Magnetic Reconnection Driven by Gekko XII Lasers with a Helmholtz Capacitor-Coil Target

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Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coil target

Article in Physics of Plasmas · March 2016 DOI: 10.1063/1.4944928

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All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Youichi Sakawa letting you access and read them immediately. Retrieved on: 04 November 2016 Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coil target X. X. Pei, J. Y. Zhong, Y. Sakawa, Z. Zhang, K. Zhang, H. G. Wei, Y. T. Li, Y. F. Li, B. J. Zhu, T. Sano, Y. Hara, S. Kondo, S. Fujioka, G. Y. Liang, F. L. Wang, and G. Zhao

Citation: Physics of Plasmas 23, 032125 (2016); doi: 10.1063/1.4944928 View online: http://dx.doi.org/10.1063/1.4944928 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/23/3?ver=pdfcov Published by the AIP Publishing

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Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coil target X. X. Pei,1,2 J. Y. Zhong,3,4,a) Y. Sakawa,5 Z. Zhang,6 K. Zhang,1 H. G. Wei,1 Y. T. Li,4,6 Y. F. Li,6 B. J. Zhu,6 T. Sano,5 Y. Hara,5 S. Kondo,5 S. Fujioka,5 G. Y. Liang,1 F. L . Wang,1 and G. Zhao1,a) 1Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China 2University of Chinese Academy of Sciences, Beijing 100049, China 3Department of Astronomy, Beijing Normal University, Beijing 100875, China 4IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China 5Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan 6Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China (Received 4 November 2015; accepted 16 March 2016; published online 25 March 2016) We demonstrate a novel plasma device for magnetic reconnection, driven by Gekko XII lasers irradiating a double-turn Helmholtz capacitor-coil target. Optical probing revealed an accumulated plasma plume near the magnetic reconnection outﬂow. The background electron density and magnetic ﬁeld were measured to be approximately 1018 cm3 and 60 T by using Nomarski interferometry and the Faraday effect, respectively. In contrast with experiments on magnetic reconnection constructed by the Biermann battery effect, which produced high beta values, our beta value was much lower than one, which greatly extends the parameter regime of laser-driven magnetic reconnection and reveals its potential in astrophysical plasma applications. VC 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4944928]

I. INTRODUCTION jets, the competition of thermal-plasma collisions, and the reconnection dissipation. The required conditions for laser- Producing strong magnetic fields in a laboratory by driven MR are an explicitly controlled magnetic field and a using lasers is advantageous for doing research in various low-beta plasma environment, meaning that the magnetic fields of physical science such as materials science,1 atomic pressure is dominant. and molecular physics,2 plasma and beam physics,3 and Producing a magnetic field by focusing laser beams on a astrophysics.4–7 One example of its use is simulating plasma planar foil target has many generation mechanisms,20 and phenomena such as plasma magnetic reconnection (MR).8–13 compared with this magnetic field, the magnetic field pro- MR, a topological rearrangement of magnetic field lines, is a duced by a coil is simple and explicit. Recently, Fujioka fundamental physics process that occurs in many plasma et al.21 reported that a B-field near 1.5 kT can be produced phenomena such as solar flares, Earth’s magnetosphere, star by driving a 1-kJ laser into a millimeter-scale capacitor-coil formation, gamma ray bursts, and laboratory-produced target, as first proposed by Daido et al.22 Building on this fusion plasma.14–17 The most important feature of MR is that work, here we produced magnetic reconnection by using it can efficiently convert stored magnetic energy into kinetic double-turn Helmholtz capacitor-coil targets, making our energy by accelerating and heating plasma particles, leading experiment quite different from earlier ones.8–13 to a new equilibrium configuration at a lower magnetic Here, we use a Helmholtz capacitor-coil target consisting energy. MR has been observed in detail in experiments such of two Ni disks connected by two Ni wires bent into circles. as disruptions of tokamak discharges, and the relaxation Adding photoionized and Joule-heated plasma near the two processes in reversed field pinch and spheromak plas- wires moving toward each other, together with the magnetic mas.16,18,19 Moreover, laser-driven MR experiments, mostly fields, could induce magnetic reconnection. Optical probing done by inducing a self-generated magnetic field on the sur- revealed an accumulated plasma plume near the magnetic face of the laser target through the Biermann battery effect, reconnection outflow. Contrasting the most recent laser- have been widely performed in recent years.8–13 These driven MR experiment, which produced a high plasma beta experiments have revealed many notable features consistent value, our design produced a plasma beta value of much lower with MR, such as magnetic field annihilation,9,13 jet forma- than one, which might be similar to the environments of some tion,8,10–12 electron energization,12 and the stretched current astrophysics plasmas such as the magnetosphere. sheet.13 However, because high-energy-density laser plasmas have complex three-dimensional effects, laser-driven II. EXPERIMENT SETUP MR still has issues, including the production of high-speed Our experiments were performed at the Gekko-XII Laser Facility. Figure 1 shows the experimental setup and target a)Electronic addresses: [email protected] and [email protected] configuration. The target, made of Ni, consisted of two disks

