the CN detector with eight channels. The thicknesses of the Al filters were 2 pm (transmitted proton energy> 180 keV) for the first channel, 4pm (380 keV) for the second, 6.5 pm (550 keV) for the third, 10 pm (770 keV) for the fourth, 13 pm (925 keV) for the fifth, 20 pm (1.2 MeV) for the sixth, 30 pm (1.6 MeV) for the seventh and no filter for the eighth which formed tracks of protons below 200 keV. Such an arrangement allowed us to obtain the track densities for eight energy ranges in one discharge. Fig. 2 shows the proton energy spectrum at z = 0.8 m, which is clearly divided into two components described by the following experimental formulas: 9
dn/dE
=
10 exp(—8.8 E)
(E ~ 1 MeV),
=
3 X iO~exp(—0.9 E)
(E> 1 MeV)
4 May 1981
respectively, where n is the proton density (cm—2) at z = 0.8 m and E is the proton energy (MeV). The former, low-energy, component may not be so exact, since it is possibly distorted by atomic reactions of the protons with the background gas. Integrating this energy spectrum up to 2 MeV, we estimated that the proton density below 2 MeV was 9 X l0~cm2 at z = 0.8 m. To measure the ejected cone of protons, 6 cm diameter CN films with 6.5 and 10 pm thick Al foils and without a filter were located at z = 10 and 15 cm. The protons transmitted through a 10 pm thick Al foil (770—970 key) were distributed in a cone with a half-angle of 10°,as shown in fig. 3. This angle shows the edge of the cone. Both the protons between 550 keV and 750 key through the 6.5 pm Al foil and the protons below 200 keV produced tracks over the whole surface of the CN film located at z = 10 cm. So the ejection cone of protons below 750 keV was not yet confirmed precisely. The ratio of the track density between 550 keV and 750 keVto that between 770 keVand 970 keV was 30. While the latter was only twice that between 925 and 1125 keV. From this result and the measurement of the ejected cone, we can say that the protons have two energy components with the following properties: the low-energy component below 750 keV was produced mainly and was ejected in a cone of half-angle wider than 10°,whereas the high-energy component above 750 keV was localized in a cone of 10°. In conclusion, the protons below 2 MeV consist of two energy components and each component has a different angular distribution. This result is useful for the investigation of the particle acceleration mechanism in a plasma focus device.
1O~
\ ~ 108
~ 10~
~ 10~ 8
0
I
I
0.5
1.0
Proton
1.5
2.0
Energy (MeV)
Fig. 2. Proton energy spectrum below 2 MeV at z 0.8 m. dn/dE = lO9exp(—8.8 E) forE < I MeV and dn/dE = 3 X lO5exp(—0.9 E) forE ~ I MeV.
10
io° 0
~ 10 15 20 Distance from anode(cnl
Fig. 3. Half-angle of protons between 770 and 970 keV ejected ahead of the anode.
Volume 83A, number 1
PHYSICS LETTERS
More detailed information concerning the energy spectra may be obtained from a measurement of the angular distribution over the whole solid angle. This experiment will be done in the future.
References [1] R.L. Gullickson and H.L. Sahlin, J. AppI. Phys. 49 (1978) 1099; R.L. Gullickson, J.S. Luce and H.L. Sahlin, J. Appl. Phys. 48(1977) 3718; R.L. Gullickson and R.H. Barlett, Adv. X-ray Anal. 18 (1975) 184. [2] H. Conrads, P.Cloth, M. Demmeler and R. Hecker, Phys. Fluids 13(1972)209. [3] IF. Belyaeva and N.V. Filippov, Nuci. Fusion 13 (1973) 881.
4 May 1981
[5] G. Gourlan, J.P. Rager, M. Samuelli and C. Strangio, Frascati Report GI. R/PLADI74.I0/E (October 1974), unpublished. [6] V. Nardi, in: W.H. Bostick, J. Feugeas, andphysics C. Coritese, Proc. 7th Intern. Conf.W. onPrior Plasma and controlled nuclear fusion research (Innsbruck, 1978), Vol.11 (IAEA, Vienna, 1979) p. 143. [7] H. Krompholz, E. Grimm, F. Ruhl, K. Schönbach and G. Herziger, Phys. Lett. 76A (1980) 255. [8] Y. Kitagawa, Y. Yamada, A. Ishizaki, M. Naito, M. Yokoyama and C. Yamanaka, Intern. Conf. on Plasma physics (Nagoya, 1980), unpublished; M. Yokoyama, Y. Kitagawa, Y. Yamada, C. Yamanaka and K. Hirano, 8th Intern. Conf. on Plasma physics and controlled nuclear fusion research (Brussels, 1980), IAEA-CN-38/G-l-3. [9] R.L. Gullickson, W.L. Pickles, D.F. Price and H.L. Sahlin, preprint UCRL-81962 (1979), unpublished.
