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Transport of an intense pulsed light-ion beam through a plasma channel in the Nagaoka etigo-I
Volume89A, number 5
17 May 1982
PHYSICS LETTERS
TRANSPORT OF AN INTENSE PULSED LIGHT-ION BEAM THROUGH A PLASMA CHANNEL IN THE NAGAOKA ETICO-I
Kiyoshi YATSUI, Katsumi MASUGATA, Tadao NAKYAMA and Masao MATSUI Laboratory of Beam-Fusion
Technology,
The Technological
University of Nagaoka, Nagaoka, Niigata 949-54, Japan
Received 18 February 1982 Revised manuscript received 22 March 1982
The transport efficiency of an intense pulsed proton beam has been studied experimentally through a zdischarge plasma channel (50 cm long) produced by an exploding wire. At Vch (charging voltage of channel) = 30 kV and I,h (channel current) = 50 kA, a good transport efficiency has been obtained, yielding an efficiency of more than 80%.
Recently, considerable attention has been given in the literature to inertial-confinement fusion (ICF) by use of an intense pulsed light-ion beam (LIB) [ 11. Concerning the transport [2] of the focused LIB, several ideas have been proposed and tested to form plasma channels by an exploding wire [3], a wall-confined discharge [4] or a laser-guided discharge [ 51. As a preliminary experiment towards an independent confirmation of these studies, we have injected the focused LIB into the plasma channel formed by an exploding wire [6,7]. In this paper, we report some experimental data associated with the LIB transport. The experiments were carried out in the Nagaoka ETIGO-I [8,9]15 kJ LIB generator at Tech. Univ. of Nagaoka. It consists of a Marx generator (43 kJ of stored energy), a pulse-forming line (PFL) of 5 Q and an ion diode. The diode utilized here is a spherically shaped magnetically-insulated diode (MID). Fig. 1 shows the outlines of the LIB-transport experiment. As reported elsewhere, there appeared a voltage reflection due to an impedance mismatch between PFL and MID; the diode works at a relatively high impedance in the first phase (phase I), while the impedance decreases in the second phase (phase II) presumably due to a residual anode-sheath plasma. We briefly summarize the typical experimental parameters, where the suffix I and II refer to phases I and II, respectively. We have chosen the gap length d-between anode (polyethylene) and cathode as d = 10 mm *I. The diode voltage and 0 031-9163/82/0000-0000/$02.75
0 1982 North-Holland
AC
Fig. 1. Schematic diagram of LIB-transport experiment.
current are PdO % 590 kV and Fd@, c 270 kV, and Zd” = 30 kA and ZdO = 70 kA, respectively. The beampulse width is =80 ns (FWHM). The total ion current is Zio w 9 kA and Zim a 15 kA. The enhancement factors of 3-5 and 30-43 of the ion current over the space-charge limiting current are obtained at phases I and II, respectively. We have operated the MID at B/B, = 3.6 (phase I) and 5.7 (phase II), where B and B, are the transverse magnetic field strength and the critical magnetic field strength above which the electron flow is insulated, respectively. The diameter of the flashboard anode is 110 mm, where -1100 neck pins (copper) are buried. To obtain a geometric focusing *l For convenience, the experimental conditions at d = 10 mm are called case (c), compared with case (a) (d = 25 mm) and case (b) (d = 15 mm) [6,9].
