STATUS AND PROSPECTS OF GOL-3 MULTIPLE-MIRROR TRAP.

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1 STATUS AND PROSPECTS OF GOL-3 MULTIPLE-MIRROR TRAP A. Burdakov a,c, A. Arzhannikov a,b, V. Astrelin a, V. Batkin a, V. Burmasov a,b, G. Derevyankin a, V. Ivanenko a, I. Ivanov a, M. Ivantsivskiy a,c, I. Kandaurov a, V. Konyukhov a, K. Kuklin a, S. Kuznetsov a, A. Makarov a, M. Makarov a, K. Mekler a, S. Polosatkin a,b, S. Popov a, V. Postupaev a,c, A. Rovenskikh a, A. Shoshin a, S. Sinitsky a,c, V. Stepanov a, Yu. Sulyaev a, Yu. Trunev a, L. Vyacheslavov a, Ed. Zubairov a a Budker Institute of Nuclear Physics, 11 Lavrentjev Avenue, Novosibirsk 639, Russia b Novosibirsk State University, 2 Pirogova Street, Novosibirsk 639, Russia c Novosibirsk State Technical University, 2 Karl Marx Avenue, Novosibirsk 6392, Russia A.V.Burdakov@inp.nsk.su The paper reviews recent experimental results from GOL-3. Currently efforts are focused on further development of a physical database for multiple-mirror confinement systems and also on an upgrade of plasma heating systems of GOL-3 device. In general, current GOL-3 parameters demonstrate good prospects of a multiple-mirror trap as a fusion reactor. I. INTRODUCTION Fusion research program based on the concept of a multiple-mirror confinement suggested by Budker, Mirnov and Ryutov 1 is developing in the Novosibirsk BINP. Multiple-mirror trap is a set of linked magnetic mirror cells that form corrugated magnetic field. The plasma undergoes a kind of a friction force which decelerates its axial expansion along the system. If free path length is close to corrugation period l, a longitudinal plasma motion becomes diffusion-like, and plasma confinement time increases in respect to a free expansion time 1 ~ L/v Ti, where L is length of the trap and v Ti is an ion thermal velocity. The reason of this phenomenon is that the plasma ions consist of two groups with comparable densities: transiting ions with small pitch angles and trapped ones. Transiting particles became trapped due to scattering and then transiting again, with random direction of motion. Corresponding diffusivity coefficient can be estimated as D ~ l v Ti. Particle 2 confinement time L lvti 1N becomes proportional to number of magnetic mirrors N L/ l 1. This concept allows increasing confinement time for multiple-mirror traps with N 1 sufficiently, making it suitable for a multiple-mirror fusion reactor. 2 In the proposed reactor scheme the hot plasma of m -3 density is confined in solenoid with corrugated magnetic field of 1 1 m length. Following the initial proposal an experimental check and theoretical development of this method were done. 3,4 Such a way, the multiple-mirror confinement method based on classical binary particle collisions was validated in theory and partly in the experiment. In present a very important role of collective effects in multiple-mirror plasma confinement systems becomes evident. Generally, for most of magnetic systems these effects are negative and decrease energy confinement time. A situation in multiple-mirror systems is quite different. The main feature for them is a small free path comparing with length of a system, in contrast to simple classical mirror trap, just as to tokamaks, stellarators and other magnetic confinement systems. Development of turbulent processes during the plasma heating and confinement causes reduction of free path for particles. This makes possible to reduce a length of a separate magnetic cell for reaching the best confinement condition eff ~ l ( eff is effective free path of particles in respect to scattering on turbulent fields). As a result the total system length can be considerably reduced. The turbulence noticeably increases transverse particle transport, but the main problem of open traps is still too large axial energy and particle losses, which can be significantly reduced by the turbulence. There are some experimental hints on minimization of total losses at the certain level of turbulence. Similar problems appeared and were solved in other types of open traps by different methods. 5,6 The primary concept of pulsed fusion reactor 7 was based on classical binary particle collisions. Two principles were used: slow diffusive expansion of plasma to vacuum and transversal wall confinement of high- TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB

