Multiple electron stripping of heavy ion beams

Laser and Particle Beams ~2002!, 20, 551–554. Printed in the USA. Copyright © 2002 Cambridge University Press 0263-0346002 $16.00 DOI: 10.10170S0263034602204036 Multiple electron stripping of heavy ion beams D. MUELLER,1 L. GRISHAM,1 I. KAGANOVICH,1 R.L. WATSON,2 V. HORVAT,2 K.E. ZAHARAKIS,2 and Y. PENG 2 1 2 Princeton University Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA ~Received 26 May 2002; Accepted 24 October 2002! Abstract One approach being explored as a route to practical fusion energy uses heavy ion beams focused on an indirect drive target. Such beams will lose electrons while passing through background gas in the target chamber, and therefore it is necessary to assess the rate at which the charge state of the incident beam evolves on the way to the target. Accelerators designed primarily for nuclear physics or high energy physics experiments utilize ion sources that generate highly stripped ions in order to achieve high energies economically. As a result, accelerators capable of producing heavy ion beams of 10 to 40 MeV0amu with charge state 1 currently do not exist. Hence, the stripping cross sections used to model the performance of heavy ion fusion driver beams have, up to now, been based on theoretical calculations. We have investigated experimentally the stripping of 3.4 MeV0amu Kr17 and Xe111 in N2; 10.2 MeV0amu Ar16 in He, N2, Ar, and Xe; 19 MeV0amu Ar18 in He, N2, Ar, and Xe; 30 MeV He11 in He, N2, Ar, and Xe; and 38 MeV0amu N16 in He, N2, Ar, and Xe. The results of these measurements are compared with the theoretical calculations to assess their applicability over a wide range of parameters. Keywords: Inertial fusion; Cross-sections; Heavy ion; Electron 1. INTRODUCTION sections for electron stripping cross sections of the heavy ion beams in order to estimate these effects on the transport of the beam through the chamber to the target. In a previous experiment using Kr 17 and Xe111 beams at 3.4 MeV0amu, the first experimental demonstration that such energetic ions undergo multiple electron stripping events in a background gas of N2 was made ~Mueller et al., 2001!. As a result, the charge state of the ion beam increases more rapidly than would be the case if only a single electron were lost in each encounter. This effect must be included in the transport calculations of heavy ion beams. Unfortunately, there are currently no accelerators capable of accelerating heavy ions ~A ; 200! with charge state 1 to the energy range envisioned ~20 to 40 MeV0amu!, so the electron stripping cross sections cannot be measured directly by experiment and must be obtained from theoretical calculations. We have performed additional measurements, which along with those made by Olson et al. ~2002!, provide experimental data for comparison with theory over a range of beam energy, beam species, and gas target species. Agreement of calculations with the measurements would provide a measure of confidence that the theory might be applicable in a regime that is not yet experimentally available. Use of energetic heavy ion beams as a driver for inertial confinement fusion is a technique that is under development. The heavy ion beam focused on an indirect drive target would produce X rays, which then compress the deuterium–tritium target ~Bangerter, 1996!. The target chamber gas in one reference design, HYLIFE-II, would probably be comprised of vapor from FLIBE, a salt of fluorine, lithium, and beryllium. The gas density of beryllium difluoride in this design is expected to be 5 3 10 13 cm23 ~Callahan, 1996!. As the heavy ion beam propagates through this background gas, it would undergo electron stripping reactions that would raise the space-charge density of the beam. This can cause the beam spot to expand, reducing the power density on the target. Ionization of the background gas would supply space-charge-neutralizing electrons, which could compensate for the space-charge defocusing force on the positive ion beam. It is important to have accurate cross Address correspondence and reprint requests to: D. Mueller, Princeton University Plasma Physics Laboratory, Princeton University, P.O. Box 