- Email: [email protected]
On measurements of stopping power in explosively driven plasma targets
Nuclear Instruments and Methods in Physics Research A 415 (1998) 715—719
On measurements of stopping power in explosively driven plasma targets V. Mintsev!,*, V. Gryaznov!, M. Kulish!, V. Fortov!, B. Sharkov", A. Golubev", A. Fertman", N. Mescheryakov", W. Su¨{#, D.H.H. Hoffmann#, M. Stetter$, R. Bock$, M. Roth%, C. Sto¨ckl%, D. Gardes& ! Institute of Chemical Physics in Chernogolovka, Chernogolovka, Moscow reg. 142432, Russia " Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117259 Moscow, Russia # Universia( tt Erlangen-Nu( rnberg, D-91058 Erlangen, Germany $ Gesellschaft fu( r Schwerionenforschung, D-64220 Darmstadt, Germany % Technische Universtia( t Darmstadt, D-64289 Darmstadt, Germany & Institute Nacional de Physique Nucleare, 91406 Orsay, France
Abstract A new method to generate plasma targets with electron densities n 51020 cm~3 behind strong shock waves to study % the energy loss of protons and heavy ions is discussed. Estimates on the stopping power in hydrogen, xenon and argon were carried out. It is shown that shock velocities of more than 20 km/s in xenon and argon and of more than 60 km/s in hydrogen are needed to get a major contribution of free electrons. The problems of matching of large scale accelerator facility and explosive technique are considered. It is suggested to use a small ((150 g TNT) vacuum pumped explosive metallic chambers with fast valves in such experiments. Details on the construction and performance of small-sized explosively driven generators of strong shock waves are presented. The experimental setup including the proton accelerator ISTRA-36 and the explosive chamber, which have been installed in ITEP, is also presented. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction To investigate the heating of matter by particle beams detailed knowledge of the energy loss in dense plasma at high pressures and temperatures are crucial. Experiments carried out at GSI with discharge plasmas having electron densities up to
* Corresponding author. E-mail: [email protected].
n &1019 cm~3 show a considerable contribution % of free electrons to the stopping power. With increasing plasma density the influence of the effects of the Coulomb coupling are expected to be of great importance. Shock wave techniques make it possible to produce plasmas with electron densities of up to n &1022 cm~3 [1]. For this goal explosively % driven plasma generators have been developed [2]. In such devices plasma is created behind the plane front of an intense shock wave generated by the
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 5 5 4 - 3
716
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
detonation of high explosive chemicals. Standard shock wave plasma generators contain more than 500 g of high explosives, to produce shock compressed strongly coupled plasma with temperatures of 1—10 eV, pressures of 1—200 kbar and Coulomb coupling parameters of 1—5 [1]. Explosively driven plasma targets look very attractive for beamplasma interaction experiments because of the absence of strong electromagnetic fields like in discharges, which significantly effect the beam transport. To use standard explosive devices in beam areas of accelerator facilities it is necessary to build large-scaled explosive chambers and to solve the problems of matching high vacuum beam lines with the explosive apparatus. As a first step we suggest use of small charge ((150 g TNT) explosive generators in a vacuum pumped explosive metallic chamber with fast valves in such experiments. To optimize the explosive plasma generators numerical simulations of plasma shock compression and a special series of shock wave experiments were carried out. They show the possibility to construct small-sized linear and cumulative explosively driven generators with shock front velocities of about 6—20 km/s having a high explosive charge not exceeding 30—150 g. Using these devices we will investigate experimentally (i) the effect of strong interparticle interactions in plasma on the energy loss of fast ions and (ii) the stopping power of plasma at high ionization degrees.
2. Estimates of the stopping power behind strong shock waves To calculate the plasma parameters of the shock compressed plasma the computer code SAHA-4 [3] was used. The computing procedure is based on the chemical picture of plasma [4]. This code allows to calculate composition, the equation of state and thermodynamic functions of plasmas in a wide range of densities and temperatures. Using this code Hugoniots of hydrogen and noble gases were calculated in a range of shock velocities, typical for explosively driven plasma generators considered here.
