PLASMA DIAGNOSTICS IN THE HEAVY ION BEAM-DENSE PLASMA INTERACTION EXPERIMENT AT ORSAY. C. FLEURIER, A. SANBA, D. HONG, J. MATHIAS and J.C.

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1 Colloque C7, supplément au n 12, Tome 49, décembre 1988 C7-141 PLASMA DIAGNOSTICS IN THE HEAVY ION BEAM-DENSE PLASMA INTERACTION EXPERIMENT AT ORSAY C. FLEURIER, A. SANBA, D. HONG, J. MATHIAS and J.C. PELLICER GREMI, CNRS, Université d'orléans, BP 6759, F Orléans Cedex 2, France Résumé - On décrit la source plasma utilisée lors d'expériences d'interaction entre un faisceau d'ions lourds (C 4 * ou S 7t ) à une énergie de 2 Mev par nucléon et un plasma. Le but était de mesurer la perte d'énergie des ions dans le plasma. Les méthodes de diagnostic du plasma par spectroscopie d'émission, par interférométrie laser et par absorption laser à deux longueurs d'ondes sont présentées ainsi que les résultats. Abstract - We describe the plasma source used for interaction experiments between heavy ion beams (C* ou S 7+ ) at energies of 2 Mev per nucleon and a plasma. The purpose of these experiments was to measure the ion energy loss in the plasma. The plasma diagnostics methods by emission spectroscopy, by laser interferometry and two-wavelength laser absorption are presented as well as the results. 1 - INTRODUCTION Ion energy losses in dense plasmas have recently focussed attention due to their connection with inertial confinement fusion (ICF). It has been shown that nearly 80 per cent of the interaction of the ion beam with a solid target takes place in a dense plasma as a result of the ablation and of the heating of the target surface. Therefore the understanding of the ion transport conditions and stopping in the plasma is needed in order to optimize the beam pulse structure and possibly the target structure. A first measurement of such an interaction between a beam of moderate intensity and a plasma of moderate electron density succeeded two years ago and is reported in /l/. Shortly, this experiment exhibited an enhanced energy loss of a (C 4 *, 24 Mev ion beam in the plasma compared to the loss in the equivalent cold matter, in good agreement with theory 111 which explains that the extra energy loss can be mainly ascribed to the effect of the free electrons. More recently these first results were confirmed and improved by other experiments with different ion-beams (S 7 *, Ca 13 *, Ge 18 *, U 3, + ) at energies of about 2 Mev per nucleon, and plasmas with densities in the cm" 3 range. These results will be soon reported elsewhere, together with full description of the experimental methods. In the different experiments, an hydrogen plasma at moderate electron densities was used. It was produced by means of an electrical discharge in a quartz vessel. The choice of this kind of plasma was guided by practical reasons of simplicity and duration (> 50 \>.s) which guarantees that many incident ion bunches interact with the plasma and also by the values of the plasma parameters that are large enough to insure a high ionization degree and easily measurable energy losses. Other kind of plasmas were discarded in this first phase of investigation because of their complexity or their short life time. Besides, because the energy losses are expected to be low, a high plasma quality is required concerning the stability, homogeneity and reproducibility in order to limit the uncertainty bars. The plasma must also be uncontaminated by heavy species like Si atoms which have several bound electrons in their external layer. The different experiments required ion velocities measurements as well as plasma parameters determination. Ion velocity measurements are reported elsewhere (Hoffman and Gardes). Here we will only report on the plasma parameters measurements. Article published online by EDP Sciences and available at

