Comparative analysis of large plasma focus experiments performed at IPF, Stuttgart, and IPJ,

Transcription

1 Home Search Collections Journals About Contact us My IOPscience Comparative analysis of large plasma focus experiments performed at IPF, Stuttgart, and IPJ, Świerk This content has been downloaded from IOPscience. Please scroll down to see the full text. 989 Nucl. Fusion ( View the table of contents for this issue, or go to the journal homepage for more Download details: This content was downloaded by: eskladnik IP Address: This content was downloaded on 29/02/206 at 0:3 Please note that terms and conditions apply.

2 COMPARATIVE ANALYSIS OF LARGE PLASMA FOCUS EXPERIMENTS PERFORMED AT IPF, STUTTGART, AND AT IPJ, SWIERK H. HEROLD*, A. JERZYKIEWICZ**, M. SADOWSKI**, H. SCHMIDT* * Institut fur Plasmaforschung (IPF), Universitat Stuttgart, Stuttgart, Federal Republic of Germany ** Soltan Institute for Nuclear Studies (Instytut Problemdw Jadrowych, IPJ), wierk near Warsaw, Poland ABSTRACT. The paper presents a comparison of the main characteristics and experimental results obtained in plasma focus (PF) experiments in the 500 kj facility in Stuttgart and the PF-360 kj device in Swierk. Parameters of various electrodes and insulators are given, and studies on the evolution of the discharges are summarized. Selected data on X-ray, ion and neutron emission are given. Also presented are recent experimental results a maximum neutron yield of up to 2.5 x 0" for 500 kj/80 kv runs with a new ceramic insulator in and an average neutron yield of.2 x 0" for operation at 7 kj/36 kv in PF-360. Particular attention is paid to the neutron scaling and the saturation effects observed at higher energy and current levels. Proposals are made for new experimental studies which can facilitate further progress in PF research.. INTRODUCTION Plasma focus (PF) facilities have been known for many years [-8] to be powerful sources emitting pulses of soft and hard X-rays, fast electron and ion beams as well as fusion reaction products. One of the most important characteristics of PF devices is the neutron emission from fusion reactions. Experimental and theoretical studies on the scaling of neutron yield Y n, as a function of input energy W o and/or of discharge current I, have been performed in many laboratories all over the world [4-7, 9]. Although a promising scaling (Y n ~ Wg, where a = 2-2.2) has been observed for energies ranging up to approximately 500 kj, it is found that the neutron yield from a PF facility saturates or even decreases when the charging voltage and thus the initial energy input as well as the discharge current are increased to values above certain limits. This saturation effect was investigated in two large PF facilities: the 500 kj device operated at the IPF, Stuttgart [0-2], and the 360 kj machine PF-360 at the IPJ, Swierk [3, 4]. In, the saturation effect was observed at an operation level above kj [5], while in PF-360 this effect was apparent above 200 kj [6]. Generally, saturation effects seem to occur at nearly half of the nominal bank energy. For a long time, when the operational conditions as well as the configuration and dimensions of the electrodes were changed, no fundamental improvement was achieved, neither in Stuttgart [7-9] nor in Swierk [20, 2], The application of additional pre-discharges in PF-360 did not improve the neutron emission, as was also found in other experiments [22], although some valuable information was gained [2]. However, the recent replacement of the glass insulator in by a ceramic insulator as used in PF-360 improved the situation considerably [23]. When the energy was increased from 280 kj (at 60 kv) to 500 kj (at 80 kv), only partial saturation of the neutron emission was observed and the neutron yield reached a value of 2.5 x 0". Further optimization and neutron scaling to higher energy levels (> 500 kj) are still open questions. Thus, it seems to be of common interest to compare the main characteristics and results for The ceramic insulators (57% A 2 O 3 ) were supplied by the IPJ $wierk, within an agreement on scientific co-operation and exchange with the IPF Stuttgart. NUCLEAR FUSION. Vol.29, No.8 (989) 255

3 HEROLD et al. EXPERIMENT ion-pinhole camera if proton spectrometer time-integrated X-ray pinhole camera; X-ray pinholes, microchannel plates, smear camera laser interferometer optical spectrometer 2 monochromators, photomultipliers 6 optical telescopes, *" ^slight-pipes + phodiodes HV cables FIG.. Schematic of the facility with the most important diagnostic equipment (not to scale). Not indicated is the standard equipment for measuring the initial voltage, total current, neutron yield (end-on and side-on) and time resolved neutron signals. and PF-360, which at the moment are probably the largest PF experiments in Europe. The main aim of this paper is to present such a comparative analysis of the experiments to show similarities and basic differences, which can be of importance for further progress in PF research. We compare only experimental results obtained independently in the two devices and do not include theoretical considerations. are installed in PF-360 to damp the current during the second half-period of the discharge [23a]. is operated mostly with a glass (Pyrex) insulator and a squirrel-cage cathode (24 bar), while PF-360 is operated with a tubular cathode and a ceramic insulator (containing about 57% of A 2 O 3 ). 2. EXPERIMENTAL ARRANGEMENTS PF-360 EXPERIMENT photomullipliers Schematic diagrams of the and PF-360 facilities are presented in Figs and 2. Both facilities are equipped with Mather-type coaxial electrodes made of vacuum annealed copper, but the dimensions of the electrodes (0 208/0 270 x 470 mm) are larger than those of the PF-360 electrodes (0 00/ 0 50 x mm). The vacuum chambers of both devices are made of stainless steel. Numerous diagnostic techniques have been applied in these experiments and the main results are discussed in the following sections. Important differences in the electrical scheme and construction are as follows: The capacitor bank has a two-stage Marx generator and thus its nominal voltage is twice that of PF-360. Non-linear damping resistors 280/JF kv HV cable* photomultiplier* odoy cogt FIG. 2. Schematic of the PF-360 facility with the X-ray and ion diagnostic equipment (not to scale). Not shown is the conventional equipment for V(t), I(t), YJt) and Y n measurements as well as the laser interferometric system. 256 NUCLEAR FUSION, Vol.29, No.8 (989)

