The concepts of communicative space, media sphere and public sphere are sometimes used like synonyms one of the other. However, according to us, they are three different concepts: public sphere and media sphere are two distinct spaces symbolic systems which, both, are anchored in communicative spac

Bimbot, R

Deutsch, C

Fleurier, C

Gardes, D

Hong, D

Maynard, G

Rivet, M F

Servajean, A

Gardes, D.

Bimbot, R.

Rivet, M.F.

Servajean, A.

Fleurier, C.

Hong, D.

Maynard, G.

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Stopping of energetic sulfur and bromine ions in dense hydrogen plasma

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Particle Accelerators, 1992, Vols. 37-38, pp. 361-370Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomSTOPPING OF ENERGETIC SULFUR ANDBROMINE IONS IN DENSE HYDROGEN PLASMAD. GARDES, R. BIMBOT, M. F. RIVET, A. SERVAJEAN(lPN, CNRS: Groupe de Recherche 918, Orsay, France)C. FLEURIER, D. HONG(GREMI, Orleans, France)C. DEUTSCH and G. MAYNARD(LPGP, Orsay, France)(Received 3 December 1990)The stopping power of sulfur and bromine ions traversing a fully ionized hydrogen plasma has beendetermined by coupling a plasma target to the heavy ion beam of the Orsay Tandem accelerator. Measuredenergy losses are well reproduced by the Standard Stopping Model and they clearly demonstrated theenhanced stopping power of a plasma (free electrons) relative to cold matter (bound electrons). Thedetermination of mean effective charge state during the stopping process is discussed in relation totheoretical predictions.INTRODUCTIONBeam-plasma interaction experiments allow one to investigate a new domain of theion-matter interactions that is of fundamental interest for atomic physics and plasmaphysics. Beyond the interest of fundamental investigations, the knowledge of theenergy deposition of intense beams in ionized matter is essential with respect to heavyion inertial confinement fusion (ICF) studies. In both cases of direct or indirect drivefor ICF scenario, the first step of the energy deposition on the ICF pellet correspondsto a transfer of the kinetic energy of the incident beam in collective modes, generatingcompression and heating of the pellet surface, and the stopping process takes placein a corona plasma resulting from the rapid ablation of the target surface. Preciseknowledge of the energy deposition process is of prime importance in order todetermine the thickness of the target tamper.There has been many theoretical approaches of this problem, but only a fewexperimental data were available to confirm these theories. The experimental pro-gram described here presents a complete set of stopping power measurementsobtained during the last four years in a joint collaboration between French andGerman laboratories.361362 D. GARDES et aleRelativistic---------------classical20 25log ne{cm-3)15~:. gas puff~i~'7linear ~~...discharge~ /// // // Collective/ Degenerated110510I--eno:::.::=o-FIG URE 1 Temperature-density diagram for the plasmas in this study.The basic idea was to couple an external plasma target to an accelerator beamline and let the heavy ions flow through the plasma. This simulation of the interactionbetween an intense ion beam and the ablation plasma is only valid because of theso-called reduction principle l which stipulates that even with intense beams, theaverage ion-ion distance within the beam remains two order of magnitude largerthan the electron Debye screening length. This effect suppresses largely collectiveeffects in the entrance channel, and allows one to treat the interaction as a simpleion-electron or ion-ion collision.Figure 1 shows the characteristics of the plasma target that have been involved inthese experiments.In this temperature~density diagram, are reported experimental plasmas comparedto the ablation-compression plasma related to the ICF process. All these plasmasare supposed to be relevant of typical classical, non-degenerated plasmas.This paper is mainly devoted to interactions with a hydrogen plasma produced inSTOPPING OF SULFUR AND BROMIDE 363linear discharge, which have been extensively studied during the first step of thisprogram. 2- 42 THEORETICAL BACKGROUNDSwift heavy ions penetrating into a plasma column interact mainly with free electrons,inducing inelastic processes. The energy transfer is much more efficient than thecorresponding interaction with bound electrons. Two reasons explain such a beha-vior:• Free electron orbitals show the most efficient stopping due to their flexibility torespond to the incoming electrostatic field.