2 2 - PLASMA SOURCE The plasma tube is schematically represented in fig.1. It consists in a cylindrical quartz tube, 54mm 0.d. and 50mm i-d., 440 mm long. Electrodes are made out of stainless steel and are placed at each end of the tube which is filled with hydrogen at pressures ranging from 4 to 10 Torrs. In order to allow the ion beam crossing, each electrode has a 3 mm diameter hole in its center. In this configuration, the plasma tube was connected to the beam line through special ports allowing pressure adjustement between tube and line by means of powerful1 differential pumping systems. The plasma tube axis was carefully adjusted on the beam line axis. The electrical energy was delivered to the plasma through a set of six coaxial cables connected to the electrodes in a cylindrical geometry. The cables were coupled to a bank of 4 capacitors, 25 KV, 15 pf each, yielding a total energy of 6.75 KJ when operated at 15 KV. The total inductance of the circuit was about 1.4 ph, and the half-period of the current was 29 ps. A maximum current of 60 KA was measured with a Rogowski coil while the voltage determined with fast high voltage probes was of the order of 200 V. The plasma ignition was controlled by means of an ignitron or a spark-gap. A preliminar study showed that the maximum ionization of the hydrogen plasma occured 15 to 20 ps after plasma ignition, consequently the discharge parameters were optimized in order that the current maximum occur during this period of time. In doing so,the plasma was kept very clean all along the time discharge while it showed to be strongly contaminated by wall materials when the current maximum occured after the maximum hydrogen ionization. This maximum ionization time is determined by the initial pressure of the gas, by the tube diameter and by the current derivative, which represents the speed of energy transfer to the tube. These quantities set the dynamical and thermodynamical behaviour of the plasma by ruling the competition between the ohmic heating of the plasma, the energy losses by radiation and thermal conductivity and the pinch effect resulting from the inward magnetic force (cross product of the current with the self consistent azimuthal magnetic field) acting on the plasma. This pinch effect while being strongly inhibited by the thermal pressure of the plasma, in our particular conditions, confined slightly the plasma during the first half-period of the current, preventing direct contacts of the plasma with the walls and improving further the plasma purity. It is very common in the operation of such discharges to use a stabilizing axial magnetic field, generated by a solenoid coil surroundind the plasma. Plasma instabilities of different kind are systematically met in Z-discharges, after the plasma compression. In our case only m = 1 helical instabilities seem to occur as seen on instant photographies. Be calculated that an axial magnetic field of the order of 4000 Gauss was enough to stop their growing. In fact, experiments made with or without this stabilizing field did not show any differences on the measured plasma parameters as well as on the ion- beam energy losses, this led to the conclusion that the observed instabilities were not strong anyway. Finally, the problem of the on-line plasma diagnostics was difficult to solve because the optical methods that were used needed measurements along the tube axis, which was incompatible with the ion beam propagation. Geometrical modifications of the line by means of magnets as in /3/ are possible but even so, optical measurements across a series of small orifices turned out to be uncertain. Thus the main plasma diagnostics were made off-line in the best optical and spectroscopical conditions as possible. In this case the beam-plasma coupling ports were replaced by optical windows. However, in order to insure the validity of our off-line measurements, the plasma was monitored with the light emitted transversally by the plasma. This light contains a large number of informations on the plasma state, stability and reproducibility. Furthermore one of the diagnostic method could be performed on-line by means of removable mirrors placed inside the beam line (fig. 5), we were working then in real conditions except that, in this case, the beam could not go into the plasma. However its effect on the plasma can certainly be neglected if we consider the relative energies of the ion bunches and of the plasma. 3 - EMISSION SPECTROSCOPY The ligth emission of atoms, ions in a plasma is strongly influenced by the plasma state, specially by the electron density and temperature. This influence is caused by the collisions between particles and by the action of electrical fields. As a consequence, a detailed study of the ligth spectrum yields, via thermodynamical and line broadening models, large informations about the plasma. Particularly, the spectral lines are of paramount importance, as their shape

3 Fig. 1 - Plasma tube and coupling port. Both sides are identical. A Zm spectrometer I 17 m spectrometer c 1200 I/mm qratinq Fig.2 - Optical set-up for the different diagnostic methods nm wavelength (InmWiv) Fig. 3 - Self absorbed Hot line profiles and effect of inhomogeneity.

4 Fig. 4 a-d - HP line profiles at different times during the discharge. Here the temperature is larger than 25000K and the self-absorption is week. remcwabk mirrors differential pumping argon ion laser photo-diodes Fig. 5 - Optical arrangement for the two wavelengths laser absorption method when performed on the ion-beam line.

