- Email: [email protected]
Influence of the gap-target configuration on the measured energy loss of C-ions in Ar-gas and -plasma
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370 www.elsevier.com/locate/nima
Influence of the gap-target configuration on the measured energy loss of C-ions in Ar-gas and -plasma K. Weyricha,, H. Wahla, A. Golubevb, A. Kantsyrevb, M. Kulishc, S. Dudinc, D.H.H. Hoffmanna,d, B. Sharkovb, V. Mintsevc a
GSI, Plasmaphysik, Planckstrasse 1, D-64291 Darmstadt, Germany b ITEP, Moscow, Russia c IPCP, Chernogolovka, Russia d Technical University, Darmstadt, Germany Available online 21 February 2007
Abstract Plasma targets for measuring energy loss and charge state distribution of heavy ions in non-ideal Ar-plasmas ðG-parameters 0.55–1.5) have been developed and the interaction with several ion species, C, Ar, Xe at 5:9 MeV=u, was studied. Here the results for 5:9 MeV=u C-ions are presented which have been understood just recently by considering the gas dynamics in the target due to the special configuration with the gap interaction zone for the plasma, which can be also simulated by the VarJet code. r 2007 Elsevier B.V. All rights reserved. PACS: 52.27.Gr; 52.40.w; 52.40.Mj; 52.50.Lp Keywords: Non-ideal plasma; Shockwave-produced plasma; Ion–beam interaction; Stopping power
1. Motivation Plasmas which have recently found enhanced scientific interest are non-ideal plasmas in the ‘warm dense matter’ regime, since they are important for laboratory astrophysics, inertial confinement fusion [1–3] and the design of fusion targets [4]. The non-ideal plasmas are also strongly coupled and are characterized by a G-parameter with G ¼ e2 =kTrD X1. For ideal plasmas the energy loss of interacting charged particles exhibits a Z 2eff -proportionality, but for non-ideal plasmas a proportionality of the stopping power or energy loss of charged particles in the plasma of Z beff with bo2 [5–8] is predicted. A detailed knowledge of the physics of energy deposition of a heavy ion beam in dense matter is of prime interest for the fields mentioned above. While a variety of experimental data has been gathered concerning the stopping of ions in cold matter and in ideal plasmas [9–16] no experimental
Corresponding author.
E-mail address: [email protected] (K. Weyrich). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.02.032
database exists for the interaction of heavy ions with non-ideal plasmas. The stopping power of dense non-ideal plasmas is affected by correlation effects due to strong coupling: multiple scattering, dynamic screening, bound states and lowering of the ionization energy. These nonideality effects can lead to strong deviations from the Z2eff -dependence of the stopping power, especially for low ion energies with vpvth of the plasma electrons [6,7]. In our regime of several MeV=u they are expected to be small.