4 LARGE PLASMA FOCUS EXPERIMENTS TABLE I. DIMENSIONS (in mm) OF ELECTRODES USED IN Set No. 0i Inner electrode Configuration Material Insulator 4s 0o Outer electrode Configuration 3 48 Tube, 020, hole Pyrex rods, Tube, 020, hole Pyrex rods, Tube, conic, end Tube, conic, end Pyrex Pyrex rods, rods, Tube, 060, hole Pyrex rods, Tube, 060, hole Ceramic rods, 020 TABLE II. DIMENSIONS (in mm) OF ELECTRODES USED IN PF-360 Set No. 0i Inner electrode <s Configuration Material Insulator 0o Outer electrode <b Configuration 00 Tube, 090, hole Alumina Tubular Tube, 090, hole Tube, 090, hole Tube, 090, hole Ceramic Ceramic Ceramic Tubular Tubular Tubular Tube, 060, hole Tube, 000, hole Ceramic Ceramic rods, 02 6 rods, 02 Unsuccessful tests because the mechanical strength was too low. TABLE III. SELECTED CHARACTERISTICS OF AND PF-360 Characteristic parameters Notation Units Pyrex Ceramic PF-360 Ceramic Operational voltage Electrical field Current derivative at breakdown V o E ins dl/dt kv kv-cm' TAs" Maximum current Insulator circumference Line current density Stored energy Optimum pressure Energy-to-gas ratio 'max *0ins j W o Po W o / Po V MA cm ka-cm" kj mbar J-mbar"'-cm" NUCLEAR FUSION, Vol.29, No.8 (989) 257

5 HEROLD et al. In, the insulator encloses the inner electrode; in PF-360, the outer diameter of the insulator and that of the inner electrode are equal. This difference is considered to be important [24]. Optimization studies in were carried out with various electrode configurations [2, 7, 23], as shown in Table I. The main optimization tests in PF-360 were also performed with various electrode sets [3, 6, 25], as shown in Table II. It should be of interest to compare data related to the geometrical dimensions and the experimental conditions for the two facilities. Table III gives approximate values of the electrical field strength along the insulator, estimated without taking into acount calculations of the actual potential distribution. Also given are values of the line current density on the anode circumference at the time of maximum current (j = I m ax/("''0i))- For a comparison of the energy-to-gas ratios, the volume, V = T(TI - rj 2 )-^, of the electrode gap is taken into account. Considering the geometry of the two experiments, it should be noted that, contrary to the dimensional scaling in small facilities [26, 27], the ratio W o /po^ (for notations see Tables I to III) is not constant but increases with the stored energy W o. For PF-360, this ratio is equal to kj/mbar^cm, while for it reaches.5-.9 U/mbar-cm. For a smaller device in Stuttgart (Nessi, U) the ratio was found to be only 0.2 kj/mbar-cm [27]. It should also be noted that the anode radius scaling based on a two-dimensional MHD model [28] is no longer valid. With the anode radius, Rj ~ (C o V o ) l/2 po l/4, for the two experiments in question, the ratio should be Rj ()/ Ri (PF-360) =.25, but it is actually 2.. Since the neutron yield in PF-360 is relatively high, in spite of the lower energy, it is possible that the yield in could be improved by using electrodes of a smaller diameter. On the other hand, it should be noted that in the optimization of PF facilities different parameters are involved, such as the geometry and structure of the inner and outer electrodes, the dimensions of these electrodes, the material and configuration of the insulator, and the initial pressure. These parameters are interrelated in a complicated way and no general relations have been found so far. Therefore, a final optimization can be achieved only by experimental tests. 3. TIME EVOLUTION OF PF DISCHARGES So far, the ignition phase of the two experiments has not been investigated sufficiently and only scarce results are available for a comparison. For PF-360 [29, 30] it is found that the breakdown voltage between the electrodes depends on the storage bank capacitance and rises with an increase in the capacitance value. This suggests that, at some energy level, radial breakdown between the electrodes above the top of the insulator may occur, causing current sheath inhomogeneity or even preventing current sheath formation. Reducing the breakdown voltage along the insulator by using a shorter insulator is not PF tips) FIG. 3. Typical current and voltage wave-forms taken at the collector plates of (for a shot at 4 mbar D 2, 70 kv, 380 kj, Y n =.2 x 0") and PF-360 (for a shot at 2 mbar D 2, 35 kv, 7 kj, Y n = 8.2 x 0 w ). 258 NUCLEAR FUSION, Vol.29, No.8 (989)