5 can be easily related to the electron density and their intensities allow, most of the time, the temperature determination. However, one has to be very careful in doing the measurements that plasma inhomogeneities or light absorption do not perturb them.in our operating conditions in hydrogen, another difficulty arises because the spectral lines become so broad that they partially overlap and are difficult to separate from the underlying continuum. Nevertheless a few emission spectroscopy methods have been employed in our diagnostics. The spectroscopy measurements are performed by means of spectrometers, fig.2, equipped in their focal planes either with an optical multichannel analyser or with several optical fibers connected to photomultipliers. The electrical signals are then digitized and stored on magnetic supports. First, the Balmer Ha line turns out to be strongly self-absorbed when observed along the tube axis and, in its central part, radiates like a black body. Thus a simple absolute intensity measurement give the temperature by means of the Planck law : I. (A) = C1 As ( exp (C21T) - 1) C1 = lo-' cgs, c2 = cgs, A in cm and T in K. Note that this measurement is a good check of the plasma homogeneity as shown in fig. 3, where the Ha profile is exhibited. The flat part at the top of the profile A, taken during the dense phase of the plasma, corresponds to the black body radiation which is constant over this short spectral range and the small reversal in the line-center corresponds just to a very limited inhomogeneity caused by the 3 mm apertures in the electrodes in which there is a slight plasma flow during the shot. If there were large inhomogeneities along the plasma axis, we would have obtained profiles like B or C in fig. 3, these two curves having been recorded in the late time of the discharge which is of no interest in our experiments. Secondly, the Balmer Ha line is considered to be a standard for electron density determination. Its line-width is very sensitive to the density and a semi-empirical formula relating the two quantities was derived from Vidal et al. tables /4/, with AA in A. The uncertainty of the measurement, when correctly done, is of the order of 10%. Because this line is easily self-absorbed, it could be used only for high temperature discharges (T > 25000K) or at the end of high pressure discharges when the temperature was low, that is, in both cases, when the absorption coefficient is small enough to allow the measurement. Figures 4 a-d show several H, profiles observed axially at different times during the discharge. In the same conditions, the temperature was deduced from the ratio of the Hp line intensity to the intensity of the adjacent continuum, integrated over a spectral range of, for example, 100 A as in Griem 151. In this method, the continuum radiation must be strong enough to allow the neglect of the superimposed line-wing intensity of the same spectral region. These diagnostic methods based on line profile and line intensity measurements were performed on our first plasma discharge, results are reported in /I/. Finally, emission spectroscopy was also used to check the purity of the hydrogen plasma by looking at the position of the most important spectral lines of atoms or ions of different elements that could be expected in the discharge (0, Si, Fe, Cu). Only traces of oxygen ion lines were observed in the late time of the discharge which, thus, could be asserted to be free of impurity.

6 4 - TWO WAVELENGTHS LASER ABSORPTION Ligth travelling across an absorbing medium is transmitted with the differential law : where a is the absorption coefficient at the wavelength 1. In an homogeneous medium having a length L, it comes then : where 10 is the incident intensity of the ligth. The coefficient a depends on the medium, and in a plasma it depends on whether the incident wavelength corresponds to a spectral line of one of the atomic species present in the plasma or just falls in the continuum. In any cases, it depends on the electron density and on the temperature but with different laws. We used a method originally developed by Billman and Stallcop /6/ which consists in the simultaneous absorption measurement at two different wavelengths. Then, by a judicious choice of the two wave-lengths, it is possible to determine the electron density and the temperature from the two absorption coefficients. This measurement was made possible in the hydrogen plasma by use of an Argon ion laser operated in a multi-line mode. The first line at 488 nm is mainly absorbed in the plasma by the n = 2 up to n = 4 transition of the hydrogen atom. which transition corresponds to the Hp line. A second laser line at nm is mainly absorbed by photoionization and by inverse bremstrahlung, which corresponds to an absorption in the continuum. Because this laser is continuous, the absorption measurements could be performed on the whole duration of the discharge, yielding the temporal evolution of the temperature and the density of the plasma. The absorption coefficient in the HB line is given by : where A = 488 cm, ro = , feu is the absorption oscillator strength between levels 8 = 2 and u = 4, n# is the t = 2 level density in ~ m-~, P (AA) is the emission line profile in cm-l, and the last factor represent the induced emission where nu, nt are the populations of levels u and t with statistical weights gu and ge. With the assumption of local thermodynamical equilibrium, which is largely justified in our conditions, and with the use of the Saha law, we get finally : where S(a) is the reduced Stark profile of the line (see /4/). Te and neare the plasma temperature and electron density respectively. For the continuum absorption, we considered first the electronion free-free absorption (inverse bremstralhung) with the formula (from /7/) : and F(ne, T) = A3 (1 - exp (hc/h kt) nt/~'/~

7 488 nm 516.5nm 15 KV 10 Torrs HZ Tim c after plasma gnition (,us) Fig. 6 - Typical absorption curves for the two wavelengths of the ion Argon laser. I I I I I I I 1' I I m 400 Timc after plasma ignition (ps) Fig. 7 - Fringes obtained during a shot at 15 KV and 9 torrs. The above curve represents the time expansion (x 25) of the period 25 ps-45 ps. Note that the density variation frequency is close to the limiting value of 10 MHz, consequently the fringes have small amplitudes. Fig. 7a - Electron density variation during a shot from absorption and interferometry measurements (15KV. 9torrs). Fig. 7b - Temperature variation during a shot from absorption measurements (15KV, 9torr)-