2 beta plasma. Vacuum insulation at the ends was supposed for decreasing energy loss by electron channel when effect of a strong turbulent suppression of electron heat flow at beam-plasma interaction was unknown. The "wall" confinement with a direct contact of fusion plasma with a conducting wall should provide MHD stability of the plasma column. Significant drawbacks of the initial proposal were extremely high plasma density and pressure in the system which challenges practical implementation of this idea. Plasma parameters in the GOL-3 facility at the heating by the electron beam were found much higher 8 than expected from classical estimates even in conditions of low beta and of thermal contact of the plasma with solid edges. Temperatures of 3 5 kev at plasma density ~1 21 m -3 and energy confinement time ~1 ms were achieved. It becomes possible due to excitation of plasma turbulence that sufficiently decreases longitudinal losses thereby eliminate this weakness of open traps. II. ROLE OF TURBULENCE IN PLASMA HEATING AND CONFINEMENT AT GOL-3 In basic configuration of GOL-3 the deuterium plasma with density of m -3 is confined in a 12- meter-long solenoid, which comprises 52 corrugation cells with mirror ratio B max /B min = 4.8/3.2 T see Fig. 1. The plasma in the solenoid is heated up to ~2 kev temperature by a high power relativistic electron beam (~.8 MeV, ~2 ka, ~12 μs, ~12 kj) injected through one of the ends. 8 sheet beam diode U-2 generator of the electron beam exit unit solenoid with corrugated magnetic field Fig. 1. Layout of GOL-3. It was shown earlier in theory and experiment that beam-plasma interaction under certain conditions can excite Langmuir turbulence. In our case it may be complicated by close electron cyclotron frequency and lower frequencies of ion sound and MHD processes. All this in presence of gradients of magnetic field and plasma density makes theoretical analysis of instabilities very hard. Nevertheless some main features of turbulent phenomena in the beam-heated multiple-mirror trap and their consequences were understood based on experimental findings. These features are: Beam electrons lose in average up to 4% of their initial kinetic energy in the plasma 9 during the beamplasma interaction; A turbulent Langmuir waves of high amplitude are excited by the beam in the plasma if increment of twostream instability exceeds the electron collision rate, therefore high beam current density and good beam quality are required; Effective heating of plasma electrons occurs in turbulent fields; At the same conditions axial electron thermal conductivity is suppressed in ~3 orders of magnitude; 1-12 Due to two last effects strong gradients of electron pressure arise in corrugated magnetic field during the beam pulse; 13 Fast collective acceleration of ions following by thermalization occurs together with total plasma flow along the system due to electron pressure gradients; 12,13 Interaction between trapped and transit ions in highpressure-gradient zones excites an ion bounce instability 8,14 that increases exchange between two mentioned particle groups and suppresses axial plasma flow. Another class of collective phenomena is connected with formation of helical magnetic field by axial currents. Complex radial structure of currents is formed in the plasma with turbulent resistivity. Main data are the following. 15 Rotational transform factor was measured. Sheared helical magnetic field gets formed in GOL-3 with a zeroazimuthal-field magnetic surface inside the plasma. Magnetic shear was shown to be the important factor for achievement of stable operation regimes and good plasma confinement in GOL-3. Computations show that tearing-like instability could exist inside the plasma column. Global plasma stabilization in GOL-3 is achieved by control of the magnetic shear. Confinement in multiple-mirror trap is also improved by collective effects. Best energy confinement time in the multiple-mirror trap GOL-3 (~1 ms) corresponds to theory predictions but it is achieved at much lower density, than predicted. Good confinement at low density indicates that effective collision frequency in the plasma exceeds the classical value by a factor of few tens. This fact is beneficial for multi-mirror-trap-based fusion reactor concept. 8 New class of plasma oscillations in the cells of multiple-mirror trap GOL-3 that can decrease axial particle and energy losses is observed. The oscillations were identified as bounce instability of trapped ions. 64 TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB. 29

3 This simple listing of collective phenomena shows that plasma dynamics in multiple-mirror trap is mainly determined by turbulent processes. Turbulent scattering of electrons increases an effective collision rate up to some level. Enhanced collision rate can saturate the two-stream instability which determines efficiency of the beam relaxation in the plasma. In the case of bounce instability the free path length of transit ions decreases and particle exchange between transit and trapped ions occurs more effectively in the end cells of the trap. Combined effect of collective effects allows to get high plasma parameters in GOL-3. Importance of these mechanisms is that optimal for the best confinement plasma density significantly decreases comparing to the classical model and therefore becomes more suitable for a feasible fusion reactor. Conceptual