451, Princeton, NJ 08543, USA. E-mail: [email protected] 551 552 D. Mueller et al. 2. EXPERIMENT 16 18 Beams of 10.2 MeV0amu Ar , 19 MeV0amu Ar , 30 MeV0 amu He11, and 38 MeV0amu N16 were extracted from the Texas A&M K500 superconducting cyclotron. The beams were directed through a 228 deflection magnet located 10 m in front of the target chamber. The beam was collimated by three 1-mm-diameter apertures followed by a 2-mm-diameter collimator before entering a differentially pumped gas cell. The number of collimators was reduced for the most energetic beams because the range of the beams was greater than the collimator’s thickness, and the extra collimators resulted in a large low energy tail on the beam distribution. The gas cell of effective length 0.0208 m was filled with gas ~He, N2 , Ar, or Xe! to pressures from 1 to 250 mTorr, as measured by a capacitance manometer, and maintained by an automatic fill valve to about 60.3 mTorr accuracy. The background pressure in the beam line and target chamber were monitored with ion gauges and ranged between 1.5 and 5.0 3 1026 Torr, depending on target cell pressure and vacuum history. After exiting the gas cell, the beam passed between the poles of another magnet to disperse the charge states and on to a position-sensitive microchannel plate detector ~PSD!. Data were also taken with no flow in the gas cell to assess the stripping of the beam in the background gas and scattering from beam collimators. To avoid ratedependent gain changes and extraneous peaks due to pulse pile-up, the counting rate was kept below 1500 counts0s. The charge distributions were measured until the statistical uncertainties of the number of counts in the peaks representing less than four-electron loss were better than 2%. 3. RESULTS Tables 1 through 3 list the cross sections for electron stripping of the incident beams in the various target gases. The statistical uncertainty is listed in the tables for each case. There is also an estimated error of about 20% for cross sections below 10218 cm 2 and 10% for cross sections above that, due to differences in assessing the background for each of the peaks in the measured beam charge distribution and in the effective length of the target cell. It is clear that the average number of electrons lost per encounter increases with increasing target atomic number. This effect is most pronounced for 19 MeV Ar 18 where the average charge Table 2. The measured cross sections per atom for electron stripping of 19 MeV0amu Ar 18 in various gases in units of 10218 cm 2 Target gas He N2 Ar Xe 1e 2e 3e 4e 5e 1.2 6 0.1 6.33 6 0.04 16.3 6 0.1 22.8 6 0.2 0.12 6 0.01 0.57 6 0.01 4.29 6 0.05 9.86 6 0.07 0.05 6 0.01 0.10 6 0.01 1.50 6 0.03 5.53 6 0.04 0.030 6 0.003 0.59 6 0.02 3.12 6 0.03 0.02 6 0.01 0.27 6 0.02 1.82 6 0.02 change per encounter rises from 1.16 for He to 2.01 for Xe. The weighted cross sections shown in Table 3 are the cross sections weighted by the number of electrons lost in the encounter. Figures 1 and 2 show the cross section versus number of electrons lost in the various gases by 10.2 MeV0 amu Ar 16 and 19 MeV0amu Ar 18, respectively. The weighted cross sections for all of the beams we have used so far are summarized in Figure 3 as a function of target atomic number. The cross sections increase with increasing target Z and decrease with increasing beam energy in a broad energy and beam species range. The cross sections, labeled “Born” and “Classical” in Table 3, are the results of calculations described by Kaganovich et al. ~2001!. The Born approximation, which results in overestimates of the cross sections, should be valid for Z T e 2 ,, \V, where Z T is the target atomic number and V is the velocity of the beam ion relative to the target atom. The classical trajectory calculations do not account for tunneling transitions allowed by quantum mechanics. Neither approach is expected to perform well across a wide spectrum of beams and targets. Aspects of one must be included in the other to address shortcomings in the underlying assumptions. In the calculations for argon, we used simplistic assumptions based on one-electron ion scaling and ionization Table 3. The total electron-loss-weighted cross