8 where g is the Gaunt factor /8/ and secondly the electron-atom free-free absorption the expression of which can be found in /7/. This term can be neglected at temperatures above K. The photoionization processes were also considered by the relation from /7/ : E, is the ionization energy of hydrogen. The other absorption mechanisms.can be neglected in our conditions. The calculation from typical absorption curves as shown in fig. 6 must take the plasma length into account and consider that at each wavelength there exists a combined action of line and continuum absorption, thus the following system of equations must be solved : The above equations and (6) (7) and (8) show that there are two unknown quantities, namely T and ne, in this non-linear system and two datas in the left side of the equations. The system was solved by an iteration procedure. In this manner density and temperature curves as shown in fig.7 a-b were obtained. The accuracy of the method is limited by the weakness of the continuum absorption at nu, specially at times over 100 ps after plasma ignition, when the absolute errors affect strongly the measurements. We found also that at times t > 100 ps an extra absorption could happen probably because of metal atoms, vaporized from the electrodes and localized around the 3 mm apertures of the electrodes. Great care must also be taken in the alignment of the laser beam, in its transmission across the different small size apertures and in the ligth collection, (fig. 51, that must consider possible deviations of the beam as a consequence of density gradients in the plasma. The ligth is converted in an electrical signal by a fast diode and analyzed by a fast digitizing oscilloscope. From the absorption curves, when density and temperature are known, it is also possible to determine the neutral hydrogen density by means of the equation (5) and the Saha equation.the accuracy is of the order of f 50% due to the large energy gap between the ground state and the n = 2 state appearing in eqn LASER INTERFEROMETRY If a part of the ligth of an He-Ne laser beam is reflected in the laser cavity by an external mirror, the intensity of the laser will be modified depending on the phase and on the intensity of the reflected beam. In this manner if the optical length is modified along the path of the reflected beam, the modifications can be detected from the laser intensity fluctuations. Because the intensity depends on the respective phases of the direct and reflected beams, this system is called interferometer. With the optical set-up shown in fig. 8, we have been able to measure the modifications of the optical length caused by the plasma during a shot. Here, the optical length depends only on the refraction index of the plasma which can be shown to be fully determined by the electron density in our operating conditions. When the density varies, temparal fringes are produced like in fig. 9, and one fringe corresponds to a density variation of : loi3 ne = cm-3 (10) L A with A = lo-' cm and L = 30 cm.

9 From curves such as in fig. 9, the electron density can be directly calculated by counting the number of fringes. Difficulties with this method arise first from the origin determination and secondly from the localization of the inversion points, where the density variation changes its sign. These two difficuties can be removed if one uses simultaneously the total light signal of the discharge which is roughly a function of the square of the density. In addition there exists a minimum detection time which is imposed by the length of the interferometer and the propagation time of the light. Typically this time is of the order of 10 ns, which means that only fluctuations with time variations well above 10 ns can be detected. This consideration leads to a limiting frequency of about 10 MHz for the application of the method. When this limit is approached the fringe amplitudes decrease rapidly, (fig. 9). Hore elaborated methods of laser interferometry allow to go beyond this limiting frequency but they are also more difficult to settle. Density curves like in fig. 7a (curve B) are obtained. As seen in this figure, a good agreement is observed with the density obtained from absorption measurements. 6 - CONCLUSION The present review of the different plasma diagnostic methods and of the corresponding results shows that the plasma is well diagnosed with an accuracy large enough to allow a good theoretical interpretation of the measured energy losses of the ion beam in the plasma. The plasma stability, homogeneity and reproducibility have been checked by these methods and turn out to be excellent, and are in full agreement of the initial requirements concerning the plasma in this interaction experiment. REFERENCES /1/ C. Fleurier, A. Sanba, J. Hathias and J.C. Pellicer, Proc. XVIIIth ICPIG, 174 (1987) Swansea, Wales /2/ C. Deutsch Ann. Phys. Fr. 11, (1986) 1, 111. /3/ D.H.H. Hoffman, K. Weyrich, H. Wahl and J. Jacoby 2. Phys. D, to be published (1988) /4/ C.R. Vidal, J. Cooper and E.W. Smith Astrophys. J: Suppl. 214, 25, (1973) 37, 116. /5/ H.R. Griem in Plasma Spectroscopy (McGraw Hill, New York)(1964) /6/ K.W. Billman and J.R. Stallcop Appl. Phys. Lett, 22, (1973), 565 /7/ F. Cabannes and J. Chapelle in Reactions under plasma conditions, M. Venugopalan ed. (John Wiley) (1971) 367. /8/ W. Neumann. in Progress in Plasma and Gas Electronics, Vol 1 R. Rompe, M. Steenbeck ed. (Akademie verlag, Berlin) (1975). 1 laser photo - cavity n- diode M2 v osziuecope optics --- fiber plasma Fig. 8 - Optical set-up for interferometry measurements. M!?