6 LARGE PLASMA FOCUS EXPERIMENTS [mm] 30 20' 0 30' to I I I I a i b PF-360 0' tins) tins) FIG. 4. Streak pictures of the plasma radial compression during good PF discharges, taken for at the z = 20 mm plane [23] and for PF-360 at the z = 7 mm plane [6]. was operated at 80 kv/500 Id, (a) with a Pyrex insulator, Y n = 5. X 0 0, and (b) with a ceramic insulator, Y n = 9.2 x W'. PF-360 was operated at 35 kv/7 kj, with a ceramic insulator: (c) the bright spots correspond to auxiliary laser time-markers, Y n = 8.2 x 0' ; (d) operation without a filter, Y n = 4.8 X 0 0. effective because a certain length is needed to prevent secondary breakdowns at a given charging voltage. On the basis of optical observations in the gap between the electrodes during the ignition and rundown phases, as well as analyses of tracks on the surfaces of the electrodes and insulators used in the experiments, it is concluded that discharges were initiated with good azimuthal symmetry. In the initial phase, no distinct differences between the Pyrex insulators and the ceramic insulators were observed. However, considering the evident differences in the final results, it seems necessary to make detailed measurements and optical observations during ignition and current sheath formation in both facilities. Preliminary results obtained in [3] suggest that the impurity content of the plasma or the amount of impurities deposited on the electrodes depend, besides other factors, on the insulator material and can have a major impact on the final plasma ionization and compression efficiency. Hence the neutron yield can be greatly influenced. Current and voltage wave-forms, which have been measured for a long time in both facilities (Fig. 3), have provided important information. An analysis of the energy dissipation in PF-360 showed that during discharges without neutron emission (i.e. near the upper energy limit), only a small fraction of the stored energy is transferred to the plasma [30]. The rundown phase of the discharges has been investigated to some extent in with miniature Rogowski coils on cathode rods and with magnetic probes placed near the anode and cathode surfaces [7, 9]. Also, preliminary measurements have been made in PF-360 with a Rogowski coil embracing the anode core at different z-planes [25, 30]. In, the thickness of the current sheath on the anode was found to be smaller than that on the cathode rods, amounting to 25 mm and 45 mm, respectively [7]. A complex structure of the current sheath in both experiments and the formation of current loops in have been observed. No special differences have been found so far, but a characteristic feature of the PF-360 discharges seems to be the appearance of distinct radial filamentation during the rundown and compression phases; this can also be observed with an X-ray pinhole camera directed at the electrode edges [6]. Experimental observations made in have not given sufficient evidence for the appearance of such radial filaments. The axial velocity of the luminous sheath has been measured in with photodiodes looking at NUCLEAR FUSION, Vol.29, No.8 (989) 259

7 HEROLD et al y s PF mbar 30 k V, 2(5 kj a relatively high value about.5 x 0 5 m-s' while for bad discharges the velocity amounts to only (6-8) x 0 4 m-s'. The radial velocity decreases slightly when the operational pressure is increased. In general, with higher radial velocity and smaller pinch diameter, a higher neutron yield is obtained in both devices [6] (see Fig. 5). With the streak pictures, it has been possible to observe a re-expansion followed by a second pinching and breakup of the plasma column. Also, the time interval between compression and breakup [6] as well..2.3 r [l0 7 cm/s] PF-36O 0 \ ( p : mbar 30 kv, 26kJ -50 ns ns 0 : -* r minh + 50 ns FIG. 5. Neutron yield versus radial compression velocity and versus minimum pinch radius, as observed in PF-360 operated at 2 mbar D 2, 30 kv, 26 kj [6]. different z-planes [7]; it increases considerably during the rundown phase, from 8 x 0 4 to.6 X 0 5 m-s". Analogous measurements in PF-360 are under preparation for the squirrel-cage cathode [25], but no comparative data are available yet. In PF-360, the average velocity of the current sheath has been estimated from the values of U/I max and (tp - ) (where t t is the ignition time and tp is the pinch time), and from Rogowski coil signals. Sheath velocities of x 0 5 m-s' have been obtained for shots with high neutron yield [30]. The compression phase has been investigated in detail in the two facilities. For this purpose, different diagnostic methods have been used, in particular high speed photography as well as laser interferometry and shadowgraphy. Figure 4 shows typical streak pictures of the plasma radial compression, taken through narrow slits perpendicular to the z-axis. For good PF shots (i.e. shots with high neutron yield) the radial compression velocity in the two experiments reaches +00 ns +200 ns +250 ns FIG. 6. Schlieren pictures showing the behaviour of the plasma column, for operated at 3 mbar D 2, 60 kv/280 kj [2], and for PF-360 operated at 0 mbar D 2, 35 kv/7 Id [6]. The times given refer to the moments of maximum compression (t = 0). The anode edge is on the left side of the frames. 260 NUCLEAR FUSION, Vol.29, No.8 (989)

8 LARGE PLASMA FOCUS EXPERIMENTS -4ns + ns PF-360 FIG. 7. Comparison of interferometric pictures taken in and PF-360 during the radial compression phase and during the development of MHD instabilities. was operated at 70 kv/380 kj [34] and PF-360 was run at 35 kv 7 Id [32]. -40 ns + 00 ns as its correlation with the neutron yield can be determined. It is found that the smaller this time interval the higher the neutron yield. More detailed information can be gained with interferometric methods. A comparison of Schlieren pictures for [2] and for PF-360 [6, 32, 33] is presented in Fig. 6. In both experiments, when maximum compression is achieved at approximately ixs after ignition, a quasi-stable pinch column is formed with a weakly marked instability (surface ripples). The pinch diameter, as shown in streak pictures, interferograms and shadowgrams, is approximately two times larger in than in PF-360 and is proportional to the value of the current. The MHD instabilities of the pinch grow until one or more distinct m = 0 instabilities pinch off the plasma column. Such strong instabilities develop after ~ 20- ns in and after ~ 50- ns in PF-360. It should be noted that there is some macroscopic similarity of the shadowgrams and interferograms for the two experiments, but in general the MHD instabilities are irreproducible. On the other hand, phenomena PF mbar 2 mbar 5 mbar 5 mbar 8 mbar FIG. 8. Soft X-ray pinhole pictures, showing the filamentary structure of the pinch column as observed at different D 2 filling pressures for (60 kv/280 kj [35]) and for PF-360 (35 kv/7 kj [36]). The anode edge is on the left side of the frames. In both cases, the pinhole diameter was 200 urn and 0 fim thick beryllium filters were used. NUCLEAR FUSION, Vol.29, N0.8 (989) 26

