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Energy loss and charge state measurements of heavy ions
passing a hydrogen plasma
J. Jacoby, S. Miyamoto, K. Weyrich, E. Boggasch, K.-G. Dietrich, B. Heimrich, D.
H. H. Hoffmann, W. Laux, A. Tauschwitz, and H. Wahl
Citation: AIP Conference Proceedings 274, 448 (1993);
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Published by the American Institute of Physics
Energy loss and charge state measurements of heavy ions
passing a hydrogen plasma
J. Jacoby, S. Miyamoto*, K. Weyrich, E. Boggasch, K.-G. Dietrich,
B. Heimrich, D.H.H. Hoffmann, W. Laux, A. Tauschwitz, It. W a h l
G S I - D a r m s t a d t , Postfach 110552, D-6100 D a r m s t a d t , G e r m a n y
* permanent address: Institut of Laser Engineering Osaka, Japan
The accelerator facilities at GSI, consisting of a high current radio frequency
quadrupole accelerator (MAXILAC), a rf-linac (UNILAC), a heavy ion synchrotron
(SIS) and an experimental storage ring (ESR) provide unique opportunities to study
beam target interaction phenomena over a wide range of beam energy. This range extends from 45 KeV/u up to 2 GeV/u. Thus if-accelerator and beam-target interaction
physics issues for heavy-ion-beam driven inertial confinement fusion can be studied.
Recent results of stopping power experiments at UNILAC verified an enhanced energy
loss of heavy ions in a plasma of a factor three at beam energies of 1 - 6 MeV/u. Even
bigger differences in stopping power and charge state between ionized and cold matter arc expected for beam velocities of the same order of magnitude as the thermal
electron velocities in a plasma. A new energy loss experiment in this parameter range
has been started at MAXILAC. First results indicate a tremendous enhancement of
energy loss in a plasma at beam energies below 45 keV/u.
The consideration of heavy ion beams as drivers for inertial confinement fusion is a
rather recent development [1, 2, 3]. Today ion beam fusion is viewed as an alternative
route to energy production from thermonuclear fusion. At GSI-Darmstadt an experimental
program is carried out that makes use of the worldwide unique accelerator facilities to
address key issues of inertial confinement fusion.
The potential of heavy ion beams to generate dense plasmas and to heat matter to
extreme conditions of temperature and pressure is expressed by the deposited specific
energy per mass unit:
c =
where as is the energy loss of heavy ions in matter in units of g/:---~-r~,
N is the number of
beam ions and rr02 is the focal spot area of the beam on target.
These parameters, relevant for beam matter heating, are investigated in different experimental programs. Advanced focusing techniques for charged particle beams are developed using a plasma lens [4] for a strong radial symmetric focusing. The intense beams
of the high current radio frequency quadrupole accelerator MAXILAC have been used to
generate for the first time plasmas by heavy ion beams and to study the hydrodynamic
behavior in ion beam produced plasmas [5].
9 1993 American Institute of Physics
J. Jacoby et al.
Energy Loss at 1 - 6 MeV/u
The measurement of energy loss and charge state enhancement of heavy ions in a
hydrogen plasma has been one of our major tasks in recent years. Experiments have been
carried out with the UNILAC accelerator at energies between 1 - 6 MeV/u for a large
variety of ion species from Ar to Uranium [6, 7].
Densities of 1.5. 101%m -s became accessible for experiments with a z-pinch discharge
plasma. After passing the plasma the ions are seperated according to charge state and
momentum in a magnetic field. The charge state and momentum seperated ions cause
light emission at different positions on a fast scintillator. This light is imaged on the
cathode of a streak camera, which allows high time resolution and space resolution. In
addition, for the measurement of ion energy loss in the z-pinch a time-of-flight system is
" Hpta,s~=
I ~
Eo = 291 MeV
.,~ 6O
/he dl. [1019cm-2]
Figure 1: Energy loss of Pb-ions in a fully ionized hydrogen plasma.