235
Volume 89A, number 5
PHYSICS LETTERS
17 May 1982
Channel currant (Ioh) vertical;
8.5
lcA/cliv.
horizontal; 500 nsec/div.,
Magnetic probe (I$), vertical; 600 Gldiv., horizontal; 500 nsec/div., lrr - 35 mm from center Fr = 15 mm from center Fig. 2. Wave forms of (a) channel current and (b) magnetic probe.
of the LIB (mainly protons according to other measurements [lo]), the beam-extraction side of both anode and cathode are spherically shaped. The radii of curvatures of anode and cathode are 160 mm and 150 mm, respectively. Around the geometric focusing point, we have obtained the maximum ion-current density of Jim a 3.2 kA/cm2 and J/w = 1.5 kA/cm2. At phase I, the distribution shows a peak at the center, while indicating the presence of an annular distribution at phase II. Such an annular distribution seems to be due to a space-charge effect [6,10]. The focusing diameter is d(I) c 10 mm and d(Io = 30 mm. The maximum injection angle with respect to the plasma channel is estimated to be Bm = 0.35 rad at the focusing point. As a preliminary experiment, we have first fired a 1 m long, 0.16 mm diameter copper wire by a 2 kJ fast condenser bank (1.6 PF, SOkV) in a good vacuum (p = 1 X lo-4 Torr), and measured the behavior of the plasma channel. Fig. 2 shows the typical wave forms of (a) channel current (I,& and (b) azimuthal magnetic field (B,). When the channel switch is first closed (t = 0), the current increases to a small peak at t = 1 W. The wire bursts and the resistance falls rapidly, allowing Ich to attain the maximum value at t = 3.2 /.Ls,as seen in fig. 2a. From fig. 2b, the magnetic field behaves similarly as I,_h, but it is noticed that at t > 2.5 PS the B, field strength in the outer region is larger than that in the inner region. Fig. 3 shows the 236
B, field distribution in the r direction. At t = 1 ~.t.s, the B, field decreases monotonically. At t = 3.4 /.Ls, on the other hand, it first increases until r = 30 mm, and then decreases, clearly indicating the production of a plasma channel approximately 60 mm in diameter. Because of the good vacuum in the chamber, the plasma should be mainly composed of copper atoms evaporated from the wire. Furthermore, using the experimental wave forms observed, we have estimated the resistance of the plasma channel to be ~4 X 1O-4 Q/cm, giving
OL 0
I
10
20 >
30 40 r (mm)
50
Fig. 3. Radial distribution of B,g field strength at t = 1 ps and 3.4 ps after closure of channel switch.
PHYSICS LETTERS
Volume 89A, number 5
Table 1 LIB-transport efficiency through a 50 cm long plasma channel, where Eb (energy at the geometric foCUSin&! point) -250 J; cf. experiments with an un-melting wire (Copper, lmm diameter). Ir,h = 25-30 kV,zch = 42-48 kA, Eext = 95-105 J, n = 38-42%. Charging voltage Pch (kv)
.Channel current zch (kA)
Beam energy at channel exit Eext (JJ
0 20 2.5 30
0 36 40 50
180 190 203
-
Transport efficiency
rl (%I a3 a) 72 76 81
a) Transport efficiency calculated from proton number by use of carbon activation technique.
17 May 1982
(ii) The efficiency increases with increasing channel current. (iii) The maximum transport efficiency of more than 80% is achieved at Ich = 50 kA. (iv) The efficiency decreases in the case of an un-melting wire (1 mm diameter), but is better than that without the wire. It seems to be due to the fact that the B, field produced by the axial current confines the LIB to a certain extent. References [l] S. Humphries Jr., Nucl. Fusion 20 (1980) 1549; and references therein.
[ 21 P.F. Ottinger, D. Mosher and A. Goldstein, Phys. Fluids 23 (1980) 909.
[ 31 J.N. Olsen, D.J. Johnson and R.J. Leeper, Appl. Phys. Lett. 36 (1980) 808.
for the resistivity of the 60 mm diameter plasma ~0.112 s2 cm. Under the assumption of a singly charged state, the electron temperature may be evaluated to be = 3 eV. Based upon the basic data of the preliminary experiment mentioned above, we then inject the focused LIB into the plasma channel, and study the transport efficiency. Table 1 summarizes the result. Here, the plasma channel was produced by a 50 cm long, 0.16 mm diameter wire fired by a 5 kJ condenser bank (4 E.IF, 50 kV) at p m 1 X 10W4Torr, as shown in fig. 1. The transport efficiency has been determined by a calorimeter or a nuclear activation technique [ 11,I 21. The nuclear reaction utilized here is 12C(p, r)13N(Cr+)13C by use of a coincident T-ray detection technique. The diameter of the carbon target is 33 mm. The experimental results may be summarized as follows: (i) Approximately 3% of the incident number of protons transports even in the absence of the plasma channel.