9 HEROLD et al. such as the formation of a quasi-stable pinch column, the development of MHD instabilities and the generation of 'bubbles' (see Fig. 6, 200 ns and 250 ns) are common features of PF discharges. This is confirmed by a comparison of the interferometric pictures taken during different phases of PF discharges in and PF-360, as shown in Fig. 7. In general, the electron density distribution can be determined on the basis of interferometric measurements, but exact evaluations of the electron density profiles inside the pinch column are sometimes questionable because of the complex character of interferometric pictures and the often observed lack of axial symmetry. It is found that the maximum electron density in the two experiments is in the range of 0 l9 cm' 3. During the lifetime of the pinch the density on axis is about 3 x 0 l8 cm' 3. Important information about the structure of PF discharges can be obtained from X-ray emission studies. Examples of time integrated X-ray pinhole pictures for the two experiments are presented in Fig. 8. As interesting phenomena, axial filaments extending over the whole pinch column have been observed for the first time in [5, 35] and in PF-360 [36, 37], Depending on the experimental conditions (pressure, current, etc.), the axial filamentation can be very distinct (see Fig. 8, 5 mbar) or it can be blurred (Fig. 8, 2 mbar). Soft X-ray pinhole pictures show the compressed plasma, which seems to contain several filaments, including mostly also a central filament. When the operational pressure is increased, the central filament becomes more distinct [5, 35]. Since the registered internal or peripheral filaments are rather sharp, it can be concluded that the motion of such plasma structures during their lifetime is restricted and the time integrated pictures are snap shots taken at peaks of the X-ray emission. A confirmation of this hypothesis has been obtained in both experiments with time resolved X-ray measurements performed with miniature scintillation detectors placed at the end-plane of the X-ray pinhole camera [35, 36]. On the basis of time resolved measurements performed with scintillators and gated microchannel plates [35] it was found that the filaments, and in particular the central filament, appear about ns before the maximum compression and exist for up to ns after it. In spite of numerous experimental observations [35, 36] and theoretical models [38, 39], the opinions regarding the role played by the filamentation in PF processes differ. The filamentary structure seems to have a weak effect on the properties of the pinch, particularly on the neutron yield [40]. On the other hand, theoretical considerations [39, 4] suggest that the internal filaments may be a source of high energy deuterons, which are important for the fusion processes. It should be noted that at lower pinch currents (< 800 ka) the structures of PF discharges as shown by X-ray pinhole pictures are different. In such cases, no continuous filaments are observed, but there are small brilliant regions called 'hot spots' [35, 42, 43]. Since the appearance of hot spots and/or filaments seems to be an interesting non-linear process in high current discharges, further detailed investigations should be performed to reveal the physical mechanisms of the phenomena in question. 4. ACCELERATION AND EMISSION OF IONS Studies on the generation of fast ions (E > 00 kev) in PF facilities are important for the understanding of neutron production mechanisms [40, 44, 45]. Therefore, the ion emission characteristics have been determined in and PF-360 [2, 36, 46] with different diagnostic techniques. To determine ion angular distributions, solid state nuclear track detectors fixed to supports placed at some distance from the focus region have been used. A rough energy analysis of the ions has been performed, utilizing PM355 and CR39 films covered with additional aluminium foil filters [40]. Examples of the angular distributions are shown in Fig. 9. The data for high energy ions (> 0.4 MeV) were taken at large angles in [2] and at smaller angles in PF-360. Therefore, a detailed comparison is impossible, but some features of the angular distributions can be easily observed. The ions are emitted predominantly in the forward direction (0 < a < 40 ). The anisotropic character of the ion emission can be explained by the influence of the azimuthal magnetic field [40]. Also, axial magnetic fields as well as a non-symmetric distribution of the ion microsources [44, 45] and/or inhomogeneity of local electromagnetic fields [46] may play a role. Very high energy ions ( MeV < E < 6 MeV) are emitted from small spots and lower energy ions from more extended regions, as found in. The occurrence of microsources was also confirmed by space resolved measurements performed with different ion pinhole cameras. The ring shaped ion pinhole pictures with numerous quasi-central ion beam spots [3, 44] demonstrated the unrecurring nature of 262 NUCLEAR FUSION, Vol.29, No.8 (989)

10 LARGE PLASMA FOCUS EXPERIMENTS PF-36O [deg] [deg] d 2 N dedn io rr b 3 "V \ i T \\ 3 ml -sat ur ai i onr n saturation- AT 8 mbar 30kV, 26 kj A 0* 60 cv i 280 k, i -ba :k 0 und.background. I I 0' E[MeV] E[MeV] FIG. 9. Angular and energy distributions of fast deuterons (dimension in (sr-mev)' ), measured end-on in (60 kv/280 kj [2, 47] and in PF-360 (30 kv/26 kj [46]). The energy spectra are corrected for atomic processes occurring on their flight path to the detector (except for the mbar data). the emission of deuteron beams. Usually, together with the numerous low energy ion beams some narrow (divergence < 8 ) bunches of high energy (> 00 kev) deuterons are emitted [36, 48]. For mass and energy analyses of the high energy ions emitted from PF discharges, a filter technique was used in and a Thomson parabola analyser was used in PF-360, where the measurements were restricted to below 0.5 MeV [46]. Some exemplary energy spectra measured end-on are presented in Fig. 9. In general, with an increase in the bank energy (and current) the ion energy spectrum drops more rapidly; for example, in the deuteron emission at 3 mbar decreases by more than two orders of magnitude in the range 0.5- MeV. This rapid drop may be connected with the slower pinch dynamics in a greater plasma volume and a predominance of different acceleration processes. From measurements in, it is found that the high energy deuteron spectra may vary from shot to shot and that the spectral distribution of the high energy deuterons (50 kev < E d < 00 kev) is correlated with the neutron yield [40, 47]. Time resolved ion measurements, performed with scintillation detectors placed behind a pinhole or a Thomson spectrometer, revealed a coarse modulation NUCLEAR FUSION, Vol.29, No.8 (989) 263