• The effective charge of the incident ion is rapidly increased up to the equilibriumcharge and recombination mechanisms are strongly inhibited.The framework of the stopping standard model (SSM) consists of a generalizationof the well-known Bethe-Bohr-Bloch theory, extended to ion-plasma interaction.Based on a few assumptions5 this model predicts a stopping power enhanced byabout a factor of two relative to the same conditions of electron density in coldmatter.When the projectile velocity exceeds the thermal velocity of the free electrons inthe plasma (high-velocity approximation), the general expression of the stoppingpower takes the form-(dE/dx) = 4·(Z*e2)2nt./vl .L(~, Z*)where z* is the mean ionization state of the ion during the stopping process, nt isthe plasma density, and ~ is the ion velocity. The stopping number L(Vi , Z*) includesbound and free electron contributions L = Lg + L~ (atomic units):Lg = Log 2vl/Iwhere I is the mean ionisation potential energy for hydrogen, andL~ = Log2vl/wp ,where wp is the plasma frequency. As long as wp ~ I, the free electron contributionis dominant in the stopping process.Beyond the more efficient energy exchange due to free electrons, the enhancedprojectile effective charge amplifies this effect. Numerical simulation of the charge-state evolution during interaction with free electrons of a plasma predicts that thebalance between ionisation processes and bound-bound, radiative and dielectronicrecombinations leads to an intense stripping of the incident ion due to the relativelylow cross sections of the recombination processes. For the 1017 e- /cm3 plasma inthese experiments, we thus expect to observe a rapid increase of the effective chargeat the entrance of the plasma target; then the charge remains unchanged during thestopping process, except at the end of the trajectory when the velocity of the iondecreases rapidly.P.A.-N364 D. GARDES et al.12001000~~IbOSOOeC\:teCJ=> 600QJ~......".~IIIIIII'I'II:I:i,;iI)10000.1----------------------..,_.....--O~---_t_---_t_---__t_---__+_-...t---___+_--0.012001 10 100distance (ern)FIGURE 2 Calculated stopping power dE/dx for a Br ion in H plasma (3 eV line) of a cold H2 gas target(1 eV line).The addition of these two effects (efficiency of free electron stopping and enhancedionization of the projectile), results in a range-shortening of heavy ions in the plasmacompared to the same density of cold-gas bound electrons. Figure 2 presents thecalculated stopping power dE/dX for a bromine ion penetrating a cold hydrogen gastarget (1 eV line) or the corresponding plasma target (density 2 x 1017 e- /cm 3 ; 3 eVline).In the case of a plasma target, the Bragg curve exhibits a pronounced maximumat the end of the trajectory, allowing an intense and localized energy deposition. Sucha behavior is particularly favorable in the perspective of energy deposition in an ICFpellet, where we expect to deposit the projectile energy in a narrow range in orderto avoid preheating of the target core.STOPPING OF SULFUR AND BROMIDE3 BEAM-PLASMA INTERACTION EXPERIMENTS3653.1 Experimental Set-upThe basic feature of the experimental set-up is coupling of a hydrogen plasma columnwith the beam line of a heavy ion accelerator. Two similar devices have been usedsimultaneously at Orsay in France and at Darmstadt in Germany, in order toinvestigate beam-plasma interactions over a wide range of incident ions (from carbonup to uranium) and velocities.A specific effort has been made on the design of the following elements:• Plasma diagnostics.• Coupling ports between plasma target and the beam line.• Time of flight measurements and acquisition process.3.1.1 Plasma Target A steady flow of hydrogen gas (5-10 mbar) is establishedinside a quartz cylinder (5 cm diameter, 40 cm length). The complete ionization ofhydrogen gas is achieved by ohmic heating using a powerful electric discharge yieldinga total energy of 5 kJ when operated at 15 kV.The maximum ionization of the hydrogen plasma occurred 10 to 20 J.1S afterignition, depending on the intensity of the electrical discharge. The magnetic fieldproduced by the current flowing through the plasma tends to constrict the plasmacolumn (Z-pinch effect). This effect is partially inhibited in our case by the thermalpressure of the plasma. This magnetic confinement prevents direct contact with thewalls of the plasma tube and improves the stability and purity of the plasma duringabout 60 J.1S. In return, this magnetic field structure induces deflection forces on theion beam. As a result we observe a succession of focusing and defocusing configura-tions in the plasma tube which alter or on the contrary improve the transmission ofthe beam through the plasma column. These plasma lens effects have been discussedin previous papers2 and we are paying great attention to the focusing properties ofthe linear Z pinch in prospect of future applications; the properties of the Z-pinchcolumn are already investigated for achieving the final focusing of intense bunchesof the SIS-ESR beams. 