sections per atom compared with the calculated cross sections in units of 10218 cm 2 Target gas Experiment Born Classical Experiment 10.2 MeV0amu Ar 16 Table 1. The measured cross sections per atom for electron stripping of 10.2 MeV0amu Ar 16 in various gases in units of 10218 cm 2 Target gas He N2 Ar Xe 1e 2.18 6 0.09 10.77 6 0.05 27.4 6 0.2 50.1 6 0.5 2e 3e 4e He N2 Ar Xe 4.23 30.7 106 358 30 MeV0amu He 11 5e 1.65 6 0.05 0.044 6 0.02 8.96 6 0.03 1.10 6 0.01 0.221 6 0.006 0.047 6 0.003 19.5 6 0.1 6.17 6 0.08 2.64 6 0.04 1.02 6 0.03 33.1 6 0.3 13.4 6 0.2 7.8 6 0.1 5.6 6 0.1 5.61 6 0.23 3.13 33.4 6 0.2 23.8 100.5 6 0.4 104 215 6 3 633 He N2 Ar Xe 0.49 6 0.07 1.92 6 0.10 7.3 6 0.4 23 6 1 0.30 2.4 9.0 47 0.69 4.1 11.5 36 Born Classical 19 MeV0amu Ar 18 1.59 6 0.14 1.00 8.04 6 0.09 7.73 34.4 6 0.4 36.4 89.4 6 0.8 234 1.13 9.55 40.3 157 38 MeV0amu N 16 0.06 6 0.01 0.34 6 0.04 1.64 6 0.03 6.29 6 0.04 0.044 0.34 1.58 10.30 0.046 0.36 1.58 6.50 Multiple electron stripping of heavy ion beams 553 Fig. 1. Cross section per atom versus number of electrons lost in He, N2 , Ar, and Xe by a 10.2 MeV0amu Ar 16 beam. Fig. 3. Weighted cross section per atom versus atomic number of the target gas for the various beams used. Fig. 2. Cross section per atom versus number of electrons lost in He, N2 , Ar, and Xe by a 19 MeV0amu Ar 18 beam. potential for the electron distribution function. Therefore, as expected, agreement with experiment is not as good for Ar as it is for N16, where these functions are exact. More accurate calculations that account for the exact orbital electron distribution functions and ionization probabilities will be performed in the near future. The approximation used neglects the larger ionization potential for removal of multiple electrons. This leads to an overestimate in the calculation, especially for the case of Xe, where multiple electron events are more important. Table 3, which includes the total weighted cross sections for each gas and the three beam ion species used in this experiment, shows that both approximations give good estimates, except for the Xe case. The good agreement with experiment for the classical calculation suggests that tunneling transitions do not provide a major contribution to the cross section. This is not expected to be the case for ionized targets and0or low ionization potentials of the projectile ~see, e.g., He data in Table 3!, where classical calculations would strongly underestimate the cross section ~Kaganovich et al., 2002!. New experiments are needed to further check these theoretical predictions. 554 D. Mueller et al. 4. CONCLUSIONS REFERENCES We have measured the electron stripping cross sections for a variety of ion beam energies and species using different target gases. Together with the Xe data of Olson et al. ~2002!, this provides a broad range over which to test the theoretical calculations of the electron stripping of proposed heavy ion drivers for inertial confinement fusion by background gas in the target chamber. Bangerter, R.O. ~1996!. The US heavy ion fusion program. J. Fusion Eng. Des. 32 – 33, 27–32. Callahan, D.A. ~1996!. Chamber propagation physics for heavy ion fusion. J. Fusion Engineering and Design 32 – 33, 441– 452. Kaganovich, I.D., Startsev, E. & Davidson, R.C. ~2001!. Evaluation of ionization cross-sections in energetic ion– atom collisions. Proc. 2001 Particle Accelerator Conference, pp. 1649–1651 to be submitted to Phys. Rev. A., and preprint available at http:00accelconf.web.cern.ch0AccelConf0p010 PAPERS0TPAH314.PDF . Mueller, D., Grisham, L., Kaganovich, I., Watson, R.L., Horvat, V, Zaharakis, K.E. & Armel, M.S. ~2001!. Multiple electron stripping of 3.4 MeV0amu Kr 71 and Xe 111 in nitrogen. Phys. Plasmas 8, 1753–1756. Olson, R.E., Watson, R.L., Horvat, V. & Zaharakis, K.E. ~2002!. Projectile and target ionization in MeV m 21 collisions of Xe ions with N2 . J. Phys. B 35, 1893–1907. ACKNOWLEDGMENTS We gratefully acknowledge the staff at the Texas A&M Cyclotron Institute, especially Don May and George Kim, who proved invaluable in providing the desired ion beam species. This research was supported by U.S. DOE Contract No. DE-AC0276CH03073 and the Robert A. Welch Foundation.