11 HEROLD et al. (with ns intervals) and a fine multi-spike structure (with FWHM < 4 ns) of the ion pulses in both experiments [36, 48]. By comparing time resolved signals of ions of different energy, it was found that the first deuteron pulse corresponds usually to the stable (quiet) phase of the pinch, while the other pulses are generated during the unstable phase. It was also observed that in the unstable phase mostly high energy deuterons (from 00 kev up to several MeV) are emitted. However, these very energetic deuterons escaping from the system do not contribute appreciably to the fusion yield. There is a certain correlation between hard X-radiation dose and neutron yield for both devices. 5. FUSION REACTION PRODUCTS Since the d-d fusion reactions have two branches, information on their kinetics and efficiency can be obtained from studies of reaction protons in the energy range of about 2-4 MeV [47], as well as from measurements of neutrons in the energy range of about MeV [34]. In fact, very extensive studies on the reaction protons have been performed in [5, 40], but only preliminary proton measurements have been carried out so far in PF-360. Therefore, a comparative analysis of the proton results is impossible and particular attention has to be paid to the neutron measurements. The highest neutron yields were obtained in with the electrode sets Nos 5 and 6 [23], and in PF-360 with the electrode set No. 2 [6, 2, 25]. The average values of the neutron yield as a function of the operational pressure are presented in Fig. 0. Investigations performed with the PF-360 electrode sets Nos 2 and 5 did not reveal any influence of the electrode shape on the neutron yield. It is well known that the scaling of the total neutron yield as a function of the condenser bank energy is not substantiated by physical models and it is more reasonable to study the scaling of the neutron yield with the total discharge current [26] or the pinch current [49]. Nevertheless, the scaling of neutron yield with energy has been used previously [7, 26, 27], because of its simplicity and because of the relation between the supplied energy and the current amplitude. Since the total discharge current can be measured very exactly, the current scaling is also used in many laboratories. A comparison of the energy and current scalings for and PF-360 is presented in Fig.. It can be seen that for both experiments the neutron yield scaling laws, Y n ~ W o 2 " 2 and Y n ~ ft 2 ; 33, are valid only up to values at which saturation effects appear. For, the neutron yields obtained with a ceramic insulator [23] are considerably higher than those obtained with a Pyrex glass insulator, but deviations from the scaling are again observed at higher energy and current levels. In the scaling experiments PF-360 xo kj / I /" *\ \ \ \ xlo kv AS 280 kjyfj n r\ pf_mbar] pfjnbar] FIG. 0. Neutron yield as a Junction of the initial deuterium pressure, as observed in for electrode set No. 6 with a ceramic insulator, and in PF-360 for electrode set No. 2. Only the experimental points with good statistics are shown and the observed tendency is indicated by broken lines. 264 NUCLEAR FUSION, Vol.29, No.8 (989)

12 LARGE PLASMA FOCUS EXPERIMENTS Yn s 2 s 2 id > 2 glass insu ator 0 ceramic irtsulatc>r V- / \ t PF ' ' E[ kj] Yn 2 id 5 2 id Yn' /f f if // // ifka] 5 IfkA] FIG.. Neutron yield as a function of stored energy (WQ) and as a function of the discharge current (I max is the maximum value, I c is the value corresponding to the maximum compression and I pp is the value found after the collapse). At higher energies and currents there are deflections from the scaling as well as saturation and degradation effects. the electrode length and/or the filling pressure were adjusted to the current rise time, i.e. the neutron yield was optimized. Unfortunately, no method is available yet for direct measurement of the pinch current during maximum compression. In high current experiments, local probe measurements are almost impossible and the Faraday rotation method is troublesome and gives space integrated results [50]. The computation of the pinch current from the measured angular distribution of protons [2, 47] is also troublesome and not very accurate. On the other hand, only a fraction of the total current (~ 70-90%) flows through the current sheath at pinch time. Therefore, it is not easy to obtain the scaling of neutron yield with pinch current, and use is made of other features of the current, for example NUCLEAR FUSION, Vol.29, No.8 (989) 265

13 HEROLD et al. PF-360 I ni Q f r- rays 'neutrons Musi - Pyrex insulator 3 mbar 60 kv, 280 kj Ceramic insulato 'i mbnr 60 kv, 280 kj 0 mbar 30 kv, 26 kj Y = 8.0 x Pyrex i n!>ul ator 6 mbnr 8O kv, kj Y =.0 x 0-2 mbnr 30 kv, 26 kj Y_ = 7.3 x 0 0 I a. u.l i n.u.l - Ceramic insulator 6 mbnr 80 kv, "JOO kj Y =.6 x O - 2 mbnr 35 kv, 7 kj Y n =. x 0 2- [a.u.l 0 3- I o.ul 0 I a.u.l - 5 mbar 35 kv, 7 kj Y n = 9.8 x Ceramic insulntor 6 mbar 80 kv, "5OO kj Y = 2.0 x 0 Y n I a. u.l FIG. 2. Comparison of time resolved neutron signals obtained from and PF-360. In, the measurements were performed at a distance of 4.5 m (Pyrex and/or ceramic insulator, kv/ U) and in PF-360 at a distance of 2.4 m (ceramic insulator, kv 26-7 U). the total current immediately after the collapse (I pp ) or the current corresponding to maximum compression (I c ). Such a scaling is also shown in Fig.. Above some level of the bank energy, the average neutron yield decreased to very low values, which could not be measured. This may be due to technological factors, such as the dimensions, shapes and materials of the electrodes, the insulator and the vacuum chamber. When these parameters were changed, it was possible to operate the devices with elevated neutron emission levels, but the characteristic saturation limit could only be shifted to higher bank energies. When predischarges were used in PF-360, the energy limit was shifted to a higher level, but the 266 NUCLEAR FUSION, Vol.29, N0.8 (989)