1070-664X/2016/23(3)/032125/5/$30.00 23, 032125-1 VC 2016 AIP Publishing LLC

III. RESULT AND DISCUSSION The Faraday rotation angle h is linearly proportional to the magnetic field component in the direction of the probe beam incident according to h ¼ L V B,whereL is the length of the optical path in a birefringent medium: L ¼ 0.5 mm in our experiment. Figure 2(a) shows the streak image used to measure the magnetic field and gives the evolution of the horizontal IH and vertical IV components of the probe beam, from which we could determine the intensity ratio as IH/(IH þ IV). Figure 2(b) shows the variation of the intensity ratio. FIG. 1. Experimental setup: Two bunches of long-pulse (0.5 ns) lasers are focused onto the rear disk of a Helmholtz coil target through a hole in the First, the large variation, as noted by the dashed box in front disk. Shadowgraphy and interferometry assess the plasma evolution by Fig. 2(b), was caused by the main lasers and the probe light using a long-pulse (10 ns) probe beam (shown as a green line). MR occurs overlapping with the main lasers during 7.0–7.5 ns. Then, af- between the two coils and is also detected by visible self-emission cameras. ter the lasers irradiated the target, the intensity ratio varied The inset shows the topology of the magnetic reconnection in the Helmholtz coil target. twice, as noted by the black and red triangles in Fig. 2(b), which we believe, were caused by Faraday rotation. The two connected by two U-turn coils. The distance between the two rotations occurred are probably because the circuit currents disks was 600 lm, and the thickness was 100 lm. Both the in the two U-turn coils were not synchronized, so both of the thickness and width of the U-turn coils were 100 lm, the dis- rotations were caused by the hot-electron current produced tance between the connecting coils was 600 lm, and the by the electrical potential of the two disks. Alternatively, the length of the straight-line portion of the U-turn coils was 900 first rotation was caused by the synchronized circuit currents lm. We used laser beams to ablate the rear disk of the target in the two coils, and the second rotation was caused by the through a hole in the front disk with a diameter of 1600 lm. These experiments employed a laser configuration of two beams with wavelength of 1.05 lm, total energy of 1.0 kJ, pulse duration of 0.5 ns, and a 300–lm-diameter focal spot. With an irradiance of 3 1015 W/cm2, the laser beams ablated the rear disk, producing hot electrons with energies exceeding 10 keV in front of the rear disk, contrib- uting to the electrical potential between the two disks; this potential difference drove two synchronized currents in the two U-turn coils. According to the Biot–Savart law, strong magnetic fields are generated near the coils. The magnetic field between the two coils is antiparallel and moves towards each other along with low-density plasmas, which were produced from the coils by X-ray radiation from the disks and Joule heating. The pulsed, oppositely directed B-fields along with those expanded plasmas encounter at the middle plane where the MR can occur, as shown in the inset of Figure 1. The evolution of the plasma was investigated with optical probing, including shadowgraphy and interferometry, with a 10-ns probe laser beam (kL ¼ 0.532 lm). The B-field was measured by assessing the Faraday rotation of the polarization direction of the probe laser beam. The laser probe was incident along the coil axis and passed through a 0.5- mm-thick birefringent terbium gallium garnet (TGG) crystal with a Verdet constant of V ¼ 11.35/(T mm) placed 3.6 mm from the middle plane of the two coils. To protect the TGG crystal from the X-ray radiation of the disks, we placed a 100–lm-thick Ta shield plate between the disks of the target and the TGG. The Faraday effect was assessed by measuring FIG. 2. (a) Shadow image from the optical streak camera, showing the evo- the two perpendicular components of the probe beam inten- lution of the horizontal component IH and vertical component IV of the probe sity, separated by a Wollaston prism and recorded by a time- beam. (b) The variation of the probe beam’s intensity ratio IH/(IH þ IV). The resolved optical streak camera. The MR occurred between large variation, shown by the dashed box, was caused by the main lasers and the two coils, and the self-emission of the reconnection out- the probe light overlapping with the main lasers during 7.0–7.5 ns. The first Faraday rotation, noted by the black triangle, is caused by hot-electron cur- flow was also imaged by an intensified charge-coupled de- rent along the coils, and the second rotation, noted by the red triangle, is vice (ICCD) camera. caused by the reconnection electric field accelerating the electrons.