Computer simulations of explosively driven plasmas show that using the simplest (linear) scheme of shock tube makes it possible to obtain an ionization degree a'1 and high electron densities for gases with a rather high molecular weight. For example, shock waves with a velocity of 6 km/s in xenon at an initial pressure of 1 bar produce a plasma with an electron density of more than n &1020 cm~3 and an ionization degree of about % a&1. Higher plasma densities can be reached without any problem by increasing the initial pressure of the investigated gas. Shock compression of lighter gases, for example, argon gives lower electron densities at the same shock velocities and one needs a more powerful explosive charge to obtain the same ionization degree. Higher electron densities can be reached if the technique of reflected shock waves is used. With hydrogen, due to its low molecular weight, a noticeable ionization degree after a direct shock heating can be obtained only at front velocities of more than 40 km/s. These shock velocities are too high for linear plasma generators. Using the reflected shock technique would only slightly increase the ionization degree. The required high shock velocities cannot be obtained with simple linear plasma generator. In this case the cumulative version of the plasma generator can be used, which allows shock waves of very high velocities of up to 100 km/s to be produced. The stopping power of explosively produced plasma was calculated within the framework of approximation [3]. All calculations were carried out for the Xe probe ions. Results of these calculations for xenon plasma are presented in Fig. 1 (n — concentration of Xe atoms). Shock velocities X% of D"18 km/s are typical for the cumulative plasma generator and they allow to investigate experimentally the effects of the Coulomb interparticle interaction on the stopping power. The calculated contribution of the free electrons to the energy loss is comparable to that of the bound ones only in the low-energy limit of the probe ion but the estimated range (about a few millimeters) makes it technically difficult to measure the contribution of free electrons for these kind of plasma targets. The effects of free electrons can be studied much better in a hydrogen plasma
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
717
Fig. 3. Linear explosively driven shock tube.
Fig. 1. Energy loss of Xe ion beam in Xe plasma.
Fig. 2. Energy loss of Xe ion beam in hydrogen plasma.
The detonation products in this device push a metal impactor which produces a flat shock front in the investigated gas. The glass tube remains immobile during a time of about 1 ls while the plasma passes through the tube. The flow of plasma is practically one dimensional. Shock velocities of 6—8 km/s were obtained in this generator. By applying a conical section (cf. Fig. 4) the shock velocity can be increased to &15 km/s and a plasma of cylindrical shape with a diameter of 0.5—2 cm and a thickness of about 1 cm is created. This plasma object exists for about several microseconds which is sufficient to carry out measurements with ion beams. Typical pictures of the gas dynamical flow in these generators are shown in Fig. 5. To protect the equipment from the debris of the exploding plasma generator, it is placed into a special compact vacuum pumped (up to 10~2 Torr) steel chamber with a diameter of 80 cm. This chamber allows to apply explosive charges of up to 150 g of TNT and is specially designed to provide a complete matching with the beam line of
(Fig. 2). At shock velocities of about 80 km/s, which are reachable using a compact cumulative explosive device, the ionization degree is about a&0.5 and the concentration of hydrogen atoms n & H 2]1020 l/cc. This is enough to study the free electron effects because the stopping power of the free electrons is higher than that of the bound electrons.
3. Experimental setup A schematic drawing of linear small-sized explosively driven plasma generator is shown in Fig. 3.
Fig. 4. Cumulative explosively driven shock tube.
718
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
Fig. 5. Gas dynamic flow in explosively driven shock tube.
the accelerator facility. A schematic drawing of the experimental setup with the explosive chamber and plasma generator is presented in Fig. 6. A 3 MeV,
60 ls proton beam delivered by the 148.5 Mhz ISTRA-36 RFQ linac of ITEP [5] was used for first energy loss experiments. The differential pumping proved to be sufficient to separate the high vacuum beam line from the low vacuum explosive chamber. Fast valves [6] were used to protect the beam line from the moving detonation products. They were closed before the explosive flow runs into the chamber walls. The position of the proton beam on a scintillator after the analyzer magnet was recorded by a fast shutter camera (PCO-camera). Typical picture of the experiment is shown in Fig. 7. The time gates of the PCO-camera was about 50 ls in this shot. The undisturbed position of the proton beam passing through cold xenon at an initial pressure of 10 kPa is seen to the right. The time of about 1 ls is necessary for plasma slug (D&10 km/s) to cross the beam line. The image of the proton beam interacting with the explosively driven plasma is to the left in Fig. 7. The intensity of the beam image is low because of the small time of the process and additional scattering of the beam. The estimated value of the energy loses in this experiment is *E "150 keV. %91 The plasma temperature and shock velocities were measured in the experiment. Pressure P&50 Mpa, temperature ¹&4 eV, density o&6]10~3, electron concentration n &6 1019 l/cc and nonideality % parameter C&0.35 were realized. Calculated value of the energy loses (*E "200 keV) is in reasonable # agreement with the measured one.