14 LARGE PLASMA FOCUS EXPERIMENTS neutron yield was not increased above 0 n neutrons per shot. 6. STUDIES OF TIME RESOLVED NEUTRON EMISSION To investigate time resolved neutron emission, extensive neutron measurements were performed in [2, 5, 23, 34] and in PF-360 [4, 6, 5]. Using scintillator-photomultiplier sets and appropriate collimator systems, it was found that the axial dimensions of the main neutron source in these experiments are in the range mm and the radial dimensions are in the range 0-20 mm. The dimensions of the neutron source were deduced directly from side-on neutron [34] and proton [47] pinhole measurements in and indirectly from interferometry and shadowgraphy in PF-360 [6], assuming that beam-target interaction occurs only in a dense plasma. Time resolved neutron signals usually exhibited two separate pulses, but sometimes three or even four pulses were registered [23, 5]. Figure 2 presents examples of neutron signals from and PF-360. The FWHM of the two main neutron pulses in both experiments is in the range ns, while the separation of the peaks is 20- ns in and 50- ns in PF-360. The total neutron emission in (at a filling pressure of 3-6 mbar D 2 ) usually lasts ns, while in PF-360 it is ns. In the PF-360 investigations with the electrode set No. 4, a very long emission has been observed (up to about /*s) [6, 5]. This suggests that the pinch column can exist for a longer time in PF-360 compared with. This is also consistent with the results of interferometric measurements discussed above. It should be noted that in, when the Pyrex insulator was used, the first neutron pulse gave only 5-20% of the total neutron yield, while the second pulse gave % [5, 23]. Sometimes the second pulse was shorter than the first one or multiple pulses appeared, but these cases were rare. When the ceramic insulator was used in, the contribution of the first neutron pulse (-80-85%) was predominant [23]; this was also the case in most of the PF-360 discharges [5]. This suggests that the use of a ceramic insulator changes the current sheath structure, which results in a more efficient acceleration of deuterons during the collapse and the stable pinch phases. However, this hypothesis has to be confirmed by more detailed studies, especially on the anisotropy of neutron pulses at higher energy and current levels. In general, the neutron emission from PF discharges exhibits an anisotropy which is usually characterized by the factor A = Y n end - n /Y^ide - n. Investigations in [2] and PF-360 [52] indicate that the anisotropy factor A for the first pulse is close to, which means that during this phase azimuthal as well as axial motions of fast deuterons (producing fusion neutrons) play a role. On the contrary, for the second neutron pulse the anisotropy factor A is usually higher (>.2) and increases with an increase in neutron yield. This suggests that the later phases of PF discharges in both experiments are dominated by axial motion of deuterons. 7. CONCLUSIONS From the above data and discussion, the following conclusions can be drawn: () Since a considerable increase in neutron yield has been achieved in by replacing the Pyrex insulator by a ceramic tube, the possibility of a further improvement through the use of other materials should be considered. It would also be of interest to apply other insulator/electrode configurations. (2) The two experiments are characterized by similar values of the initial electric field along the insulator and of the linear current density on the insulator circumference. Regarding the scaling, it would be of interest to perform tests in with electrodes and insulators of smaller diameters. An explanation for the dependence of the breakdown voltage on the storage bank capacitance and the dependence of the energy limit on the insulator length should be sought. New technological concepts should be developed to meet the contradictory demands resulting from these dependences. (3) Comparisons of the interferometric measurements as well as the neutron yields and the time resolved neutron signals suggest that the pinch lifetimes in PF-360 and in are comparable. For T(W) scaling (T being the lifetime of the pinch column, i.e. the time interval between maximum compression and breakup, and W being the input energy), which is of primary importance for future PF devices, more extensive and detailed investigations are required. (4) In both experiments, filamentation of the pinch column was observed and some theoretical models NUCLEAR FUSION, Vol.29, No.8 (989) 267