2. Plasma creation and diagnosis A possibility to create non-ideal, strongly coupled plasmas which are very homogenous, are shockwavedriven plasmas, where gas is compressed and becomes ionized in front of a shock front and a plasma sheath of a thickness of several millimeters is created [17,18]. The shockwave is initiated by a small amount of a high explosive material (55 g of RDX) and reaches velocities of
ARTICLE IN PRESS K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
several km/s. Flyer-plates between high explosive and gas volume improve the planeness of the shockwave. Fig. 1 shows the head of such a target for creating nonideal plasmas developed for the experiments at GSI. The interaction experiments at GSI extended for the first time experiments with non-ideal plasmas and protons done at ITEP in the MeV=u energy region to heavy ions. The first stage of the experiments at GSI involved this target development. The targets originally developed at ITEP, Moscow, for the proton experiments [17,18] were not compatible with heavy ion beams. The finally successful design was the ‘gap target’, while the lower part with the explosive, flyer-plate and gas inlets stayed unchanged as shown in Ref. [17], the target head was redesigned. In the head section of this target two small metal tubes with 1:5 mm Mylar foil windows face each other 180 opposite (diameter of target head ¼ 28 mm). Between them is a gap of several millimeters. This is the interaction zone with the plasma. The shockwave travels through the target, but the outer areas with possible inhomogeneities due to plasma–wall interaction are shaded by the tubes. The short interaction distance in the gap with the compressed gas in the plasma phase minimizes straggling losses. Two glass rods protrude with different lengths into the target head. A peak in the light emission of the plasma observed with
367
photo diodes originates from the reflection on the surface of the rods. The timely difference between the peaks recorded with an oscilloscope gives the shockwave velocity (see Fig. 1). The plasma parameters are determined by the SAHA-4 code [19], which needs initial gas pressure and shockwave velocity as input parameters to calculate the plasma properties. The code is based on the chemical model of the plasma and extensive data from spectroscopic measurements (IPCP). The plasma parameters for Ar-plasmas with initial Ar-gas pressures between 0.2 and 3 bar, which were obtained with the gap targets, are listed below: Free electron densities between 3 1019 and 1:5 1020 cm3 . Electron temperatures 2:0 eV. Compression factors of 8–10 (pressure in the plasma phase/initial gas pressure). G-Parameters between 0.55 and 1.5. Ionization degree in the plasma 50–30%.
3. Measurement of energy loss and charge state In the Z6 experimental area at GSI a set-up was installed for measuring the energy loss of heavy ions in these plasma targets. A compact vacuum-pumped steel chamber, designed for explosions up to 200 g RDX, which was originally used for the proton experiments at ITEP, was aligned to the beam line. Fast valves [20] connected to the chamber protect the high vacuum beam-line from the pressure increase and detonation products. The energy loss measurements are performed by time-of-flight (TOF) methods with a microspherical plate (MSP) or phase probes at sufficient beam intensity as stop detectors. The charge state distribution of the ions after passing the plasma was analyzed by a 10 magnetic spectrometer and a scintilator. With either a CCD camera a static charge state distribution or with a streak camera a dynamic charge state distribution can be recorded. A phase probe as stop detector in front of the magnet provides a nonbeam-destructive TOF measurement and therefore simultaneous analysis of the charge state distribution after the magnet. Fig. 2 shows the target chamber and important details of the set-up (upper picture) and the devices for the measurement of charge-state distribution (lower picture). 4. Results for different measurements with 5:9 MeV=u C-ions
Fig. 1. New designed head of the shockwave-driven gap target and principle of shock-wave velocity measurement.
Between December 2002 and November 2005 interaction experiments with C-, Ar- and Xe-ions [21] at energies 5.9 and 11:4 MeV=u were carried out. In this paper we concentrate on the results with 5:9 MeV=u C-ions. For them the plasma data were reproduced in different beam
ARTICLE IN PRESS 368
K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
Fig. 3. Energy loss of 5:9 MeV=u C-ions in Ar-plasma in different beam times and comparison to SRIM_2003 calculations for the same line density in Ar-gas.
Fig. 2. The experimental set-up: above, the target chamber and details for plasma production and diagnosis; below, the set-up for charge state measurements.
times and were understood recently due to the realization of some principle problems in understanding the cold gas data influenced by the target configuration. After the first 1:5 mm thick Mylar foil and 180 mbar Ar-gas in the target the C-ions are fully ionized. The lowest pressure used in the experiments was 200 mbar and in the plasma phase there is compression of the stopping medium, so the C-ions are fully ionized under all experimental conditions. Any observed difference in energy loss compared to cold gas is not due to changes in the charge state of the ions in the plasma. This cannot be excluded for Ar- and Xe-ions, where the data are still processed and discussed. Fig. 3 shows the energy loss of 5:9 MeV=u C-ions in Arplasma, the line density corresponds to initial gas pressures of 200–800 mbar (measured with a precision pressure gauge, Pfeiffer TPG 261) before ignition. The energy loss in the plasma is compared to the energy loss in cold gas at the same line density calculated with SRIM_2003 [22]. The linear fit curve through the data points indicates at least for the August 2004 data a slight enhancement of the energy
Fig. 4. Energy loss of 5:9 MeV=u C-ions in Ar-gas measured in different beam times and comparison to SRIM_2003 calculations.