[ 41 D. Mosher et al., in: Proc. 3rd Intern. Top. Conf. on
[5] [6]
[7]
[ 81
[9]
[lo] [ 1 l] [ 121
High-Power El. and Ion Beam Res. and Tech. (Novosibirsk, USSR, 1979) p. 576. J.N. Olsen and R.J. Leeper, in: Proc. 1981 IEEE Intern. Conf. on Plasma Scl. (Santa Fe, May, 1981). K. Yatsui et al., in: Proc. 4th Intern. Top. Conf. on HighPower El. and Ion Beam Res. and Tech. (Palaiseau, France, 1981) p. 27. K. Yatsui et al., Lab. Beam-Fusion Tech. Rep. LBFT8201, Tech. Univ. Nagaoka (Jan., 1982). K. Yatsui, in: Proc. US-Japan Workshop on Compact Toroid (Osaka Univ., 1981) p. 105 ; Lab. Beam-Fusion Tech. Rep. LBFT-8101 (Tech. Univ. Nagaoka, March, 1981). K. Masugata et al., Japan. J. Appl. Phys. 20 (1981) L347. K. Masugata et al., in preparation. F.C. Young, J. Golden and C.A. Kapetanakos, Rev. Sci. Instrum. 48 (1977) 432. K. Konno et al., Kakuyugo-Kerkyu (Nucl. Fusion Res., circular in Japanese) 46 (1981) 466; Lab. BeamFusion Tech. Rep. LBFT-8202 (Tech. Univ. Nagaoka, Feb., 1982).
237
17 May 1982
PHYSICS LETTERS
TRANSPORT OF AN INTENSE PULSED LIGHT-ION BEAM THROUGH A PLASMA CHANNEL IN THE NAGAOKA ETICO-I
Kiyoshi YATSUI, Katsumi MASUGATA, Tadao NAKYAMA and Masao MATSUI Laboratory of Beam-Fusion
Technology,
The Technological
University of Nagaoka, Nagaoka, Niigata 949-54, Japan
Received 18 February 1982 Revised manuscript received 22 March 1982
The transport efficiency of an intense pulsed proton beam has been studied experimentally through a zdischarge plasma channel (50 cm long) produced by an exploding wire. At Vch (charging voltage of channel) = 30 kV and I,h (channel current) = 50 kA, a good transport efficiency has been obtained, yielding an efficiency of more than 80%.
Recently, considerable attention has been given in the literature to inertial-confinement fusion (ICF) by use of an intense pulsed light-ion beam (LIB) [ 11. Concerning the transport [2] of the focused LIB, several ideas have been proposed and tested to form plasma channels by an exploding wire [3], a wall-confined discharge [4] or a laser-guided discharge [ 51. As a preliminary experiment towards an independent confirmation of these studies, we have injected the focused LIB into the plasma channel formed by an exploding wire [6,7]. In this paper, we report some experimental data associated with the LIB transport. The experiments were carried out in the Nagaoka ETIGO-I [8,9]15 kJ LIB generator at Tech. Univ. of Nagaoka. It consists of a Marx generator (43 kJ of stored energy), a pulse-forming line (PFL) of 5 Q and an ion diode. The diode utilized here is a spherically shaped magnetically-insulated diode (MID). Fig. 1 shows the outlines of the LIB-transport experiment. As reported elsewhere, there appeared a voltage reflection due to an impedance mismatch between PFL and MID; the diode works at a relatively high impedance in the first phase (phase I), while the impedance decreases in the second phase (phase II) presumably due to a residual anode-sheath plasma. We briefly summarize the typical experimental parameters, where the suffix I and II refer to phases I and II, respectively. We have chosen the gap length d-between anode (polyethylene) and cathode as d = 10 mm *I. The diode voltage and 0 031-9163/82/0000-0000/$02.75
0 1982 North-Holland
AC
Fig. 1. Schematic diagram of LIB-transport experiment.