Fig. 6. Experimental setup. (1) Bending magnet R"500, (2) differential pumping, (3) vacuum measurement, (4) fast valves, (5) explosive chamber, (6) explosive generator, (7) analysing magnet R"707, (8) MCP plate with szintillator, (9) PCO camera, (10) pumping.
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
719
Fig. 7. Image of 3 MeV proton beam in the beam-explosively driven plasma interaction experiment (P "10 kPa). 0
So the first shots with the weakly nonideal xenon plasma showed a reliable operation of the experimental setup.
Acknowledgements This work was supported in part by INTAS Grant N94-1638, NATO Research Grant No. 931324 and NATO Linkage Grant No. 920854.
References [1] V. Fortov, I. Yakubov, Physics of Nonideal Plasma, Hemisphere, Washington, DC, 1990. [2] V.B. Mintsev, V.E. Fortov, High Temperatures 20 (1982) 584. [3] W. Ebeling, A. Foerster, V. Fortov, V. Gryaznov, A. Polishchuk, Thermophysical Properties of Hot Dense Plasmas, Teubner, Stuttgart-Leipzig, 1991. [4] W. Ebeling, Physica 43 (1969) 293. [5] I.V. Chuvilo, et al., Proton 36 MeV, 0.5 mA Linac Istra-36 as a drive of multipurpose irradiation test facility. EPAC’96. 10—14 June 1996, Barcelona, Spain. [6] D. Gardes et al., Laser and Particle Beams 8(4) (1990) 575.
On measurements of stopping power in explosively driven plasma targets V. Mintsev!,*, V. Gryaznov!, M. Kulish!, V. Fortov!, B. Sharkov", A. Golubev", A. Fertman", N. Mescheryakov", W. Su¨{#, D.H.H. Hoffmann#, M. Stetter$, R. Bock$, M. Roth%, C. Sto¨ckl%, D. Gardes& ! Institute of Chemical Physics in Chernogolovka, Chernogolovka, Moscow reg. 142432, Russia " Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117259 Moscow, Russia # Universia( tt Erlangen-Nu( rnberg, D-91058 Erlangen, Germany $ Gesellschaft fu( r Schwerionenforschung, D-64220 Darmstadt, Germany % Technische Universtia( t Darmstadt, D-64289 Darmstadt, Germany & Institute Nacional de Physique Nucleare, 91406 Orsay, France
Abstract A new method to generate plasma targets with electron densities n 51020 cm~3 behind strong shock waves to study % the energy loss of protons and heavy ions is discussed. Estimates on the stopping power in hydrogen, xenon and argon were carried out. It is shown that shock velocities of more than 20 km/s in xenon and argon and of more than 60 km/s in hydrogen are needed to get a major contribution of free electrons. The problems of matching of large scale accelerator facility and explosive technique are considered. It is suggested to use a small ((150 g TNT) vacuum pumped explosive metallic chambers with fast valves in such experiments. Details on the construction and performance of small-sized explosively driven generators of strong shock waves are presented. The experimental setup including the proton accelerator ISTRA-36 and the explosive chamber, which have been installed in ITEP, is also presented. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction To investigate the heating of matter by particle beams detailed knowledge of the energy loss in dense plasma at high pressures and temperatures are crucial. Experiments carried out at GSI with discharge plasmas having electron densities up to
* Corresponding author. E-mail: [email protected].