reactor which includes collective effects can have sufficient advantages over the classical concept. It can be stationary, operate at beta less than unity, and have more compact dimensions. III. STRONG CORRUGATION REGIME AT GOL-3 Usually GOL-3 operates with mirror ratio of the corrugated magnetic field equal to R = 1.5. This is the hardware-limited value which cannot be intentionally changed at the chosen cell length. At the same time the physical understanding of plasma confinement in a multimirror system will benefit if other than standard configurations of the magnetic field will be studied. Classical theory of plasma confinement in a multimirror trap predicts, that particle confinement time should be proportional to a square of mirror ratio R at other parameters being equal. New experimental data is required for a margin of plasma stability in the modified system. In reality any change of the magnetic field profile in the experiment leads to related changes of some other important parameters, which govern plasma heating and confinement in the trap. A special experiment with a change in magnetic configuration of the trap was done to validate the theory predictions. A high-mirror-ratio section was formed at first 2 meters of the solenoid with 5 cells at B max /B min =6./2.2 T and 44 cm length each. Details of this work can be found in Ref. 16. Oscillations of flux of D-D neutrons from near-theend cells of the trap were found in previous experiments. 8 Period of oscillations equals to transit time of an ion through one corrugation cell. Current understanding of this effect is that the plasma motion along the corrugated magnetic field due to axial pressure gradient leads to electrostatic instability of bounce oscillations of slightly trapped ions. 14 Excitation of these oscillations leads to efficient exchange between populations of locally trapped and transit ions, therefore the axial plasma confinement in the multimirror system (which relies on a relatively short free path length for ions) improves. Experiments have shown that transport of the electron beam through the plasma is stable. Collective plasma heating to sub-fusion temperatures and emission of neutrons from D-D reactions is observed. At the same time, the absolute values of energy content and confinement time of the plasma were less, than in optimum modes at regular corrugation of the magnetic field. Such decrease of parameters can be accounted both for non-optimality of operation regime (in particular, transverse energy losses from the plasma dominated in the strong corrugation mode), and for less favourable parameters of the experiment (energy content of the electron beam, plasma volume and a ratio between the cross-sections of hot plasma and cold periphery). The further experiments with strengthened corrugation are scheduled at higher parameters of an electron beam. The important physical result from the experiments with strong corrugation of the magnetic field, is change of the period between flashes of neutron emission in an initial part of the plasma. In full conformity with current views the frequency of bounce oscillations has decreased at the transition to strong corrugation see Fig. 2. A B U, V B, T z, cm z, cm time, microseconds time, microseconds Fig. 2. Oscillations of neutron flux measured by local detector. Cases A and B correspond to the standard and strong corrugation regimes, respectively. Top: magnetic field near the detector, arrows indicate the transit distance for oscillating ions. Bottom: fragments of waveforms of the local detector. IV. PLASMA HEATING BY PROLONGATED ELECTRON BEAM The basic condition for high parameters of GOL-3 is anomalously low electron heat flow along the axis which TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB

4 is provided by small-scale Langmuir turbulence developing as a result of two-stream instability. 1,11 In order to analyze fusion prospects of electron-beam technology of plasma heating in the long solenoidal trap it is necessary to investigate experimentally the possibility of further increase of plasma parameters by the prolongation of the beam pulse. At more durable (~1 s) beam pulse of ~.1 1 GW power one can expect the maintenance of the turbulence at the level appropriate for providing suppression of the electron heat transport. That will lead to substantial decrease of ion losses and hence to the increase of the ion energy lifetime. Additionaly the beam current can contribute to MHD stability of plasma column. 