loss in the plasma. The data are reproducible within the error bars for both experiments. But the error bars 10% of the measured values and the known precision of SRIM_2003 for C-ions in the MeV/u region of 5% [22] are in the same order of magnitude. Also some density effect is indicated for the data points at higher pressures, which are located above the fit curve. For plasma the results from the beam times in December 2002 and August 2004 are reproducible within the error bars. The measured data for the energy loss in cold gas could not be reproduced. The cold gas data from December 2002 lie 15% below the plasma values and the gas data from
ARTICLE IN PRESS K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
August 2004, see Fig. 4, even if the August 2004 data coincide completely with the SRIM_2003 simulations. This was only understood in later beam times, because of the influence of the gap-target configuration and ion beam current on the line density of the Ar-gas, see Section 5. The measurements were carried out with a MSP-detector and a 500-ms-beam pulse in December 2002 and in August 2004 with phase probes and a 100-ms-beam pulse. Phase probes and MSP detector give exactly the same results for the energy loss in Ar-gas measured at the same time in the macro-pulse. This was verified in a beam time in January 2005. 5. Gas dynamics in the target due to the gap configuration The development of the energy loss in Ar-gas was systematically studied in two beam times. Fig. 5 shows the energy loss of 5:9 MeV=u C-ions measured in a 100-ms long beam pulse and at different time windows in 500 ms pulse at 300 and 600 mbar (upper picture). The later in the pulse the
Fig. 5. Energy loss at the beginning of a 100 ms-pulse and at 300 and 600 mbar in later time-windows in a 500 ms-pulse (upper picture) and simulation of the Ar-density profile in the Al-tubes in the target at different times (lower picture) with the VarJet-Code [23].
369
measurement was done, the lower is the energy loss. The energy deposition of the ion beam heats the gas which expands out of the interaction zone through the gap, reducing the line density. Simulations with the VarJet code [23] verify this. Fig. 5 also shows the density profile of the Ar-gas along the radius of the tubes at three different times in the macro-pulse calculated with VarJet (lower picture), which exhibits the same behaviour as the energy loss, which is proportional to the line density. In the last beam time in November 2005 this behaviour was systematically investigated for 5.9 MeV/u C- and Ar-ions. Inside a 300-ms beam pulse a measuring time window of 20 ms was shifted starting at 10 ms after the beginning of the pulse to later times inside the pulse and the energy loss was measured scanning so systematically the development of the line density of the Argon gas during the heating by the ion pulse. Early in the pulse the energy loss data for C-ions from the August 2004 beam time (see Fig. 4) were reproduced. At the end of the 300 ms pulse a decrease of the energy loss compared to the beginning of the pulse of only 3–5% was measured, the 15% measured much earlier in the pulse in December 2002 and at 250 ms in January 2005 were never reached. The heating of the gas is very sensitive to the beam current. At 10 mA a decrease in the energy loss of 15% was measured at times o100 ms, at 8 mA at 250 ms (Fig. 5), and for 6 mA the decrease never reaches 10%. This does not apply to the energy loss measurement in the plasma, for the 4–5 mm thick plasma sheath travels through the gap in a few ms, which is not sufficient time to induce any gas