n &1019 cm~3 show a considerable contribution % of free electrons to the stopping power. With increasing plasma density the influence of the effects of the Coulomb coupling are expected to be of great importance. Shock wave techniques make it possible to produce plasmas with electron densities of up to n &1022 cm~3 [1]. For this goal explosively % driven plasma generators have been developed [2]. In such devices plasma is created behind the plane front of an intense shock wave generated by the
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 5 5 4 - 3
716
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
detonation of high explosive chemicals. Standard shock wave plasma generators contain more than 500 g of high explosives, to produce shock compressed strongly coupled plasma with temperatures of 1—10 eV, pressures of 1—200 kbar and Coulomb coupling parameters of 1—5 [1]. Explosively driven plasma targets look very attractive for beamplasma interaction experiments because of the absence of strong electromagnetic fields like in discharges, which significantly effect the beam transport. To use standard explosive devices in beam areas of accelerator facilities it is necessary to build large-scaled explosive chambers and to solve the problems of matching high vacuum beam lines with the explosive apparatus. As a first step we suggest use of small charge ((150 g TNT) explosive generators in a vacuum pumped explosive metallic chamber with fast valves in such experiments. To optimize the explosive plasma generators numerical simulations of plasma shock compression and a special series of shock wave experiments were carried out. They show the possibility to construct small-sized linear and cumulative explosively driven generators with shock front velocities of about 6—20 km/s having a high explosive charge not exceeding 30—150 g. Using these devices we will investigate experimentally (i) the effect of strong interparticle interactions in plasma on the energy loss of fast ions and (ii) the stopping power of plasma at high ionization degrees.
2. Estimates of the stopping power behind strong shock waves To calculate the plasma parameters of the shock compressed plasma the computer code SAHA-4 [3] was used. The computing procedure is based on the chemical picture of plasma [4]. This code allows to calculate composition, the equation of state and thermodynamic functions of plasmas in a wide range of densities and temperatures. Using this code Hugoniots of hydrogen and noble gases were calculated in a range of shock velocities, typical for explosively driven plasma generators considered here.
Computer simulations of explosively driven plasmas show that using the simplest (linear) scheme of shock tube makes it possible to obtain an ionization degree a'1 and high electron densities for gases with a rather high molecular weight. For example, shock waves with a velocity of 6 km/s in xenon at an initial pressure of 1 bar produce a plasma with an electron density of more than n &1020 cm~3 and an ionization degree of about % a&1. Higher plasma densities can be reached without any problem by increasing the initial pressure of the investigated gas. Shock compression of lighter gases, for example, argon gives lower electron densities at the same shock velocities and one needs a more powerful explosive charge to obtain the same ionization degree. Higher electron densities can be reached if the technique of reflected shock waves is used. With hydrogen, due to its low molecular weight, a noticeable ionization degree after a direct shock heating can be obtained only at front velocities of more than 40 km/s. These shock velocities are too high for linear plasma generators. Using the reflected shock technique would only slightly increase the ionization degree. The required high shock velocities cannot be obtained with simple linear plasma generator. In this case the cumulative version of the plasma generator can be used, which allows shock waves of very high velocities of up to 100 km/s to be produced. The stopping power of explosively produced plasma was calculated within the framework of approximation [3]. All calculations were carried out for the Xe probe ions. Results of these calculations for xenon plasma are presented in Fig. 1 (n — concentration of Xe atoms). Shock velocities X% of D"18 km/s are typical for the cumulative plasma generator and they allow to investigate experimentally the effects of the Coulomb interparticle interaction on the stopping power. The calculated contribution of the free electrons to the energy loss is comparable to that of the bound ones only in the low-energy limit of the probe ion but the estimated range (about a few millimeters) makes it technically difficult to measure the contribution of free electrons for these kind of plasma targets. The effects of free electrons can be studied much better in a hydrogen plasma
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
717
Fig. 3. Linear explosively driven shock tube.
Fig. 1. Energy loss of Xe ion beam in Xe plasma.
Fig. 2. Energy loss of Xe ion beam in hydrogen plasma.
