The GDT is an axisymmetric plasma confinement device (see Fig.1) with the high mirror ratio variable in the range of 12.5-100. To provide MHD stability of the ...
GAS-DYNAMIC TRAP EXPERIMENT: STATUS AND PERSPECTIVES.
A.A.Ivanov, A.N.Karpushov , and K.V.Lotov Budker Institute of Nuclear Physics, Prospect Lavrent’eva 11, 630090, Novosibirsk, Russia
INTRODUCTION The paper reports on the present status of the Gas Dynamic Trap (GDT) experiment and plasma parameters recently achieved with increased neutral beam power. Also discussed in the paper are possible future experiments in the GDT that would contribute to establishing the required plasma physics database for the GDT-based neutron source [1] and its Hydrogen Prototype as well. The GDT is an axisymmetric plasma confinement device (see Fig.1) with the high mirror ratio variable in the range of 12.5-100. To provide MHD stability of the entire plasma axisymmetric min-B cells are attached from both ends of the device. The bulk plasma confined in the central cell is essentially collisional. Additionally, it contains a hot ion minority produced by oblique injection of Neutral Beams (NBs) at the center of the device. The plasma in the External Stabilizing Cells (ESCs) is fed by collisional plasma losses from the central cell. The two types of the ESCs were used in the experiments: cusp end cell and expander end cell with gradually decreasing magnetic field. As a matter of fact most of the experimental data were obtained with the cusp end cell because it provided more robust stabilization of MHD perturbations. Studies of low beta (~5%) plasma confinement in GDT are reported elsewhere [2-4]. Recently, plasma parameters in the GDT experiment were significantly improved with higher injected NB power and improved vacuum conditions [5]. EXPERIMENTAL DESCRIPTION Main parameters of the GDT device are listed in Table I. The recent technical improvements of GDT mainly involved optimization of the injection system and extensive application of Ti evaporation to reduce outgassing of the central cell first wall. The injected currents of the six NB injectors were equalized by their simultaneous increase to 37 eq. A. (for 15.5 –17.5keV
beam energies) as it was for the better running injector. It provided full injection power of up to 4.2MW in 1.1ms pulses. Due to equalizing the injected currents and beam energies for the injectors located at different azimuths higher axial symmetry of the injection was achieved and thereby disturbances and induced losses of the target plasma during the initial stage of neutral beam heating were significantly reduced. This was accompanied in the GDT by an access to higher plasma parameters. Heating up of bulk electrons to temperatures of 110eV was observed. Reduction of charge-exchange losses and increase of target plasma temperature leads to increase of fast ion density (5-8keV energies) approaching to 1013 cm-3 near the turning points. Table I. The parameters of GDT experiment Parameter Magnetic field at midplane Total length Trapped NBI power (15.5-17keV beam energies) Fast ion energy content Bulk plasma energy Electron temperature Radiated power Electron drag power in shadow of limiters Power released on limiters Plasma radius Bulk plasma density
Value 0.22 T 11m 2.4-2.6 MW 520-700 J 170-230 J 85-110eV 20-50kW 220kW 300kW 8.5cm 1.0-10×1013 cm-3
Fig.1 General layout of GDT device
FAST ION CONFINEMENT In order to establish if there are any anomalous losses of energetic ions in the high performance shots, the temporal variation of their energy content was compared with that predicted by the numerical codes (Fig.2). As it seen from Fig.2 the predicted behavior as well as the absolute value of fast ion energy accumulated during NBI, by Monte-Carlo and Fokker-Planck codes, reasonably coincide with the experimentally measured ones. Comparison of the experimental data with the simulation results enable us to conclude that significant increase of the fast ion density and plasma beta, more than three times, as illustrated by Fig.2, did not give rise to any additional energy losses.