dynamics by the deposited beam energy. The measurements have shown that the gap target is optimized for the measurements in plasma and the results are reproducible in the case of 5:9 MeV=u C-ions. It is necessary to compare the energy loss in the non-ideal plasmas to the measured one in Ar-gas, as it was done in previous experiments for ideal plasmas. It was realized that the gap-target configuration, necessary for the plasma measurements, and high beam intensities are contradictory. But high beam intensities are needed to get good signals from the stop detectors at high gas pressures and in the plasma phase. Gas dynamics through the gap and the decrease of gas line density must be excluded to provide reproducible experimental conditions. This can be done by measuring very early in the pulse and by keeping the beam intensity below a limit for sufficient heating of the gas to expand out of the interaction zone. The development of more sensitive stop detectors, diamond detectors are very promising candidates to be sensitive to low beam currents and are tested at the moment, is essential. With them the experimental conditions could be made reproducible to compare measurements in Ar-gas with the Ar-plasma for detecting any non-ideality effects, which are predicted to be very small [6] in our parameter regime. They are inside the error bars of the measurements presented here with phase probes and MSP detectors, where also a small enhancement of the energy loss in the plasma over the gas may
ARTICLE IN PRESS 370
K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
partly be compensated by non-ideality effects with a less than Z2eff -dependence of the energy loss. References [1] P.T. Leon, S. Elizier, M. Piera, M.J. Martinez-Val, Laser Part. Beams 23 (2005) 193. [2] B.G. Logan, R.O. Bangerter, D.A. Callahan, et al., Fusion Sci. Technol. 49 (3) (2006) 399. [3] B.G. Logan, F. Bionisek, C. Celata, et al., Nucl. Instr. and Meth. A 544 (1-2) (2005) 1. [4] E.R. Koresheva, I.E. Osipov, I.V. Aleksandrova, Laser Part. Beams 23 (2005) 563. [5] G. Zwicknagel, C. Toepffer, P.G. Reinhardt, Laser Part. Beams 13 (1995) 311. [6] G. Zwicknagel, Phys. Rep. 309 (1999) 117. [7] D.O. Gericke, M. Schlanges, J. Phys. IV France 10 (2000) Pr5–481. [8] D.O. Gericke, M. Schlanges, Th. Bornath, Phys. Rev. E 65 (2002). [9] K. Weyrich, D.H.H. Hoffmann, J. Jacoby, et al., Nucl. Instr. and Meth. A 278 (1989) 52. [10] D.H.H. Hoffmann, K. Weyrich, H. Wahl, et al., Phys. Rev. A 42 (1990) 2313. [11] D.H.H. Hoffmann, K. Weyrich, H. Wahl, et al., Z. Phys. D 16 (1990) 229.
[12] K.-D. Dietrich, D.H.H. Hoffmann, E. Boggasch, J. Jacoby, et al., Phys. Rev. Lett. 69 (1992) 3623. [13] D.H.H. Hoffmann, J. Jacoby, W. Laux, et al., Nucl. Instr. and Meth. B 90 (1–4) (1994) 1. [14] C. Sto¨ckl, M. Roth, W. Su¨X, H. Wetzler, et al., Fusion Technol. 31 (1997) 169. [15] M. Roth, C. Sto¨ckl, W. Su¨X, O. Iwase, et al., Eur. Phys. Lett. 50 (2000) 28. [16] V. Turtikov, A. Golubev, A. Fertman, et al., Nucl. Instr. and Meth. A 464 (2001) 247. [17] V. Mintsev, V. Gryaznov, M. Kulish, E.V. Fortov, et al., Nucl. Instr. and Meth. A 415 (1998) 715. [18] V. Mintsev, V. Gryaznov, M. Kulish, E.V. Fortov, Contrib. Plasma Phys. 41 (2) (2001) 119. [19] W. Ebling, E.V. Fortov, et al., Thermophysical Properties of Hot Dense Plasmas, Teubner, Stuttgart-Leipzig, 1991. [20] D. Gardes, et al., Laser Part. Beams 8 (1990) 575. [21] D.H.H. Hoffmann, A. Blazevic, P. Ni, O. Rosmej, et al., Laser Part. Beams 23 (2005) 47. [22] hhttp://www.SRIM.orgi. [23] V.L. Varentsov, D. Varentsov, A.A. Ignatiev, Laser Part. Beams 24 (2006), accepted for publication.
Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370 www.elsevier.com/locate/nima
Influence of the gap-target configuration on the measured energy loss of C-ions in Ar-gas and -plasma K. Weyricha,, H. Wahla, A. Golubevb, A. Kantsyrevb, M. Kulishc, S. Dudinc, D.H.H. Hoffmanna,d, B. Sharkovb, V. Mintsevc a
GSI, Plasmaphysik, Planckstrasse 1, D-64291 Darmstadt, Germany b ITEP, Moscow, Russia c IPCP, Chernogolovka, Russia d Technical University, Darmstadt, Germany Available online 21 February 2007
Abstract Plasma targets for measuring energy loss and charge state distribution of heavy ions in non-ideal Ar-plasmas ðG-parameters 0.55–1.5) have been developed and the interaction with several ion species, C, Ar, Xe at 5:9 MeV=u, was studied. Here the results for 5:9 MeV=u C-ions are presented which have been understood just recently by considering the gas dynamics in the target due to the special configuration with the gap interaction zone for the plasma, which can be also simulated by the VarJet code. r 2007 Elsevier B.V. All rights reserved. PACS: 52.27.Gr; 52.40.w; 52.40.Mj; 52.50.Lp Keywords: Non-ideal plasma; Shockwave-produced plasma; Ion–beam interaction; Stopping power
1. Motivation Plasmas which have recently found enhanced scientific interest are non-ideal plasmas in the ‘warm dense matter’ regime, since they are important for laboratory astrophysics, inertial confinement fusion [1–3] and the design of fusion targets [4]. The non-ideal plasmas are also strongly coupled and are characterized by a G-parameter with G ¼ e2 =kTrD X1. For ideal plasmas the energy loss of interacting charged particles exhibits a Z 2eff -proportionality, but for non-ideal plasmas a proportionality of the stopping power or energy loss of charged particles in the plasma of Z beff with bo2 [5–8] is predicted. A detailed knowledge of the physics of energy deposition of a heavy ion beam in dense matter is of prime interest for the fields mentioned above. While a variety of experimental data has been gathered concerning the stopping of ions in cold matter and in ideal plasmas [9–16] no experimental
Corresponding author.
E-mail address: [email protected] (K. Weyrich). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.02.032
database exists for the interaction of heavy ions with non-ideal plasmas. The stopping power of dense non-ideal plasmas is affected by correlation effects due to strong coupling: multiple scattering, dynamic screening, bound states and lowering of the ionization energy. These nonideality effects can lead to strong deviations from the Z2eff -dependence of the stopping power, especially for low ion energies with vpvth of the plasma electrons [6,7]. In our regime of several MeV=u they are expected to be small.
2. Plasma creation and diagnosis A possibility to create non-ideal, strongly coupled plasmas which are very homogenous, are shockwavedriven plasmas, where gas is compressed and becomes ionized in front of a shock front and a plasma sheath of a thickness of several millimeters is created [17,18]. The shockwave is initiated by a small amount of a high explosive material (55 g of RDX) and reaches velocities of
ARTICLE IN PRESS K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
several km/s. Flyer-plates between high explosive and gas volume improve the planeness of the shockwave. Fig. 1 shows the head of such a target for creating nonideal plasmas developed for the experiments at GSI. The interaction experiments at GSI extended for the first time experiments with non-ideal plasmas and protons done at ITEP in the MeV=u energy region to heavy ions. The first stage of the experiments at GSI involved this target development. The targets originally developed at ITEP, Moscow, for the proton experiments [17,18] were not compatible with heavy ion beams. The finally successful design was the ‘gap target’, while the lower part with the explosive, flyer-plate and gas inlets stayed unchanged as shown in Ref. [17], the target head was redesigned. In the head section of this target two small metal tubes with 1:5 mm Mylar foil windows face each other 180 opposite (diameter of target head ¼ 28 mm). Between them is a gap of several millimeters. This is the interaction zone with the plasma. The shockwave travels through the target, but the outer areas with possible inhomogeneities due to plasma–wall interaction are shaded by the tubes. The short interaction distance in the gap with the compressed gas in the plasma phase minimizes straggling losses. Two glass rods protrude with different lengths into the target head. A peak in the light emission of the plasma observed with
367
photo diodes originates from the reflection on the surface of the rods. The timely difference between the peaks recorded with an oscilloscope gives the shockwave velocity (see Fig. 1). The plasma parameters are determined by the SAHA-4 code [19], which needs initial gas pressure and shockwave velocity as input parameters to calculate the plasma properties. The code is based on the chemical model of the plasma and extensive data from spectroscopic measurements (IPCP). The plasma parameters for Ar-plasmas with initial Ar-gas pressures between 0.2 and 3 bar, which were obtained with the gap targets, are listed below: Free electron densities between 3 1019 and 1:5 1020 cm3 . Electron temperatures 2:0 eV. Compression factors of 8–10 (pressure in the plasma phase/initial gas pressure). G-Parameters between 0.55 and 1.5. Ionization degree in the plasma 50–30%.