A typical result for the energy loss measurement is shown in Figure 1. A Pb beam
of 291 MeV was injected into the z-pinch plasma. With increasing density during the
pinch compression phase the measured energy loss increases likewise. The solid line is
a theoretical prediction of the energy loss in ionized matter, taking into account the
increasing charge state from initially 30+ to a final charge state of 38+ in the dense
plasma phase [8]. The prediction for the corresponding energy loss in cold hydrogen gas is
given by the dashed line [9]. This experimental result demonstrates beyond experimental
uncertainties a higher energy loss in a plasma compared to cold matter. We find an
enhancement of about a factor three.
Energy Loss at 45 keV/u
As soon as beam ions and plasma electrons achieve velocities of the same order of
magnitude even bigger differences in energy loss and charge state between a plasma and
cold matter are predicted [10]. For feasible plasma temperatures of about 5 eV this
E n e r g y Loss a n d C h a r g e S t a t e M e a s u r e m e n t s
condition requires beam velocities offl = 0.005, respectively ion energies of 10 keV/u. An
energy loss experiment in this parameter range allows to test energy loss models at the
maximum difference between hot and cold matter. In addition, due to the enhanced ion
charge state of more than twice the effective charge in cold matter a fully ionized plasma
provides a new effective ion stripper at low beam energies.
To investigate the energy loss at low beam energies a new experiment is on the way
at the RFQ-accelerator MAXILAC. This RFQ accelerates high currents (several mA) of
singly charged heavy ions to an energy of 45 keV/u. Important features of the setup are
the linear hydrogen plasma, the differential pumping, and an electrostatic deflector to
seperate the produced charge states. At initial hydrogen gas pressures of 0.5 to 2 mbar
electron densities of up to 1017cm -3 are produced by a small discharge.
Figure 2: Energy loss of Kr-ions in a hydrogen plasma (upper Figure) and the corresponding electron density in the plasma (lower Figure).
J. Jacoby et al. 451
First results of the energy loss measured by a time-of-flight system and the corresponding electron density obtained from spectroscopy of the Balmer emission are shown
in Figure 2. Due to the microstructure of the MAXILAC beam, every 75 ns a time-of-flight
signal of the ions is obtained. The intensity of the time-of-flight signal shows periodical
intensity variations following the discharge current of the pulsed power generator. This is
caused by the focusing and defocusing of the ions, due to the well known lens effect of the
discharge current [4], which is likely to be the reason for the vanishing ion beam signals
at the begin of the discharge. Data for the energy loss in cold hydrogen are available from
reference [9] and can also be obtained from the time-of-flight signals before the discharge.
The comparison of energy loss and electron density with the stopping power in cold gas
indicates a difference between hot and cold matter of more than one order of magnitude.
These results are already very encouraging, also the data analysis is still in progress.
The experiments reported here were performed in an international collaboration of a
number of institutes, including also MPQ-Garching, IPN-Orsay, GREMI-Orleans, LPGPOrsay, FHG-ILT-Aachen, Univ. Giessen, Univ. Erlangen, and I T E P Moscow.
The funding for the German institutes collaborating with GSI was provided by the
federal ministry for research and technology (BMFT).
[1] D. Keefe: Ann. Rev. Nucl. Part. Sci. 32, 391 (1982)
[2] R. Bock et al.: Nucl. Sci. Appl. 2, 97 (1984)
[3] C. Deutsch: Ann. Phys. 11, 1 Paris (1986)
[4] E. Boggasch et al.: Phys. Rev. Lett. 66, 1705 (1991);
Appl. Phys. Lett. 60, 2475 (1992)
[5] J. Jacoby et al.: Phys. Rev. Lett. 65, 2007 (1990)
[6] D.It.H. Hoffmann et al.: Z. Phys. A30, 339 (1988)
[7] K.-G. Dietrich et al.: Z. Phys. D16, 229 (1990);
K.-G. Dietrich: Report GSI-91-24 (1991)
[8] D.H.H. Hoffmann et al.: Phys. Rev. A42, 2313 (1990)
[9] L.C. Northchffe and R.F. Schilhng: Nucl. Data Tables A7 (1970)
[10] T. Peter, J. Meyer-ter-Vehn: Phys. Rev. A43, 1998 (1991)
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