current are PdO % 590 kV and Fd@, c 270 kV, and Zd” = 30 kA and ZdO = 70 kA, respectively. The beampulse width is =80 ns (FWHM). The total ion current is Zio w 9 kA and Zim a 15 kA. The enhancement factors of 3-5 and 30-43 of the ion current over the space-charge limiting current are obtained at phases I and II, respectively. We have operated the MID at B/B, = 3.6 (phase I) and 5.7 (phase II), where B and B, are the transverse magnetic field strength and the critical magnetic field strength above which the electron flow is insulated, respectively. The diameter of the flashboard anode is 110 mm, where -1100 neck pins (copper) are buried. To obtain a geometric focusing *l For convenience, the experimental conditions at d = 10 mm are called case (c), compared with case (a) (d = 25 mm) and case (b) (d = 15 mm) [6,9].
235
Volume 89A, number 5
PHYSICS LETTERS
17 May 1982
Channel currant (Ioh) vertical;
8.5
lcA/cliv.
horizontal; 500 nsec/div.,
Magnetic probe (I$), vertical; 600 Gldiv., horizontal; 500 nsec/div., lrr - 35 mm from center Fr = 15 mm from center Fig. 2. Wave forms of (a) channel current and (b) magnetic probe.
of the LIB (mainly protons according to other measurements [lo]), the beam-extraction side of both anode and cathode are spherically shaped. The radii of curvatures of anode and cathode are 160 mm and 150 mm, respectively. Around the geometric focusing point, we have obtained the maximum ion-current density of Jim a 3.2 kA/cm2 and J/w = 1.5 kA/cm2. At phase I, the distribution shows a peak at the center, while indicating the presence of an annular distribution at phase II. Such an annular distribution seems to be due to a space-charge effect [6,10]. The focusing diameter is d(I) c 10 mm and d(Io = 30 mm. The maximum injection angle with respect to the plasma channel is estimated to be Bm = 0.35 rad at the focusing point. As a preliminary experiment, we have first fired a 1 m long, 0.16 mm diameter copper wire by a 2 kJ fast condenser bank (1.6 PF, SOkV) in a good vacuum (p = 1 X lo-4 Torr), and measured the behavior of the plasma channel. Fig. 2 shows the typical wave forms of (a) channel current (I,& and (b) azimuthal magnetic field (B,). When the channel switch is first closed (t = 0), the current increases to a small peak at t = 1 W. The wire bursts and the resistance falls rapidly, allowing Ich to attain the maximum value at t = 3.2 /.Ls,as seen in fig. 2a. From fig. 2b, the magnetic field behaves similarly as I,_h, but it is noticed that at t > 2.5 PS the B, field strength in the outer region is larger than that in the inner region. Fig. 3 shows the 236
B, field distribution in the r direction. At t = 1 ~.t.s, the B, field decreases monotonically. At t = 3.4 /.Ls, on the other hand, it first increases until r = 30 mm, and then decreases, clearly indicating the production of a plasma channel approximately 60 mm in diameter. Because of the good vacuum in the chamber, the plasma should be mainly composed of copper atoms evaporated from the wire. Furthermore, using the experimental wave forms observed, we have estimated the resistance of the plasma channel to be ~4 X 1O-4 Q/cm, giving
OL 0
I
10
20 >
30 40 r (mm)
50
Fig. 3. Radial distribution of B,g field strength at t = 1 ps and 3.4 ps after closure of channel switch.