3. Measurement of energy loss and charge state In the Z6 experimental area at GSI a set-up was installed for measuring the energy loss of heavy ions in these plasma targets. A compact vacuum-pumped steel chamber, designed for explosions up to 200 g RDX, which was originally used for the proton experiments at ITEP, was aligned to the beam line. Fast valves [20] connected to the chamber protect the high vacuum beam-line from the pressure increase and detonation products. The energy loss measurements are performed by time-of-flight (TOF) methods with a microspherical plate (MSP) or phase probes at sufficient beam intensity as stop detectors. The charge state distribution of the ions after passing the plasma was analyzed by a 10 magnetic spectrometer and a scintilator. With either a CCD camera a static charge state distribution or with a streak camera a dynamic charge state distribution can be recorded. A phase probe as stop detector in front of the magnet provides a nonbeam-destructive TOF measurement and therefore simultaneous analysis of the charge state distribution after the magnet. Fig. 2 shows the target chamber and important details of the set-up (upper picture) and the devices for the measurement of charge-state distribution (lower picture). 4. Results for different measurements with 5:9 MeV=u C-ions
Fig. 1. New designed head of the shockwave-driven gap target and principle of shock-wave velocity measurement.
Between December 2002 and November 2005 interaction experiments with C-, Ar- and Xe-ions [21] at energies 5.9 and 11:4 MeV=u were carried out. In this paper we concentrate on the results with 5:9 MeV=u C-ions. For them the plasma data were reproduced in different beam
ARTICLE IN PRESS 368
K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
Fig. 3. Energy loss of 5:9 MeV=u C-ions in Ar-plasma in different beam times and comparison to SRIM_2003 calculations for the same line density in Ar-gas.
Fig. 2. The experimental set-up: above, the target chamber and details for plasma production and diagnosis; below, the set-up for charge state measurements.
times and were understood recently due to the realization of some principle problems in understanding the cold gas data influenced by the target configuration. After the first 1:5 mm thick Mylar foil and 180 mbar Ar-gas in the target the C-ions are fully ionized. The lowest pressure used in the experiments was 200 mbar and in the plasma phase there is compression of the stopping medium, so the C-ions are fully ionized under all experimental conditions. Any observed difference in energy loss compared to cold gas is not due to changes in the charge state of the ions in the plasma. This cannot be excluded for Ar- and Xe-ions, where the data are still processed and discussed. Fig. 3 shows the energy loss of 5:9 MeV=u C-ions in Arplasma, the line density corresponds to initial gas pressures of 200–800 mbar (measured with a precision pressure gauge, Pfeiffer TPG 261) before ignition. The energy loss in the plasma is compared to the energy loss in cold gas at the same line density calculated with SRIM_2003 [22]. The linear fit curve through the data points indicates at least for the August 2004 data a slight enhancement of the energy
Fig. 4. Energy loss of 5:9 MeV=u C-ions in Ar-gas measured in different beam times and comparison to SRIM_2003 calculations.