15 HEROLD et al. were developed. Since the role of the filaments in the production of high energy deuterons is not yet known, more detailed experimental studies are required to explain the physical nature and role of the filamentation as well as to verify the theoretical models. (5) Important information on the nature and dynamics of plasma processes in PF discharges can possibly be gained from studies of high energy electron beams emitted in the up-stream direction. Preliminary electron measurements have been performed only in [5, 53]. Of special importance would be correlation studies of the X-ray, electron and ion emissions as well as space resolved measurements of the electron beams from high current PF discharges. Recent measurements in a small PF device showed that the ion and neutron emission in the first phase is associated with a high energy electron beam [54]. ACKNOWLEDGEMENTS The paper is the outcome of scientific co-operation between the IPF Stuttgart and the IPJ Swierk. The authors wish to express their gratitude to H.J. Kaeppeler for valuable discussions. They also wish to thank K. Kociecka, J. Kucinski, M. Shakhatre and the other members of the experimental teams for their help in collecting the experimental data. One of the authors (M.S.) would like to express his gratitude to the Humboldt Stiftung for assistance granted during his stay at the IPF Stuttgart. REFERENCES [] FILIPPOV, N.V., FILIPPOVA, T.J., VINOGRADOV, V.P., in Plasma Physics and Controlled Nuclear Fusion Research (Proc. st Int. Conf. Salzburg, 96), Nucl. Fusion, 962 Suppl., Part 2, IAEA, Vienna (962) 577. [2] MATHER, J.W., Phys. Fluids 8 (965) 366. [3] PEACOCK, N.J., WILCOCK, P.D., SPEER, R.J., MORGAN, P.D., in Plasma Physics and Controlled Nuclear Fusion Research 968 (Proc. 3rd Int. Conf. Novosibirsk, 968), Vol. 2, IAEA, Vienna (969) 5. [4] DECKER, G., HEROLD, H., KAEPPELER, H.J., et al., in Plasma Physics and Controlled Nuclear Fusion Research 978 (Proc. 7th Int. Conf. Innsbruck, 978), Vol. 2, IAEA, Vienna (979) 35. [5] NARDI, V., BOSTICK, W.H., FEUGEAS, J., PRIOR, W., CORTESE, C, ibid., p. 43. [6] BERNARD, A., Atomkernenergie 32 (978) 73. [7] SCHMIDT, H., Atomkernenergie-Kerntechnik 36 (980) 6. [8] JANKOWICZ, Z., JERZYKIEWICZ, A., NOWIKOWSKI, J., et al., in Controlled Fusion and Plasma Physics (Proc. 9th Eur. Conf. Oxford, 979), Culham Lab., UKAEA, Abingdon, Oxfordshire (979) 07. [9] RAGER, J.P., in Controlled Fusion and Plasma Physics (Proc. 0th Eur. Conf. Moscow, 98), Vol. 2, European Physical Society (982) 7. [0] HEROLD, H., BERTALOT, L., DEUTSCH, R., et al., in Plasma Physics and Controlled Nuclear Fusion Research 982 (Proc. 9th Int. Conf. Baltimore, 982), Vol. 2, IAEA, Vienna (983) 405. [] HEROLD, H., BERTALOT, L., JAGER, U., SCHMIDT, H., SCHMIDT, R., SHAKHATRE, M., in Controlled Fusion and Plasma Physics (Proc. th Eur. Conf. Aachen, 983), Vol. 7D, Part I, European Physical Society (983) 477. [2] SCHMIDT, H., and the Team, in Plasma Focus Research (Proc. 3rd Int. Workshop Stuttgart, 983), Institut fur Plasmaforschung, Universitat Stuttgart (983) 63. [3] JERZYKIEWICZ, A., BIELIK, M., JANKOWICZ, Z., et al., in Controlled Fusion and Plasma Physics (Proc. th Eur. Conf. Aachen, 983), Vol. 7D, Part I, European Physical Society (983) 485. [4] JERZYKIEWICZ, A., BIELIK, M., JANKOWICZ, Z., et al., in Plasma Focus Research (Proc. 3rd Int. Workshop Stuttgart, 983), Institut fur Plasmaforschung, Universitat Stuttgart (983) 7. [5] HEROLD, H., BERTALOT, L., HIRANO, K., et al., in Plasma Physics and Controlled Nuclear Fusion Research 984 (Proc. 0th Int. Conf. London, 984), Vol., IAEA, Vienna (985) 579. [6] JERZYKIEWICZ, A., BIELIK, M., JAKUBOWSKI, L., et al., ibid., p. 59. [7] SCHMIDT, H., BIERMAYER, W., HEROLD, H., HESSELMAIER, B., JAGER, U., SCHMIDT, R., SHAKHATRE, M., in Plasma Focus and Z-Pinch Research (Proc. 4th Int. Workshop Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985). [8] HEROLD, H., JAGER, U., KAEPPELER, H.J., SCHMIDT, H., SCHMIDT, R., SHAKHATRE, M., in Plasma Science (Proc. IEEE Int. Conf. Saskatoon, 986), IEEE, New York (986) paper 5C8. [9] HEROLD, H., KAEPPELER, H.J., SCHMIDT, H., SCHUMACHER, A., SHAKHATRE, M., WIN, T., in Plasma Focus and Z-Pinch Research (Proc. 5th Int. Workshop Toledo, 987), Imperial College, London (987) 7. [20] JERZYKIEWICZ, A., KOCIECKA, K., KOCINSKI, L., in Plasma Focus and Z-Pinch Research (Proc. 4th Int. Workshop Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985) 7. [2] JERZYKIEWICZ, A., KOCIECKA, K., KOCINSKI, L., in Plasma Focus and Z-Pinch Research (Proc. 5th Int. Workshop Toledo, 987), Imperial College, London (987) 37. [22] MATHER, J.W., AHLUWALIA, H.S., IEEE Trans. Plasma Sci. 6 (988) 56. [23] HEROLD, H., KAEPPELER, H.J., SADOWSKI, M., SCHMIDT, H., SHAKHATRE, M., Verhandlungen DPG Friihjahrstagung Dusseldorf 988, Rep. IPF--88, Institut fur Plasmaforschung, Stuttgart (988). 268 NUCLEAR FUSION, Vol.29, No.8 (989)

