On the opportunity of Laser Plasma simulation of Plasma Jets formation in moderate magnetic fields ∼ kGs

Today a number of various experiments with Laser-Produced Plasmas (LPP), done at very-high Magnetic Fields (up to B0 > MG) are using to model the physics of Astrophysical and Space Jets, a various processes of their formation and possible long-range propagation at various angles S to magnetic fields. We discuss the opportunity and present the first results of new-type experiments on simulation of Jets with LPP at KI-1 facility of ILP, at moderate magnetic field ∼ kGs, oriented quasi-transverse (S ≍ 600) of LPP-blob expansion (with velocity V 0) relative to B 0. They were done on the base of all our preliminary studies, both at large-scale, high-vacuum chamber (0120 cm of KI-1) and others devices with LPP (oriented earlier at V0 transverse to B0, with $ = 900).


Introduction
Since 1991 a lot of investigations at KI-1 facility of ILP were done [1][2][3][4][5][6][7] on the expansion of LPP across magnetic field B0, including a few cases with multi-kGs ones, of dipole type [1][2][3]5] and quasiuniform [4,6,7] of small Helmholtz Coils (  15 cm). Therefore, a data about revealed Jet-like structures (in both cases) were very preliminary, and only recently we could study initial stage of Jet formation from LPP (with the flat target) in the field B0 = 300 Gs [8] at scale ~ 100 cm, comparable with the diameter  120 cm of vacuum chamber KI-1. Now, with the purpose to develop the methods of laboratory simulation in the LPP-experiments, into the center of chamber was installed a spherical plastic target  = 10 mm with the system of its symmetrical irradiation by 4 beams (of tetrahedral scheme). As a result, we had started a new kind of experiments [10] to generate a sphericallysymmetrical plasma cloud and to study its collisionless interaction with magnetic field and magnetized plasma background (for verification of new 3D-Codes). In our previous experiment MHD [7] with a small spherical target ( = 3 mm and 2-beams irradiation), in the field 8 kGs we had observed formation of 2 Jets (along to beams) with velocity VJ ≈ 50 km/s ~ V0/3 (at radius R > 2 cm). For the initial total energy of this LPP, E0 ~ 10 J, its deceleration radius [11] by field is RB = (3E0/B0 2 ) 1/3 ≈ 1,7 cm and directed ( V0) ion Larmor radius -RL≈ 0,5 cm, so, the criterion of ion magnetization had enough small value b =RL/RB ≈0,3 < 1 for the effective interaction of LPP with B0-field at scale ~ RB, including its deceleration and cavity formation [7].

Experiments on generation and study of Jets in moderate magnetic fields 300400 Gs
Therefore we had tryed to realize in the given 4-beams scheme, attempt to form a Jet-like structure from a larger spherical target (C2H4), by using only 2 beams, from the left entrance laser window). At Figure 1 one can see the relevant part of this scheme with 2 copper mirrors (at the bottom and at left), reflected 2 beams (here) of CO2 -laser to the target above them. As a result, with magnetic field B0 = 400 Gs, we could registered by time-integrated photos (Figure 1a), a long red plasma stream from target downwards at presence of H2-gas (with pressure ≈ 1,5 mTorr). With gas, but without field, only small blue sphere appears shifted from target (Figure 1c). The photo (Figure 2) of Gated Optical Imager (GOI,) in the same field (in vacuum) have revealed (at time t = 2,5 s with exposition 1,5 s, without gas) a Jet-like structure ( of size up to S≈ 25 cm into the same down-left direction, with the velocity V0 ≈ 100 km/s, rather well corresponding to the initial expansion velocity of LPP-sphere, generated in the case of total 4 beams (when LPP energy E0≈25 J).
The red color of observed luminosity well corresponds to the Balmer Alfa (656.2 nm) of Hydrogen and we also seen such an effect before, during previous experiment Super-Jet [8] with the flat target (C2H4) and real kinetic energy of LPP 50 J (with effective energy 300 J, into 4) in the field B0 = 300 Gs. In the given case, with expected LPP kinetic energy E ≈ 12 J and B0 = 400 Gs, expected cavity radius could be RB = (3E/B0 2 ) 1/3 ≈ 13 cm and ion Larmor RL≈ 10 cm, and so the b ~ 1 and we have presented at Figure 2, a GOI-data on Jet-like structure up to the radius R ≈ 25 cm (see  Figure 2. GOI-data on the Jet-like plasma expansion (from 2 laser beams) at the moment t=2 мкс, with exposition τ = 1,5 s, (negative) in vacuum 3*10 -6 Torr at field B0=400 Gs. The dark spot in center is a slow plasma near target, while below -fast stream with a front velocity V0= R/t ≈125 km/s close to the initial velocity of LPP expansion without field. This velocity was measured by Langmuir probe (at its maximal R = 25 см ), which support are seen lower.

Magnetic probes data on propagation "2-Beams" Jets at large distances with B0 = 400 Gs
Since it was impossible to study such Jets at distances R > 25 см by GOI (to get such GOI-Photos in later times, we need to increase P more than 57 mTorr) and also by Langmuir probes (in this direction), we did additional measurements by the pair of magnetic probes (see Figure 1a, at the left). Their main data on magnetic disturbances by Jet are presented at Figures 3, 4, 5. At the Figure 5, the main moments of Jets arriving to probe RM are presented, first of all, the a-time 2 mks (of RM at distance 25 cm), which is rather close to AIP-signal at distance 26 cm (at Figure 4b). Therefore we could combine the both data into general R-t diagram at Figure 6.  (Figure 4), registered by IAP-probe, as a result, a joint R-t diagram of Jets front was found (Figure 6).

Discussion and Conclusions
Presented at Figure 6 (below), R-t diagram of Jet front, corresponds to its motion without deceleration accordingly insufficient level of ion-magnetization on b = RL/RB ≈ 1 [7]. Since in the given case ion-Larmor radius RL ≈ 10 cm and we register Jet propagation up to the length L ≈ 5060 cm (see Figure  1a,b), we can conclude that also in the case of low-level of ion-magnetization, the Jets could propagate in the Regime of Long-Range Propagation of Large-Scale Jets [12].   Figure 2). The Line corresponds to the velocity 120 km/s, rather close to the initial velocity of LPP expansion (V0 ~ 100 km/s) in the full 4-beams scheme [10], in contrast to our previous data [7], with multi-kGs field and with high-level of ion-magnetization on b≈0,3<1, While in the given case we have b ≈ 1, only.