63.1.2 Coupling Ports The typical plasma lifetime is approximately 100 J.1S. There-fore the plasma tube needs only to be open on the beam line during this short time.Special coupling ports located at both sides of the plasma tube were thereforedeveloped,9 namely two fast valves which can be retracted within 200 J.1S (totalaperture). This limits the hydrogen leakage during the measurement. During theignition of the plasma column there always remains a few centimeters of molecularhydrogen at both ends of the plasma tube. This "cold" hydrogen is of greatimportance for the exact estimation of the projectile charge state at the entrance ofthe plasma. Thus, a crucial improvement related to this shutter device is the reductionof cold gas layers at both extremities of the plasma target 7. This system allows oneto use more expensive gases such as deuterium, which have properties that stabilizethe discharge and improve the reproducibility from shot to shot.366....-..("')E 9u.........o,.- 7~~(I); 5't:Jc:oa-U 3CI)[jD. GARDES et al.15 KV 8 mbar H2::...'\\" .••. .1 ••...... r ....•.....................................6>5 CI)CI)a-::1..-4 ~CDc.ECI)3 ;E(I)as20::8 24 40 56 72Time after plasma ignition ( Jls )FIGURE 3 Time evolution of free electron density and plasma pressure.3.1.3 Plasma Diagnostic Plasma diagnostics were performed using three opticalmethods8 . The free-electron density and temperature were determined from:• Emission spectroscopy using Stark-broadening of the hydrogenic Hp line.• Simultaneous absorption measurement at two different laser wavelengths (Arlaser).• Laser interferometry. The optical length depends on the refraction index of theplasma which is fully determined by the electron density. (He-Ne laser).Figure 3 reports the measured evolutions of free electron density (solid line) andplasma temperature (dashed line) as a function of time. A good agreement is obtainedfor the density determination using the three methods. The accuracy on the densitymeasurement is on the order of 15%.The oscillatory behavior of these curves is due to the oscillating current in thedischarge circuit. At the end of the plasma shot a progressive contamination by siliconions pulled out from the quartz walls lowers the temperature and consequentlyinduces a rapid recombination of the free electrons. This particular problem has beensolved recently by replacing the quartz tube by an alumina one. In this latter casethe temperature remains constant during the complete measurement.3.1.4 Data Acquisition The energy losses of the heavy ions were measured with atime of flight (TOF) technique using each beam burst as an energy probe. Theaccelerator pulsation was fixed at 2.5 MHz, which corresponds to one beam burstevery 800 ns. During one plasma shot we were thus able to perform about 100energy-loss samplings.STOPPING OF SULFUR AND BROMIDE 36714 _---,..---,....---...------.--__----r--~-__""T--~-___,Bromine 0.95 MeV/uDeuterium plasma Bmbar, 15 ~V12 t---t---t----+-'"'IIIIIIIIIlr--1--4 1----+---...-----+---~i---_+_-__i--_t_-___i-- -___1__ 10 J----+---...-----+------l!:I"----t--____i---t----;----+-------1>!~ 8J------,jaa---...----&l~-~_\_-__+_-__i--_t_-____i--_+_-___t~"'"vc:::tIJ 61----+---...----+--~--__+_-__i_A::::::--_t_-___i--_+_-___t1101009030202""---I---"---......----l~-..&..-.....I1oo.o.._ .....--....I~-......_......10 40 50 60 70 80elapsed time since plasma ignition (p.s)FIGURE 4 Energy loss measurements for a Br beam traversing a deuterium plasma target.The plasma light emission was used to define the time zero of the acquisitionprocess. The first beam bunch arriving after this light signal triggered the acquisition.Five capacitive phase probes placed at regular intervals along the beam line providedthe timing signals. These signals were sent to a special time-to-digital-converter (TDCMultistops), which registered 255 stops following one start pulse. The time resolutionof this device was 1 ns (0.05% of the full TOF) and its contribution to the globalprecision of the velocity measurement was thus negligible.Several personal computers were devoted to the visualisation and memory storageof the data. Plasma parameters were registered simultaneously during each plasmashot.3.2. Energy Loss MeasurementsEnergy loss measurements obtained with a bromine beam traversing a deuteriumplasma target are presented in Figure 4.The measured energy losses reproduce the oscillations of the plasma density andthe error bars reflect the fluctuations between different plasma shots, demonstratingthe good reproducibility of the plasma parameters.The corresponding density curve indicates that in this particular measurement wehave explored plasmas with densities evolving from 2.1017 e - fcm 3 up to7.1017 e - fcm 3 • By coupling energy losses measurements to their correspondingdensities we are able to construct the curves, which are reported in Figure 5 fordifferent incident ions.All these have been obtained for incident ions covering a velocity range from 1 to368 D. GARDES et al.Br 0.9 Mev/uma S 1 Mev/uma10 5/ /8 f 4 II I~6 ; 3*4 f 2 ,-4,.