Fig.2 Measured and simulated temporal variation of fast ion energy content during neutral beam injection. A cross-check of the absence of non-classical channels of fast ion energy losses in the high performance shots is provided by the measurements of the local slowing down distributions (Fig.3) by the charge-exchange diagnostic installed at the GDT
Fig.3 Energy distribution of fast ions at the center of GDT midplane. Comparison of the calculated and locally measured energy and angular distributions of fast ions
also indicates that their slowing down and scattering rates are almost classical in agreement with global energy balance data presented in Fig.2. BULK PLASMA ENERGY LOSSES To evaluate the central cell energy confinement time we define
τ ⊥ = WP
cc
⊥
, where Wcc is energy stored in
the bulk plasma, determined by diamagnetic loops and P⊥ is the transverse power losses from the bulk plasma. Energy losses from collisional bulk plasma under neutral beam heating were evaluated in the high performance shots by direct bolometric measurements of the power released on the limiters, less the power released by finite-orbit fast ions in the halo plasma located in the limiter’s shadow. The former portion is not directly measured, we determined it from fast energy balance data. The magnitude of the various terms used to evaluate the global energy confinement time of the bulk plasma is given in Table I. The difference between the total power absorbed by the limiters (300kW) and Electron Drag Power (EDP) of fast ions in the halo plasma gives for the radial power flux from the core plasma the value of about 100kW. In the same experimental conditions the bulk plasma energy content amounts to 170J. This corresponds to energy confinement time of the heated plasma of 1.7ms. In accordance with the data given in Table I, taking the plasma radius a=8.5cm and B=0.2T, T=85eV, one obtains that the characteristic Bohm confinement time 2
τ Bohm = a 6 D , Bohm
where
DBohm = cT
16eB
, equals to
0.045ms and is at least 37 times less than the observed value. This observation reasonably correlates with the results obtained previously for lower plasma temperatures. Note that this scaling of plasma lifetime is considered to be favorable for the operational parameters of the GDT-based neutron source for fusion materials and components testing [9]. Axial losses from the central cell plasma were also measured and found to be in reasonable agreement with the theory applicable to a collisionless regime of plasma flow beyond the mirrors [6,7]. An assessment of power balance of the cusp stabilized central cell plasma indicates that these axial power losses account for at least 60% of the EDP for mirror ratio of 12.5. It was observed that the electron temperature profile in the high performance shots has a well pronounced minimum on axis, indicating enhanced energy losses from the near-axis region. This region is well mapped along the field lines onto the plasma gun, so it can be concluded that this temperature reduction is attributed to heat sink onto the gun. Local energy balance data indicate that about 30% EDP are lost due to this mechanism for plasma temperatures in the range of
80-110eV. For smaller temperatures this energy losses become less significant [8]. D-D NEUTRON PRODUCTION EXPERIMENT In recent years steady progress has been made in increasing the fast ion density in the GDT central cell. Diamagnetic loops data in conjunction with the measurements of the angular spread of the fast ions at the midplane indicated that the fast ion density near the turning points is near 1× 1013 cm-3. This density is high enough to generate a noticeable flux of D-D neutrons which could be used for diagnostic purposes. In order to provide the neutrons we plan to inject 20-25keV, 46MW deuterium beams into central cell and increase the magnetic field from 0.22T to 0.35-0.4T to ensure reasonable scaling from the conditions of our experiments with injection of hydrogen beams. The absolute intensity of the neutron flux is quite sensitive to D-beam energy, magnetic field and target plasma temperature. Assuming that the parameters of the target plasma and fast ions could be the same as in the high performance shots with the hydrogen beams we estimated the parameters of conceivable experiments with deuterium beams at GDT. Total neutron production was estimated to be in the range 0.1 ÷ 1 × 1012 neutrons/s. Calculated axial profile of the specific linear neutron flux is shown in Fig.4.
Fig.4 Simulated linear density of D-D neutron production rate in GDT
CONCEVABLE EXPERIMENTS WITH SYNTHESIZED HOT ION PLASMOID (SHIP) A possibility to achieve, as is required in the GDT-base neutron source, both high electron and ion temperatures of dense plasma has not been yet demonstrated experimentally in GDT. Therefore, development of approaches enabling addressing plasma physics issues related to plasma confinement in GDT at relevant conditions is mandatory. We are going to apply a stepwise approach to ”synthesize” [10] a hot-ion dense
plasma with the parameters which could be of interest for the neutron source simulations. Namely, at the GDT facility, in the regimes with neutral beam heating when electron temperature exceeds 100eV, one can synthesize the plasma with high ion temperature and density using pulsed neutral beam injection into an ancillary local axisymmetric mirror cell with a small mirror ratio. The high power density neutral beams are to be injected into the region with locally decreased magnetic field in order to provide longitudinal confinement of the trapped energetic ions. For the given experimental conditions, the lifetime of the thus synthesized plasma is essentially determined by the time during which the target plasma can be sustained, and might be of the order of 1ms. Since ion energy losses in a high enough electron temperature plasma background are quite negligible in this time scale, the averaged energy of the trapped ions is expected to be close to the injection energy. The density of this Synthesized Hot Ion Plasmoid (SHIP) can be close to or even higher than that of the target plasma. We estimated the attainable parameters of the synthesized plasma using the three sets of input parameters summarized in Table II
Fig.5 Simulated profiles of fast ion density and plasma beta (A). Table II. The parameters of conceivable experiment on SHIP production in GDT. Magnetic field at midplane of the local mirror cell First wall radius Neutral beam size Trapped neutral beam current Energy Injection duration Target plasma electron temperature
A 2.5 T
B 5T
C 5T
7cm 7x12cm2 50 eq.A
7cm 7x12cm2 50 eq.A
7cm 7x12cm2 100 eq.A
50keV 250µs 85-110eV
50keV 250µs 85-110eV
25keV 250µs 85-110eV
What will be the contribution of non-paraxial region into the driving force of the instability? b) How effective is the stabilization by remote stabilizers ? c) How effective is the stabilization by vacuum minB field in the presence of considerable (and unfavorable) finite-beta-driven distortion of field lines? d) Whether the fast azimuthal drift of hot ions noticeably affects the stability.