15 As the first step on solving this task the 1.5-fold increase of duration of the high-power part of the electron beam generated by the accelerator U-2 was completed. Conditions of durable generation in magnetically insulated diode, stable transportation and compression in the magnetic field of the high-power electron beam were found. Below we present preliminary results of study of dynamics of the plasma electron distribution function in the range of 5 kev carried out by means of 9 Thomson scattering diagnostics and the first results from multi-foil spectrum analyzer of high energy electron component (.1-1MeV) outgoing from the beam-plasma system. Since the duration of the beam generation in the magnetically insulated diode is limited by shortening of the cathode-anode gap due to plasma motion, this gap in the diode of the accelerator U-2 was increased from 1 to 12.5 cm. To optimize the beam transportation and compression in the magnetic field at the new diode geometry and also to condition high-voltage surfaces of a cathode holder of the accelerator as well, the preliminary experiments in which the beam was dumped on a collector, were carried out. This collector was installed in a vacuum chamber in the position inside the magnetic compression system where the magnetic field strength was in 15 times higher than the field near the cathode. These experiments showed that for stable generation of the beam with duration more than 1 s the diode voltage with amplitude ~.8 MV has to be increased in two stages. Firstly, it can be increased up to.5 MV in short time (<.3 s) and then its further growth to.8 MV has to be gradual in a few microseconds. It is needed in order to provide the diode current exceeding 2 ka at the time moment when the diode voltage is approaching the megavolt level. Such value of the current is appropriate for self magnetic insulation of high voltage surfaces of the cathode holder. So, by means of lengthening the total front duration from 1 to 3 s at small decrease of the diode voltage amplitude the beam pulse duration was increased from 9 to 14 s (see Fig. 3). As the prolongation of the pulse was reached due to the increase of the diode gap so the beam current decreased from 3 to 25 ka. As a result the total energy content of the beam at the collector was practically unchanged. U #7328 I beam ( ) diod ( ) # #6957 # Fig. 3. Traces of diode voltage and beam current before (dashed lines) and after (solid lines) prolongation of the electron beam. After the beam conditioned experiments were completed the investigations on heating the dense plasma in GOL-3 device started. Experiments were done under conditions described above in the Section II. Here we will discuss the experiments with a bell-shaped distribution with a density peak of (3 8) 1 2 m -3 at the middle of solenoid and three-fold density decrease to the plasma edges. Growth of the plasma energy was measured with a set of 2 diamagnetic loops placed at different coordinates along the axis of the plasma column. In general axial distribution of the plasma pressure is similar to that with the beam of shorter duration. Shape of individual waveform depends on several parameters, including the coordinate along the plasma column. Figure 4 shows diamagnetic signals from a loop placed in the input part of the plasma at Z = 44 cm. Two shown cases differ in the beam duration only. Growth of the plasma pressure occurs during the high-power part of the beam pulse, and then the plasma cooling is observed despite the continuing beam injection. As was already mentioned, the main mechanism of the plasma cooling here is axial heat loss by electron thermal conductivity which is turbulently suppressed during the heating phase. When the beam power is insufficient for keeping the plasma turbulence at a high level, the suppression of axial heat loss decreases and fast cooling of plasma electrons occurs. Important feature of waveforms presented in Fig. 4 is extended duration of the plasma heating phase with the new beam. This means that such beam provides improved plasma heating even at lower average power without a degradation of peak plasma parameters TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB. 29

5 nts,.. 1 Beam duration 9 s Beam duration 12 s t, t, s Fig. 4. Plasma diamagnetism at Z = 44 cm in the same conditions for the beam of old and prolonged duration. Fast plasma cooling at the second half of the beam injection is observed in the edge parts of the plasma column only. Near the plasma midplane the energy and particle confinement are better. Typical diamagnetic signal from the central part of the plasma is shown in Fig. 5. Unlike Fig. 4, decreased heating efficiency stays high enough to keep the pressure at almost constant level up to the end of the beam injection. Dynamics of diamagnetic signals and data from several other diagnostics indicate the possibility of substantional change of the beam-plasma interaction at approximately the peak power of the electron beam. Such change should result in the change of electron distribution function. Usually the electron distribution function is non- Maxwellian with large high-energy tails in the experiments with good efficiency of electron beam relaxation in the plasma. μm, 3 J, 15 ns) and two registration systems