--.../>CD 2~........... 0 00 0 0.5 1.0 1.5 2.0 2.5 0 0.5 1.0 1.5 2.0 2.50 S 2 MeV/uma0 S 1.5 MeV/uma5 5>- /C) /'- 4 / 4 /CDc: / /W 3 J 3 ~;I2 j// 2 -ff+/0 00 0.5 1.0 1.5 2.0 2.5 0 0.5 1.0 1.5 2.0 2.5Linear density ( 1019 I crn2 )- cold gas- --plasmaFIGURE 5 Energy loss as a function of linear plasma density for several incident ions. Solid lines referto cold gas calculation; dashed lines correspond to SSM Theory.2 cm/ns. This velocity region corresponds to the largest electronic stopping powerfor the considered ions.Solid lines correspond to energy loss calculated values as a function of plasmadensity using the SSM theory. There is a fair agreement between experimental pointsand calculated values. This indicates that both estimations of projectile eff~ctivecharge and stopping number evaluation are correctly taken into account.3.3. Experimental Effective Charge State Determination.If we consider two ions with the same velocity penetrating in the same plasma target,the only factor which differentiate their stopping powers is the square of their effectivecharge.Measuring the energy loss allows charge-state determination, provided that weknow a reference ion charge evolution. The carbon is a good candidate to be thereference ion. Due to its low atomic number, this ion reaches rapidly an equilibrium60;:5as~(I)(I).2 4>-a~CD 3cw2STOPPING OF SULFUR AND BROMIDESIC~. IIIi l -BrlS ~~ I T369oo 5 10 15Density: n.l (1018 cm-2 )20FIGURE 6 Energy loss as a function of effective charge.charge very close to that of the fully ionised ion (Zeff = 5.95). A comparison with theeffective charge state evolution of the sulfur ion is presented in Figure 6. For eachvalue of plasma density (or distance in hydrogen plasma) we can calculate the ratioof the square of the effective charge for the two ions. The corresponding curve isreported as a solid line on Figure 6.The experimental determination of the ratio of the energy losses for the same ionstraversing the same plasma densities are displayed as black points on the figure. Theagreement is quite good and an extension of this mean charge determination is shownon the same diagram for the ion couple Br/S.This method only gives access to mean values of the effective charge states andthe natural development of this study will be a more refined analysis including adiscrete charge state determination based on a magnetic analysis of the beam burstafter interaction.4 CONCLUSION AND FUTUREThis first step in the experimental approach of beam-plasma interactions has givena strong support for the validity of the SSM theory applied to hot matter. A widerange of heavy-ion species interacting with fully ionized hydrogen and deuteriumplasmas was explored, and it was clearly demonstrated that the stopping power isenhanced, due to free electrons, in agreement with theoretical expectations.The future of this experimental program is now to extend investigations in severaldirections:370 D. GARDES et ale• A progressive increase of the plasma densities and temperatures by varyingplasma sources (Energetic Z pinch, gas puff generator, impact plasma from energeticand intense beam).• A charge analysis of projectile ions after plasma interaction and the correlationwith stopping power measurements.• A more refined analysis of stopping process by varying the ionization ratio ofthe plasma target (by changing the gas support or the temperature)REFERENCES1. C. Deutsch, Ann. Phys. (Paris) 11 (1986) 1-111.2. D. Gardes, R. Bimbot, S. Della Negra, M. Dumail, B. Kubica, A. Richard, M. F. Rivet, A. Servajean,C. Fleurier, A. Sanba, C. Deutsch, G. Maynard, D. H. H. Hoffmann, K. Weyrich and H. Wahl, Europhys.Lett. 8 (1988) 7 701-705.3. D. Gardes, R. Bimbot, M. Dumail, B. Kubica, A. Richard, M. F. Rivet, A. Servajean, C. Fleurier, A.Sanba, D. Hong, C. Deutsch, G. Maynard, D. H. H. Hoffmann, K. Weyrich, and K. G. Dietrich Rad.Effects and Defects in Solids 110 (1989) 49-53.4. D. H. H. Hoffmann, K. Weyrich, H. Wahl, R. Bimbot, D. Gardes, and C. Fleurier Phys. Rev. A 42,4(1990) 2313-2321.5. C. Deutsch, G. Maynard, R. Bimbot, D. Gardes, S. Della Negra, M. Dumail, B. Kubica, A. Richard,M. F. Rivet, A. Servajean, C. Fleurier, A. Sanba, D. H. H. Hoffmann, K. Weyrich, and H. Wahl, Nucl.Instrum. Meth. A 278 (1989) 38-43.6. E. Boggasch, these Proceedings; Proceedings of the Workshop on Plasma Focusing and Plasma Lenses,(Darmstadt, Germany; January 1990).7. D. Gardes, R. Bimbot, M. F. Rivet, A. Servajean, A. Fleurier, D. Hong, C. Deutsch, and G. Maynard,Laser and Particle Beams 8, 4 (1990) 575-581.8. C. Fleurier, A. Sanba, D. Hong, J. Mathias, J. C. Pellicer, J. Phys. C 7, 49 (1988) 141-149.9. C. Fleurier, 1. Mathias, B. Dumax, 1. Pellicer, A. Bonnet, D. Gardes, B. Kubica Nucl. Instrum. Meth.B61 (1991) 236-238.

Stopping of energetic sulfur and bromine ions in dense hydrogen plasma

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