16 LARGE PLASMA FOCUS EXPERIMENTS [23a] JERZYKIEWICZ, A., KOCIECKA, K., in Plasma Focus and Z-Pinch Research (Proc. Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985) 40. [24] NOWIKOWSKI, J., JANKOWICZ, Z., JERZYKIEWICZ, A., in Controlled Fusion and Plasma Physics (Proc. 9th Eur. Conf. Oxford, 979), Culham Lab., UKAEA, Abingdon, Oxfordshire (987) 542. [25] SADOWSKI, M. (Ed.), Soltan Institute for Nuclear Studies, Dept. Nuclear Research, Annual Report 987, Rep. IPJ 2056/P-V/PP/A, Otwock-Swierk (988) 9. [26] DECKER, G., FLEMMING, L., KAEPPELER, H.J., et al., Plasma Phys. 22 (980) 245. [27] RAPP, H., Phys. Lett., A 43 (973) 420. [28] FILIPPOV, N.V., AGAFONOV, V.I., BELYAEVA, I.F., et al., in Plasma Physics and Controlled Nuclear Fusion Research 97 (Proc. 4th Int. Conf. Madison, 97), Vol., IAEA, Vienna (97) 573. [29] BRANDT, S., KOCIECKA, K., JERZYKIEWICZ, A., NAWROT, W., in Controlled Fusion and Plasma Heating (Proc. 5th Eur. Conf. Dubrovnik, 988), Vol. 2B, Part II, European Physical Society (988) 387. [30] JERZYKIEWICZ, A., BRANDT, S., KOCIECKA, K., et al., in Plasma Physics and Controlled Nuclear Fusion Research 988 (Proc. 2th Int. Conf. Nice, 988), Paper IAEA-CN-50/C-5-7, Vol. 2, IAEA, Vienna (to be published). [3] HEROLD, H., KAEPPELER, H.J., SCHMIDT, H., SHAKHATRE, M., in Z-Pinch and Plasma Focus (Proc. Workshop Nice, 988), Ecole polytechnique, Palaiseau (988) 44. [32] CZEKAJ, S., DENUS, S., KASPERCZUK, A., MIKLASZEWSKI, R., PADUCH, M., PISARCZYK, T., WERESZCZYNSKI, Z., in Plasma Focus and Z-Pinch Research (Proc. 4th Int. Workshop Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985) 24. [33] CZEKAJ, S., DENUS, S., KASPERCZUK, A., et al., in Plasma Physics and Controlled Nuclear Fusion Research 986 (Proc. th Int. Conf. Kyoto, 986), Vol. 2, IAEA, Vienna (987) 593. [34] SCHMIDT, R., Untersuchungen uber den Ablauf der Fusionsprozesse im Plasmafokus unter Verwendung von zeitaufgeloster Neutronenspektroskopie, Rep. IPF-87-4, Institut fur Plasmaforschung, Stuttgart (987). [35] SADOWSKI, M., HEROLD, H., SCHMIDT, H., SHAKHATRE, M., Phys. Lett., A 05 (984) 7. [36] SADOWSKI, M., JAKUBOWSKI, L., RYDYGIER, E., ZEBROWSKI, J., in Controlled Fusion and Plasma Physics (Proc. 2th Eur. Conf. Budapest, 985), Vol. 9F, Part I, European Physical Society (985) 538. [37] SADOWSKI, M., ZEBROWSKI, J., RYDYGIER, E., JAKUBOWSKI, L., in Plasma Focus and Z-Pinch Research (Proc. 4th Int. Workshop Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985) 04. [38] KAEPPELER, H.J., HAYD, A., MAURER, M., MEINKE, P., Die Bedeutung der Pinchphase fur die Neutronenproduktion im Plasmafokus, Rep. IPF-83-2, Institut fur Plasmaforschung, Stuttgart (983). [39] KAEPPELER, H.J., in Plasma Focus and Z-Pinch Research (Proc. 5th Int. Workshop Toledo, 987), Imperial College, London (987) 59. [40] JAGER, U., HEROLD, H., Nucl. Fusion 27 (987) 407. [4] BOSTIK, W.H., GRUNBERGER, L., PRIOR, W., NARDI, V., in Controlled Fusion and Plasma Physics (Proc. 4th Eur. Conf. Rome, 970), CNEN, Rome (970) 20. [42] SCHONBACH, K.H., MICHEL, L., FISCHER, H., Appl. Phys. Lett. 25 (974) 547. [43] RAGER, J.P., in Pulsed High-Beta Plasmas (Proc. 3rd Top. Conf. Culham, 975), Pergamon Press, Oxford (976) 39. [44] BERTALOT, L., HEROLD, H., JAGER, U., MOZER, A., OPPENLANDER, T., SADOWSKI, M., SCHMIDT, H., Phys. Lett., A 79 (980) 389. [45] SADOWSKI, M., SCHMIDT, H., HEROLD, H., Phys. Lett., A 83 (98) 435. [46] SADOWSKI, M., ZEBROWSKI, J., RYDYGIER, E., KUCINSKI, J., in Controlled Fusion and Plasma Physics (Proc. 4th Eur. Conf. Madrid, 987), Vol. D, Part II, European Physical Society (987) 520. [47] JAGER, U., Uber die Reaktionsmechanismen im Plasmafokus Untersuchungen an beschleunigten Deuteronen und Reaktionsprotonen, Rep. IPF-86-2, Institut fur Plasmaforschung, Stuttgart (986). [48] SADOWSKI, M., ZEBROWSKI, J., RYDYGIER, E., HEROLD, H., JAGER, U., SCHMIDT, H., Phys. Lett., A 3 (985). 25. [49] GENTILINI, A., MAISONNIER, Ch., RAGER, J.P., Comments on Neutron Production Mechanisms in a Dense Plasma Focus, Rep. CNEN 79.7/p, Centra Ricerche Energia Frascati (979). [50] PISARCZYK, T., Elaboration of methods and equipment for measurements of the Faraday magneto-optical effect and their use for plasma investigation, PhD Thesis, Military Academy of Technology, Warsaw (987) (in Polish). [5] KUCINSKI, J., NAWROT, W., in Plasma Focus and Z-Pinch Research (Proc. 4th Int. Workshop Warsaw, 985), Institute of Plasma Physics and Laser Microfusion, Warsaw (985) 5. [52] KUCINSKI, J., Institute for Nuclear Studies, Warsaw, personal communication, 987. [53] HESSELMAIER, B., Untersuchungen zur Elektronenemission des Plasmafokus, Institut fur Plasmaforschung, Univ. Stuttgart, Masters Thesis, 986. [54] CHOI, P., DEENEY, D., Imperial College, London, personal communication, 987. (Manuscript received 0 October 988 Final manuscript received 5 April 989) NUCLEAR FUSION, Vol.29, No.8 (989) 269