for 9 and 8 scattering angles, which enable measurements within the energy range up to 2 kev. 17 Spectra of scattered light measured with 9 -subsystem are shown in Fig. 6 for several shots and two moments of time. The upper (energy) scale is obtained using the simple relation 2 1/ 2 sin( / 2)(2E / mc ), where is the scattering angle. The distribution of plasma electrons is non-maxwellian and anisotropic. A distinguishable excess of electrons with dominated longitudinal velocities over electrons with dominated transversal velocities in the range of E ~ 1 kev is clearly seen from the small angle Thomson scattering spectrum. High-energy tails and lower bulk temperature are more evident at the second part of the beam pulse in Fig. 6. We should note that interpretation of scattered spectra is model-dependent. dn e /d, m -3 /nm E, kev μs 9-1 μs , 1 3 nm 2 U, MV Fig. 6. Scattering spectra for 4 5 (top) and 9 1 s after the beam start by 9 Thomson scattering. nt, 1 2 kev/m 3 time, μs Fig. 5. Dynamics of plasma heating in central part of the plasma column. Top: diode voltage, bottom: plasma pressure at Z = 4.75 m. Features of electron distribution function were studied with a Thomson scattering system positioned at Z = 415 cm. This system consists of a Nd-glass laser (1.58 During the period from 2 till 6 s the difference in energies of bulk and tail components is not large, the distribution functions appear similar. Then, in the second half of the beam pulse, non-maxwellian nature of the distribution function becomes more emphasized. Change in the electron distribution function occurs approximately at maximum plasma pressure mainly due to loss of a group of 1-3 kev electrons from the trap. Change of shape of electron distribution function may indicate that during the second part of the beam a level of the beaminduced microturbulence is not high enough to provide further plasma heating, but the anomalous scattering of electrons into the loss cone still exists for at least mentioned group of particles. Details of dynamics of electron distribution function are discussed in Ref.18. TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB

6 V. NEW APROACH TO LONG-PULSE ELECTRON BEAM FOR PLASMA HEATING Advance towards the creation of multi-mirror fusion reactor requires the development of new tools and techniques purposed for heating and maintenance of stability of plasma in magnetic traps. First of all, the duration of the electron beam pulse must be increased at least by an order of magnitude in comparison with that currently existing, with the retention of high current density and high brightness of the beam. An elaborated technique of high-power electron beam generation with explosive emission cathodes is inappropriate in the case of long-pulsed beams due to limitations set by accelerating gap closure with an explosive plasma. The use of electron emitter based on controllable gasdischarge plasma seems a promising solution of the problem. Accordingly, development of a suitable technology of generation of electron beams in devices with a plasma cathode is required. The aim of the work is the development of an electron beam source with a pulse length in a range of 1 s, accelerating voltage ~15 kv, current density of 1 2 ka/cm 2 and ~1 kj beam energy content. A special test bench was created for checking the concept of plasma electron emitter on the basis of highcurrent arc discharge and for the analysis of the technology of long-pulse electron beam generation. The stand comprises the electron beam source itself and the appropriate power-supply systems, vacuum system, etc. Schematic layout of the beam source is given in Fig. 7. A plasma box (1) with arc plasma generator (2) was mounted on a ceramic high-voltage insulator (3). The hydrogen was puffed in the discharge channel of arc Fig. 7. Layout of the test-bench beam source: 1 plasma box with emission electrode (cathode), 2 arc plasma generator, 3 ceramic high-voltage insulator, 4 magnetic coil, 5 beam extractor (anode), 6 beam liner evac. generator with a gas valve. The high-current arc fired between cold aluminum cathode and the plasma box, which constitutes a hollow arc anode. An electron beam extraction and acceleration occured in a double-grid system, formed with the set of circular openings drilled coaxially in a hexagonal order in the frontal plate of the plasma box (which serves as emission electrode) and in the grounded extracting electrode (5). A more detailed description of the test-bench beam source could be found in Ref. 19. An axial magnetic field was created with two coils, the first of them was mounted on the casing of the source (4), and the second was placed at the beam transport line downstream from the beam source. The value of the magnetic field could be set up to.1 T at the beam extraction region, then it rose along the beam transport line and reached its maximum up to 2 