loss in the plasma. The data are reproducible within the error bars for both experiments. But the error bars 10% of the measured values and the known precision of SRIM_2003 for C-ions in the MeV/u region of 5% [22] are in the same order of magnitude. Also some density effect is indicated for the data points at higher pressures, which are located above the fit curve. For plasma the results from the beam times in December 2002 and August 2004 are reproducible within the error bars. The measured data for the energy loss in cold gas could not be reproduced. The cold gas data from December 2002 lie 15% below the plasma values and the gas data from
ARTICLE IN PRESS K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
August 2004, see Fig. 4, even if the August 2004 data coincide completely with the SRIM_2003 simulations. This was only understood in later beam times, because of the influence of the gap-target configuration and ion beam current on the line density of the Ar-gas, see Section 5. The measurements were carried out with a MSP-detector and a 500-ms-beam pulse in December 2002 and in August 2004 with phase probes and a 100-ms-beam pulse. Phase probes and MSP detector give exactly the same results for the energy loss in Ar-gas measured at the same time in the macro-pulse. This was verified in a beam time in January 2005. 5. Gas dynamics in the target due to the gap configuration The development of the energy loss in Ar-gas was systematically studied in two beam times. Fig. 5 shows the energy loss of 5:9 MeV=u C-ions measured in a 100-ms long beam pulse and at different time windows in 500 ms pulse at 300 and 600 mbar (upper picture). The later in the pulse the
Fig. 5. Energy loss at the beginning of a 100 ms-pulse and at 300 and 600 mbar in later time-windows in a 500 ms-pulse (upper picture) and simulation of the Ar-density profile in the Al-tubes in the target at different times (lower picture) with the VarJet-Code [23].
369
measurement was done, the lower is the energy loss. The energy deposition of the ion beam heats the gas which expands out of the interaction zone through the gap, reducing the line density. Simulations with the VarJet code [23] verify this. Fig. 5 also shows the density profile of the Ar-gas along the radius of the tubes at three different times in the macro-pulse calculated with VarJet (lower picture), which exhibits the same behaviour as the energy loss, which is proportional to the line density. In the last beam time in November 2005 this behaviour was systematically investigated for 5.9 MeV/u C- and Ar-ions. Inside a 300-ms beam pulse a measuring time window of 20 ms was shifted starting at 10 ms after the beginning of the pulse to later times inside the pulse and the energy loss was measured scanning so systematically the development of the line density of the Argon gas during the heating by the ion pulse. Early in the pulse the energy loss data for C-ions from the August 2004 beam time (see Fig. 4) were reproduced. At the end of the 300 ms pulse a decrease of the energy loss compared to the beginning of the pulse of only 3–5% was measured, the 15% measured much earlier in the pulse in December 2002 and at 250 ms in January 2005 were never reached. The heating of the gas is very sensitive to the beam current. At 10 mA a decrease in the energy loss of 15% was measured at times o100 ms, at 8 mA at 250 ms (Fig. 5), and for 6 mA the decrease never reaches 10%. This does not apply to the energy loss measurement in the plasma, for the 4–5 mm thick plasma sheath travels through the gap in a few ms, which is not sufficient time to induce any gas dynamics by the