Fig.6 Simulated profiles of fast ion density and plasma beta (C).
In order to study the SHIP production, reconstruction of the mirror unit of the GDT device is required. An additional mirror coil is to be installed near the old mirror coil producing a local axisymmetric magnetic trap with the mirror ratio of 1.2-1.4 and 2.5-5 T central field thus simulating the magnetic field of GDTNS device. The neutral beam will be injected into this trap to provide SHIP build-up (Fig.7).
Some results of simulations for the cases A and C are given in Fig.5-6. In the SHIP studies there are several points of particular interest for the development of mirror-based neutron sources. Although the lifetime of the plasma is limited, a variety of involved (and important) plasma processes such as instabilities, relaxation, etc, is expected to have significantly smaller characteristic timescales and thus can be studied at relevant conditions. Particularly, the following issues could be addressed: 1. For highly anisotropic hot ion velocity distribution, plasma equilibrium turns out to be essentially nonparaxial for quite moderate values of plasma beta even though external magnetic field is paraxial. Consequently, it is important to look at following problems: a) SHIP longitudinal self-pinching in distorted magnetic field. It is important since peak hot ion density in SHIP is determined by this effect. b) Processes determining the radius of SHIP in distorted magnetic field. SHIP creates on-axis magnetic well, but increases the magnetic field at the periphery of the plasma. The field “hump” thus formed does not trap particles any more. This radial self-cutting of SHIP might considerably reduce its radius. c) Properties of thus formed non-paraxial equilibrium (in particular, profile of hot ion density and curvature of distorted field lines). 2. Theoretical and experimental studies of MHD stability of non-paraxial hot ions. Here the problems to be addressed are: a) Is the non-paraxiality of hot ion region favorable for MHD stability?
Focused NB
To cusp
To central cell
Fig.7 Experiment on the SHIP production at GDT. It is assumed to develop the specialized neutral beam injection system with the following parameters: beam energy - 25(50) keV; injection current -100(50) eq. Amps; pulse duration – 100-200µs. The scalable injector prototype with geometrical focusing of the beam is currently used in the GDT for diagnostic purposes (15 keV, 100 µs, 20 A). It provides the current density up to 0.8eq.A/cm2 in the focal plane at a distance 90cm downstream from the injector grids. CONCLUSIONS Plasma β in GDT central cell was increased up to 20% with higher NB power. Electron temperature was increased to up to 120eV and fast ion density reached 1013 cm-3 near the turning points In these high performance shots, energy balance measurements and measurements of the fast ion local distributions over energies and pitch angles were coupled to reveal possible non-classical energy losses of fast ions. The comparison
of the experimental data with the simulation results indicate that there were no noticeable anomalies in fast ion slowing down and scattering. Energy balance of the bulk collisional plasma was studied in these shots. As it was previously observed for lower plasma parameters, for mirror ratio of 12.5, energy losses from the target plasma are dominated by longitudinal ones. It was noted that in the near-axis region there is an additional channel of energy losses that presumably is connected to a heat sink to the plasma gun located in one of the end tank. This channel becomes significant for plasma temperatures exceeding 60-70eV. Transverse energy losses from the bulk plasma can be characterized by corresponding lifetime which is estimated to be ≈40 τ Bohm for the representative shot parameters. It should be emphasized that these loss rate are tolerable when scaled to the operational parameters of the GDTbased neutron source [1,7,9].
ACKNOWLEDGEMENTS The authors are grateful to the members of the GDT experimental group for their valuable comments and discussions. This work was partially supported by ISCT through research project #492-98 and also by Russian Foundation for Basic Research through grant #97-0218545-a. REFERENCES 1. A.A.Ivanov, and D.D.Ryutov, Nucl. Sci. and Eng., v.106, p.235 (1990) 2. A.A.Ivanov,A.V.Anikeev, P.A.Bagryansky, et al, Phys. Plasmas, 1(5),p.1529(1995) 3. A.V.Anikeev, P.A.Bagryansky, A.A.Ivanov and I.A.Kotelnikov, Plasma Phys. Control. Fusion 37 (1995) 1239 4. A.V.Anikeev, P.A.Bagryansky, et al, Phys. Plasmas, v.4(4), p.347(1997) 5. P.A.Bagryansky, A.V.Anikeev et al, this conference 6 Mirnov VV, Ryutov DD, Sov. Tech. Phys. Lett., 5 279 (1979) (in Russian) 7 Kotelnikov IA, Mirnov VV, Nagornyj VP, Ryutov DD, Proc. of XVI IAEA Conf. 2 309 Vienna, IAEA (1985) 8 A.V.Anikeev, P.A.Bagryansky, et al, In:Proc. 22nd EPS Conference on Controlled Fusion and Plasma Physics, V.19C, Part IV, p.193, Bournemouth, UK(1995) 9 D.D.Ryutov, Plasma Phys. Control. Fusion 32 (1990) 999 10 R.F.Post and D.D.Ryutov, Lawrence Livermore National Laboratory Preprint UCRL-JC-119341 (1995)