[4] A. Bernard, J.R. Gaconnet, A. Jolas, J.P. Ic Breton and J. de Mascureau, in: Proc. 7th Intern. Conf. on Plasma physics and controlled nuclear fusion research (Innsbruck, 1978), Vol.11 (IAEA, Vienna, 1979) p. 159.
11
PHYSICS LETTERS
4 May 1981
MEASUREMENT OF ENERGETIC PROTONS GENERATED BY A PLASMA FOCUS DEVICE Y. YAMADA, Y. KITAGAWA, M. YOKOYAMA and C. YAMANAKA Institute of Laser Engineering, Osaka University, Suita, Osaka 565, Japan Received 2 September 1980
The energy spectrum and the ejected cone of protons generated by a plasma focus device were measured by track observations in a cellulose nitrate film. The protons below 2 MeV had two energy components. The component above 750 keV was ejected in a cone with a half angle of 10°.
Plasma focus devices can produce strong electric fields within 10 ns at the end of the dense-pinch phase, which can accelerate the ions and electrons to energies many times the product of their charge and the applied voltage of a capacitor bank. Due to this field, the ions are accelerated away from the anode and the electrons are accelerated towards it. These high-energy ions and electrons have been observed at various laboratories [1—7].In this letter, measurements of the energy spectrum and angular distribution of protons by using a simple method of track observations in a cellulose nitrate (CN) film are reported. Previously [8], we measured protons with energies greater than 1 MeV by the time of flight method using silicon PIN detectors as Gullickson et al. [9]. A silicon PIN detector is, however, affected by electrons or other ions emitted from electrodes. So a CN film was used as a proton detector in the present experiment. The Mather-type plasma focus device was operated at 30 kV, 18 kJ and a hydrogen fill-gas pressure of 4.2 Torr. The copper inner electrode (anode) with a hemispherical end face had a diameter of 50 mm and a length of 250 mm, and a tantalum rod was inserted at the hemispherical end. The outer electrode (cathode) consisted of 12 copper rods (8 mm diameter and 250 mm long) which were distributed uniformly over a diameter of 100 mm. The experimental arrangement for the measurement of the proton energy spectrum is shown in fig. la. A 5 cm diameter CN film with aluminum filters was located at 0.8 m from the end of the anode with
0.8 m
~
Al
CN
40 ~F (ci)
6.5
4
10
2~im no to 20
30
( b) Fig. I. (a) Experimental set up for the measurement of the proton energy spectrum by using a CN film. (b) Arrangement of Al foils. The proton energy ranges which ~n form the ~ Mm), key (13 Mm), 1.2—1.4 MeV (20 Mm), and 1.6—1.8 MeV (30 Mm), respectively.
the surface perpendicular to the plasma axis (z-axis). The CN films used here were Kodak CA8O-1 5 and CN85 films. After the irradiation of these fIlms by the proton beam, they were chemically etched in a 10%
0 031—9163/81/0000—0000/s 02.50 © North-Holland Publishing Company
9
Volume 83A, number 1
PHYSICS LETTERS
NaOH (2.5 N) at 60°C for 15 mm, and the number of tracks was counted by using a light microscope. Under these conditions, the CN films were able to record the tracks of protons with an energy lower than 200 keV, although the track density saturatesin the film at about 108 protons/cm2. Fig. lb shows the CN detector with eight channels. The thicknesses of the Al filters were 2 pm (transmitted proton energy> 180 keV) for the first channel, 4pm (380 keV) for the second, 6.5 pm (550 keV) for the third, 10 pm (770 keV) for the fourth, 13 pm (925 keV) for the fifth, 20 pm (1.2 MeV) for the sixth, 30 pm (1.6 MeV) for the seventh and no filter for the eighth which formed tracks of protons below 200 keV. Such an arrangement allowed us to obtain the track densities for eight energy ranges in one discharge. Fig. 2 shows the proton energy spectrum at z = 0.8 m, which is clearly divided into two components described by the following experimental formulas: 9