deposited beam energy. The measurements have shown that the gap target is optimized for the measurements in plasma and the results are reproducible in the case of 5:9 MeV=u C-ions. It is necessary to compare the energy loss in the non-ideal plasmas to the measured one in Ar-gas, as it was done in previous experiments for ideal plasmas. It was realized that the gap-target configuration, necessary for the plasma measurements, and high beam intensities are contradictory. But high beam intensities are needed to get good signals from the stop detectors at high gas pressures and in the plasma phase. Gas dynamics through the gap and the decrease of gas line density must be excluded to provide reproducible experimental conditions. This can be done by measuring very early in the pulse and by keeping the beam intensity below a limit for sufficient heating of the gas to expand out of the interaction zone. The development of more sensitive stop detectors, diamond detectors are very promising candidates to be sensitive to low beam currents and are tested at the moment, is essential. With them the experimental conditions could be made reproducible to compare measurements in Ar-gas with the Ar-plasma for detecting any non-ideality effects, which are predicted to be very small [6] in our parameter regime. They are inside the error bars of the measurements presented here with phase probes and MSP detectors, where also a small enhancement of the energy loss in the plasma over the gas may
ARTICLE IN PRESS 370
K. Weyrich et al. / Nuclear Instruments and Methods in Physics Research A 577 (2007) 366–370
partly be compensated by non-ideality effects with a less than Z2eff -dependence of the energy loss. References [1] P.T. Leon, S. Elizier, M. Piera, M.J. Martinez-Val, Laser Part. Beams 23 (2005) 193. [2] B.G. Logan, R.O. Bangerter, D.A. Callahan, et al., Fusion Sci. Technol. 49 (3) (2006) 399. [3] B.G. Logan, F. Bionisek, C. Celata, et al., Nucl. Instr. and Meth. A 544 (1-2) (2005) 1. [4] E.R. Koresheva, I.E. Osipov, I.V. Aleksandrova, Laser Part. Beams 23 (2005) 563. [5] G. Zwicknagel, C. Toepffer, P.G. Reinhardt, Laser Part. Beams 13 (1995) 311. [6] G. Zwicknagel, Phys. Rep. 309 (1999) 117. [7] D.O. Gericke, M. Schlanges, J. Phys. IV France 10 (2000) Pr5–481. [8] D.O. Gericke, M. Schlanges, Th. Bornath, Phys. Rev. E 65 (2002). [9] K. Weyrich, D.H.H. Hoffmann, J. Jacoby, et al., Nucl. Instr. and Meth. A 278 (1989) 52. [10] D.H.H. Hoffmann, K. Weyrich, H. Wahl, et al., Phys. Rev. A 42 (1990) 2313. [11] D.H.H. Hoffmann, K. Weyrich, H. Wahl, et al., Z. Phys. D 16 (1990) 229.
[12] K.-D. Dietrich, D.H.H. Hoffmann, E. Boggasch, J. Jacoby, et al., Phys. Rev. Lett. 69 (1992) 3623. [13] D.H.H. Hoffmann, J. Jacoby, W. Laux, et al., Nucl. Instr. and Meth. B 90 (1–4) (1994) 1. [14] C. Sto¨ckl, M. Roth, W. Su¨X, H. Wetzler, et al., Fusion Technol. 31 (1997) 169. [15] M. Roth, C. Sto¨ckl, W. Su¨X, O. Iwase, et al., Eur. Phys. Lett. 50 (2000) 28. [16] V. Turtikov, A. Golubev, A. Fertman, et al., Nucl. Instr. and Meth. A 464 (2001) 247. [17] V. Mintsev, V. Gryaznov, M. Kulish, E.V. Fortov, et al., Nucl. Instr. and Meth. A 415 (1998) 715. [18] V. Mintsev, V. Gryaznov, M. Kulish, E.V. Fortov, Contrib. Plasma Phys. 41 (2) (2001) 119. [19] W. Ebling, E.V. Fortov, et al., Thermophysical Properties of Hot Dense Plasmas, Teubner, Stuttgart-Leipzig, 1991. [20] D. Gardes, et al., Laser Part. Beams 8 (1990) 575. [21] D.H.H. Hoffmann, A. Blazevic, P. Ni, O. Rosmej, et al., Laser Part. Beams 23 (2005) 47. [22] hhttp://www.SRIM.orgi. [23] V.L. Varentsov, D. Varentsov, A.A. Ignatiev, Laser Part. Beams 24 (2006), accepted for publication.