The detonation products in this device push a metal impactor which produces a flat shock front in the investigated gas. The glass tube remains immobile during a time of about 1 ls while the plasma passes through the tube. The flow of plasma is practically one dimensional. Shock velocities of 6—8 km/s were obtained in this generator. By applying a conical section (cf. Fig. 4) the shock velocity can be increased to &15 km/s and a plasma of cylindrical shape with a diameter of 0.5—2 cm and a thickness of about 1 cm is created. This plasma object exists for about several microseconds which is sufficient to carry out measurements with ion beams. Typical pictures of the gas dynamical flow in these generators are shown in Fig. 5. To protect the equipment from the debris of the exploding plasma generator, it is placed into a special compact vacuum pumped (up to 10~2 Torr) steel chamber with a diameter of 80 cm. This chamber allows to apply explosive charges of up to 150 g of TNT and is specially designed to provide a complete matching with the beam line of
(Fig. 2). At shock velocities of about 80 km/s, which are reachable using a compact cumulative explosive device, the ionization degree is about a&0.5 and the concentration of hydrogen atoms n & H 2]1020 l/cc. This is enough to study the free electron effects because the stopping power of the free electrons is higher than that of the bound electrons.
3. Experimental setup A schematic drawing of linear small-sized explosively driven plasma generator is shown in Fig. 3.
Fig. 4. Cumulative explosively driven shock tube.
718
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
Fig. 5. Gas dynamic flow in explosively driven shock tube.
the accelerator facility. A schematic drawing of the experimental setup with the explosive chamber and plasma generator is presented in Fig. 6. A 3 MeV,
60 ls proton beam delivered by the 148.5 Mhz ISTRA-36 RFQ linac of ITEP [5] was used for first energy loss experiments. The differential pumping proved to be sufficient to separate the high vacuum beam line from the low vacuum explosive chamber. Fast valves [6] were used to protect the beam line from the moving detonation products. They were closed before the explosive flow runs into the chamber walls. The position of the proton beam on a scintillator after the analyzer magnet was recorded by a fast shutter camera (PCO-camera). Typical picture of the experiment is shown in Fig. 7. The time gates of the PCO-camera was about 50 ls in this shot. The undisturbed position of the proton beam passing through cold xenon at an initial pressure of 10 kPa is seen to the right. The time of about 1 ls is necessary for plasma slug (D&10 km/s) to cross the beam line. The image of the proton beam interacting with the explosively driven plasma is to the left in Fig. 7. The intensity of the beam image is low because of the small time of the process and additional scattering of the beam. The estimated value of the energy loses in this experiment is *E "150 keV. %91 The plasma temperature and shock velocities were measured in the experiment. Pressure P&50 Mpa, temperature ¹&4 eV, density o&6]10~3, electron concentration n &6 1019 l/cc and nonideality % parameter C&0.35 were realized. Calculated value of the energy loses (*E "200 keV) is in reasonable # agreement with the measured one.
Fig. 6. Experimental setup. (1) Bending magnet R"500, (2) differential pumping, (3) vacuum measurement, (4) fast valves, (5) explosive chamber, (6) explosive generator, (7) analysing magnet R"707, (8) MCP plate with szintillator, (9) PCO camera, (10) pumping.
V. Mintsev et al./Nucl. Instr. and Meth. in Phys. Res. A 415 (1998) 715—719
719
Fig. 7. Image of 3 MeV proton beam in the beam-explosively driven plasma interaction experiment (P "10 kPa). 0
So the first shots with the weakly nonideal xenon plasma showed a reliable operation of the experimental setup.
Acknowledgements This work was supported in part by INTAS Grant N94-1638, NATO Research Grant No. 931324 and NATO Linkage Grant No. 920854.
References [1] V. Fortov, I. Yakubov, Physics of Nonideal Plasma, Hemisphere, Washington, DC, 1990. [2] V.B. Mintsev, V.E. Fortov, High Temperatures 20 (1982) 584. [3] W. Ebeling, A. Foerster, V. Fortov, V. Gryaznov, A. Polishchuk, Thermophysical Properties of Hot Dense Plasmas, Teubner, Stuttgart-Leipzig, 1991. [4] W. Ebeling, Physica 43 (1969) 293. [5] I.V. Chuvilo, et al., Proton 36 MeV, 0.5 mA Linac Istra-36 as a drive of multipurpose irradiation test facility. EPAC’96. 10—14 June 1996, Barcelona, Spain. [6] D. Gardes et al., Laser and Particle Beams 8(4) (1990) 575.