PHYSICS LETTERS
Volume 89A, number 5
Table 1 LIB-transport efficiency through a 50 cm long plasma channel, where Eb (energy at the geometric foCUSin&! point) -250 J; cf. experiments with an un-melting wire (Copper, lmm diameter). Ir,h = 25-30 kV,zch = 42-48 kA, Eext = 95-105 J, n = 38-42%. Charging voltage Pch (kv)
.Channel current zch (kA)
Beam energy at channel exit Eext (JJ
0 20 2.5 30
0 36 40 50
180 190 203
-
Transport efficiency
rl (%I a3 a) 72 76 81
a) Transport efficiency calculated from proton number by use of carbon activation technique.
17 May 1982
(ii) The efficiency increases with increasing channel current. (iii) The maximum transport efficiency of more than 80% is achieved at Ich = 50 kA. (iv) The efficiency decreases in the case of an un-melting wire (1 mm diameter), but is better than that without the wire. It seems to be due to the fact that the B, field produced by the axial current confines the LIB to a certain extent. References [l] S. Humphries Jr., Nucl. Fusion 20 (1980) 1549; and references therein.
[ 21 P.F. Ottinger, D. Mosher and A. Goldstein, Phys. Fluids 23 (1980) 909.
[ 31 J.N. Olsen, D.J. Johnson and R.J. Leeper, Appl. Phys. Lett. 36 (1980) 808.
for the resistivity of the 60 mm diameter plasma ~0.112 s2 cm. Under the assumption of a singly charged state, the electron temperature may be evaluated to be = 3 eV. Based upon the basic data of the preliminary experiment mentioned above, we then inject the focused LIB into the plasma channel, and study the transport efficiency. Table 1 summarizes the result. Here, the plasma channel was produced by a 50 cm long, 0.16 mm diameter wire fired by a 5 kJ condenser bank (4 E.IF, 50 kV) at p m 1 X 10W4Torr, as shown in fig. 1. The transport efficiency has been determined by a calorimeter or a nuclear activation technique [ 11,I 21. The nuclear reaction utilized here is 12C(p, r)13N(Cr+)13C by use of a coincident T-ray detection technique. The diameter of the carbon target is 33 mm. The experimental results may be summarized as follows: (i) Approximately 3% of the incident number of protons transports even in the absence of the plasma channel.
[ 41 D. Mosher et al., in: Proc. 3rd Intern. Top. Conf. on
[5] [6]
[7]
[ 81
[9]
[lo] [ 1 l] [ 121
High-Power El. and Ion Beam Res. and Tech. (Novosibirsk, USSR, 1979) p. 576. J.N. Olsen and R.J. Leeper, in: Proc. 1981 IEEE Intern. Conf. on Plasma Scl. (Santa Fe, May, 1981). K. Yatsui et al., in: Proc. 4th Intern. Top. Conf. on HighPower El. and Ion Beam Res. and Tech. (Palaiseau, France, 1981) p. 27. K. Yatsui et al., Lab. Beam-Fusion Tech. Rep. LBFT8201, Tech. Univ. Nagaoka (Jan., 1982). K. Yatsui, in: Proc. US-Japan Workshop on Compact Toroid (Osaka Univ., 1981) p. 105 ; Lab. Beam-Fusion Tech. Rep. LBFT-8101 (Tech. Univ. Nagaoka, March, 1981). K. Masugata et al., Japan. J. Appl. Phys. 20 (1981) L347. K. Masugata et al., in preparation. F.C. Young, J. Golden and C.A. Kapetanakos, Rev. Sci. Instrum. 48 (1977) 432. K. Konno et al., Kakuyugo-Kerkyu (Nucl. Fusion Res., circular in Japanese) 46 (1981) 466; Lab. BeamFusion Tech. Rep. LBFT-8202 (Tech. Univ. Nagaoka, Feb., 1982).
237