dn/dE
=
10 exp(—8.8 E)
(E ~ 1 MeV),
=
3 X iO~exp(—0.9 E)
(E> 1 MeV)
4 May 1981
respectively, where n is the proton density (cm—2) at z = 0.8 m and E is the proton energy (MeV). The former, low-energy, component may not be so exact, since it is possibly distorted by atomic reactions of the protons with the background gas. Integrating this energy spectrum up to 2 MeV, we estimated that the proton density below 2 MeV was 9 X l0~cm2 at z = 0.8 m. To measure the ejected cone of protons, 6 cm diameter CN films with 6.5 and 10 pm thick Al foils and without a filter were located at z = 10 and 15 cm. The protons transmitted through a 10 pm thick Al foil (770—970 key) were distributed in a cone with a half-angle of 10°,as shown in fig. 3. This angle shows the edge of the cone. Both the protons between 550 keV and 750 key through the 6.5 pm Al foil and the protons below 200 keV produced tracks over the whole surface of the CN film located at z = 10 cm. So the ejection cone of protons below 750 keV was not yet confirmed precisely. The ratio of the track density between 550 keV and 750 keVto that between 770 keVand 970 keV was 30. While the latter was only twice that between 925 and 1125 keV. From this result and the measurement of the ejected cone, we can say that the protons have two energy components with the following properties: the low-energy component below 750 keV was produced mainly and was ejected in a cone of half-angle wider than 10°,whereas the high-energy component above 750 keV was localized in a cone of 10°. In conclusion, the protons below 2 MeV consist of two energy components and each component has a different angular distribution. This result is useful for the investigation of the particle acceleration mechanism in a plasma focus device.
1O~
\ ~ 108
~ 10~
~ 10~ 8
0
I
I
0.5
1.0
Proton
1.5
2.0
Energy (MeV)
Fig. 2. Proton energy spectrum below 2 MeV at z 0.8 m. dn/dE = lO9exp(—8.8 E) forE < I MeV and dn/dE = 3 X lO5exp(—0.9 E) forE ~ I MeV.
10
io° 0
~ 10 15 20 Distance from anode(cnl
Fig. 3. Half-angle of protons between 770 and 970 keV ejected ahead of the anode.
Volume 83A, number 1
PHYSICS LETTERS
More detailed information concerning the energy spectra may be obtained from a measurement of the angular distribution over the whole solid angle. This experiment will be done in the future.
References [1] R.L. Gullickson and H.L. Sahlin, J. AppI. Phys. 49 (1978) 1099; R.L. Gullickson, J.S. Luce and H.L. Sahlin, J. Appl. Phys. 48(1977) 3718; R.L. Gullickson and R.H. Barlett, Adv. X-ray Anal. 18 (1975) 184. [2] H. Conrads, P.Cloth, M. Demmeler and R. Hecker, Phys. Fluids 13(1972)209. [3] IF. Belyaeva and N.V. Filippov, Nuci. Fusion 13 (1973) 881.
4 May 1981
[5] G. Gourlan, J.P. Rager, M. Samuelli and C. Strangio, Frascati Report GI. R/PLADI74.I0/E (October 1974), unpublished. [6] V. Nardi, in: W.H. Bostick, J. Feugeas, andphysics C. Coritese, Proc. 7th Intern. Conf.W. onPrior Plasma and controlled nuclear fusion research (Innsbruck, 1978), Vol.11 (IAEA, Vienna, 1979) p. 143. [7] H. Krompholz, E. Grimm, F. Ruhl, K. Schönbach and G. Herziger, Phys. Lett. 76A (1980) 255. [8] Y. Kitagawa, Y. Yamada, A. Ishizaki, M. Naito, M. Yokoyama and C. Yamanaka, Intern. Conf. on Plasma physics (Nagoya, 1980), unpublished; M. Yokoyama, Y. Kitagawa, Y. Yamada, C. Yamanaka and K. Hirano, 8th Intern. Conf. on Plasma physics and controlled nuclear fusion research (Brussels, 1980), IAEA-CN-38/G-l-3. [9] R.L. Gullickson, W.L. Pickles, D.F. Price and H.L. Sahlin, preprint UCRL-81962 (1979), unpublished.
[4] A. Bernard, J.R. Gaconnet, A. Jolas, J.P. Ic Breton and J. de Mascureau, in: Proc. 7th Intern. Conf. on Plasma physics and controlled nuclear fusion research (Innsbruck, 1978), Vol.11 (IAEA, Vienna, 1979) p. 159.
11