T at a distance ~3 cm from the beam source. voltage, kv time, s The first experiments demonstrated that the arc plasma emitter can provide electron emission current density ~ 1 A/cm 2 in axial magnetic field ~.1 T with the beam pulse duration at submillisecond range. The mean value of electric field strength in the accelerating gap was more than 1 kv/cm. Typical waveforms of the accelerating voltage and the beam current are shown in Fig. 8. Beam pulse duration and maximum current were limited in these experiments only by the capabilities of the utilized power-supply systems. The source operated in an axial magnetic field of.1 T. The accelerated beam was transported without current losses in increasing magnetic field and maximal beam current density in the magnetic mirror was achieved in the first experiments up to ~.5 ka/cm 2. The results of the test experiments seem encouraging for developing the beam source with the higher parameters. At present the efforts are undertaken current, A Fig. 8. Typical waveforms of the beam current and accelerating voltage. 68 TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB. 29

7 for the designing of the electron beam source with accelerating voltage of 15 kv and total beam current of up to 1 ka, intended for the use at the GOL-3 installation. VI. FIRST EXPERIMENTS WITH NBI AT GOL-3 A next step in the development of multimirror systems should provide significantly longer plasma lifetime. This requires development of new techniques of plasma heating in addition to existing high-power relativistic electron beam. Neutral beam injection is considered to be effective for auxiliary plasma heating in GOL-3. Special interest is in joint use of long pulse electron beam for heat conductivity suppression and highpower neutral beam injection (NBI) for creation of population of fast ions in plasma. High density, short lifetime, small plasma radius and high gas pressure near the wall are the challenges which complicate use of neutral beam injection in GOL-3. First experiments with neutral beam injection is directed to adaptation of the technique for use in the conditions of GOL-3 and to study of fast ion confinement in turbulent plasma of multi mirror trap. Neutral beam injector based on START-2 design was put into operation. Injector was installed at the central part of the solenoid for normal injection respective to the magnetic field axis. At first experiments the energy of accelerated atoms was kev, power was MW, and pulse duration was.8 ms. Beam passage across the plasma column was controlled by the multichannel current profilometer. This permits us to study radial distribution of the plasma density by attenuation of the neutral beam. I, a.u a b t, μs Fig. 9. Neutral beam attenuation by the GOL-3 plasma. Curve (a) is the current of the central collector of the profilometer, NB injector only, (b) is the same signal at NBI into the GOL-3 plasma. t = corresponds to NBI start, start of the plasma is marked by the arrow. The feature of GOL-3 is high enough concentration ( 1 21 m -3 ) of gas puffing. This results in additional losses of injected neutrals. In order to reduce such losses the facility was put into a special operation mode, with the plasma and the gas densities in the vicinity of atomic injector decreased to 1 2 m -3 while near needle valves it reached m -3. The experience of neutral beam injection is successful in general. The rate of beam atoms, trapped into the plasma, reached 84%. Beam loss due to ionization of neutrals in the gas on the way to plasma column is below 2%. The main source of non-productive loss of the beam power is cold edge plasma, which stops ¾ of injected atoms. The second problem is a fast cooling of electron component of the plasma, that restricts accumulation time of fast particles by the value of ~5 s. Other details of NBI experiments can be found in Ref. 2. In present experiments the existing grid assembly formed the beam with low angular divergence. Aperture of used for NBI input port of GOL-3 is significantly less than the beam diameter, so the most part of the beam power was lost at the construction elements before the beam arrives at plasma. In future we plan to develop the optimized grid system with geometric focusing. In the considering experiments injector was used as a diagnostic of radial profile of the plasma density. VII. SUMMARY Collective effects play important and positive role for a plasma confinement in a multi-mirror trap with a plasma heated by a high power relativistic electron beam. These effects lead to such consequences as effective beam relaxation in the plasma due to two-stream instability, strong suppression of axial electron heat flux due to enhanced particle scattering on beam-excited turbulence, effect of fast energy transfer from electrons to ions due to non-uniform plasma heating and formation of plasma flows in the mirror cells, excitation of density fluctuations in the mirror cells by axial plasma flow causing enhanced scattering of transiting ions which slows down plasma flow, MHD stabilization by a turbulence-shaped magnetic shear. Combined effect of mentioned collective phenomena results in high plasma parameters in GOL-3. Electron temperature reaches 2 4 kev at a density of m 3. Fast ion heating leads to increase of ion temperature up to ~2 kev at a density of ~1 21 m 3. Achieved energy confinement time (~1 ms) corresponds to theory taking into account collective processes. Practical significance of collective processes is that the plasma in a reactor-scale trap can be maintained at TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB

8 much lower density than existed estimates based on classical binary collisions. The best confinement regime can be realized at density levels suitable for 1 operation that greatly improves engineering and physical feasibility of multiple-mirror concept for fusion reactor. The reactor therefore might be stationary, operate at beta less than unity and have dimensions less than supposed by the original model. The data from experiments with strong corrugation and with the prolongated electron beam confirm the basic conclusions. The most important result from the presented series of experiments is the conclusion about maintenance of suppression of axial electron heat flow during full duration of prolongated electron beam. Development of technologies of plasma heating is required for further increase of plasma parameters in the multiple-mirror trap. First experiments on neutral beam injection are carried out; experimental activity on new approach to generation of high-power electron beams is in progress. ACKNOWLEDGMENTS The work was partially supported by RFBR , , , and REFERENCES 1. G. I. BUDKER, et al., "Influence of Corrugated Magnetic Field to Expansion and Cooling of Dense Plasma", JETP Letters, 14, 32 (1971) (in Russian). 2. D. D. RYUTOV, "Open-Ended Traps", Sov. Phys. Uspekhi, 31, 31 (1988). 3. G. I. BUDKER, et al., "Experiments on plasma confinement in multi mirror trap", Sov. Phys. JETP, 65, 562 (1973). 4. B. G. LOGAN, et al., "Plasma Confinement in Multiple-Mirror Systems. II: Experiment and Reactor Calculation", Phys.Fluids, 17, 132 (1974). 5. T. CHO, et al., "Observation and Control of Transverse Energy-Transport Barrier due to the Formation of an Energetic-Electron Layer with Sheared E B Flow" Phys. Rev. Lett., 97, 551 (26). 6. P. A. BAGRYANSKY, et al., "Influence of Radial Electric Field On High-beta Plasma Confinement in the Gas Dynamic Trap", Fusion Science and Technology, 51 (2T), 34 (27). 7. B. A. KNYAZEV, P. Z. CHEBOTAEV, "A pulsed Multi-Mirror Fusion Reactor: Longitudinal Confinement", Nuclear Fusion, 24, 555 (1984). 8. A. V. BURDAKOV, et al., "Plasma Heating and Confinement in GOL-3 Multiple Mirror Trap," Fusion Science and Technology, 51 (No. 2T), 16 (27). 9. A. V. ARZHANNIKOV, et al., "Recent Results on the GOL-3-II Facility", Fusion Technology, 35 (1T), 112 (1999). 1. A. V. BURDAKOV and V. V. POSTUPAEV, "Features of heat transport at beam plasma heating in experiments on the GOL-3 facility," Preprint 92-9, BINP, Novosibirsk, 1992 (in Russian). 11. V. T. ASTRELIN, et al., "Generation of Ion-Acoustic Waves and Suppression of Heat Transport during Plasma Heating by an Electron Beam", Plasma Phys. Rep., 24, 414 (1998). 12. A. V. ARZHANNIKOV, et al., "Direct Observation of Anomalously Low Longitudinal Electron Heat Conductivity in the Course of Collective Relaxation of High-Current Relativistic Electron Beam in Plasma". JETP Letters, 77, 358 (23). 13. A. V. ARZHANNIKOV, et al., "Study of the Mechanism for Fast Ion Heating in the GOL-3 Multimirror Magnetic Confinement System", Plasma Physics Rep., 31, 462 (25). 14. A. D. BEKLEMISHEV, "Bounce instability in a multi-mirror trap", Fusion Technology, 51 (2T), 18 (27). 15. V. V. POSTUPAEV, et al., "Role of q-profile for plasma confinement in the multimirror trap GOL-3", Fusion Science and Technology, 47 (1T), 84 (25). 16. V. V. POSTUPAEV, et al., "Experiments with Large- Mirror-Ratio Corrugation at Multiple Mirror Trap GOL-3", Fusion Science and Technology, this issue (29). 17. S. S. POPOV, et al., "Measurements of the Electron Energy Spectrum by Using Small-Angle Thomson Scattering", Plasma Physics Reports, 34, 212 (28). 18. V. V. POSTUPAEV, et al., "Dynamics of Electron Distribution Function in Multiple Mirror Trap GOL-3", Fusion Science and Technology, this issue (29). 19. A. V. BURDAKOV, et al., "Study of Electron Beam Emitter Based on a High Current Arc Plasma Source", in Proc. of 14 th Int. Symp. on High Current Electronics, 26, Tomsk, Russia, 78 (26). 2. S.V. POLOSATKIN, et al., "First Experiments on Neutral Injection in Multimirror Trap GOL-3", Fusion Science and Technology, this issue (29). 7 TRANSACTIONS OF FUSION SCIENCE AND TECHNOLOGY VOL. 55 FEB. 29