reconnection electric field accelerating the electrons, which field. After magnetic reconnection occurred, the magnetic we will explain later. field changed quickly and induced the electric field,24,25 which Using the first variation of the ratio and the relationship was perpendicular to the magnetic reconnection plane and between the ratio and the angle of the polarization plane IH/ exactly along the U-turn coil, as shown in Figure 3(b).The (IH þ IV) ¼ cos h/(cos h þ sin h), we determined the rotation electrons accelerated by the electric field moved along the angle h: h ¼ 1.67. In addition, even though the optical probe U-turn direction and then induced the magnetic field, which first passed through some plasma, the density of the plasma caused Faraday rotation again. The inflow plasma velocity is was too low to affect the probe’s rotation. Thus, the rotation assumed from interferometry. of the optical probe within the plasma can be neglected. In our experiment, we measured the evolution of the Using h ¼ L V B, we calculated the B-field where the plasma around the coils by using interferometry, changing TGG is placed as 0.29 T. the optical probing beam’s delay time to 1, 2, or 3 ns. At 1 Using the value of the B-field at 3.6 mm from the middle and 2 ns, the plasma around the coils was almost not pro- plane of the two coils, we easily obtained the B-field distri- duced. At 3 ns, the fringes shifted obviously, as shown in bution of the double-turn coils by using Radia code.23 This Figure 4(a). Based on the displacement distance of these code can simulate the initial shape of the double-turn coils fringes, 300 lm, and the time interval of 1 ns, we deduced with the same dimensions and material as the actual target. the velocity of the inflows to be 300 km/s. The magnetic Figure 3(a) shows the B-field distribution computed in the reconnection will occur after 1 ns, which is consistent with X-Y plane of the double-turn coils. The simulated magnetic the experimental result shown in Figure 2. The second rota- field lines at the middle plane of the coils (Z ¼ 0) have oppo- tion component is the first time an induced electric field has site directions, and along the coil direction, the B-field is been measured in a laser-driven magnetic reconnection zero. The calculated B-field at 0.25 mm from the coil center experiment. reaches as high as 60 T. Figure 4(a) shows an optical Nomarski interferometry image, taken at 3 ns after laser irradiation. The coils of the tar- In Figure 2, the magnetic field first develops at 1.5 ns after the lasers fire, which agrees with a previous experiment21 get become expanded because of the X-ray radiation from the disks and the Joule effect; the probe beam could not pass that used only one coil. In the present case, after the magnetic through them, but the fringes outside the coils shifted dramati- field was produced first, the plasmas from the ionization of cally, which indicates a change in density. The electron density the two coils move towards the reconnection region, which is outside the coils, n , can be inferred from the interferograms. 300 lm from the coils. As mentioned before, the second e To extract the phase data from the interferograms, we used the Faraday rotation may have been caused by the electrons’ Interferometric Data Evaluation Algorithms (IDEA) suite of directional movement accelerated by the reconnection electric tools.26 While the expanding plasmas are not perfectly sym- metric, we used the cylindrical symmetry approximation to ap- proximate the electron density profile based on the Abel inversion technique. Figure 4(b) shows the two-dimensional electron density map, calculated by Abel inverting the interferogram; the electron density outside the coils (noted by the yellow arrow) is on the order of 1018 cm3. Considering sym- metric expansion, the density of background plasma inside and outside of the coils should be on the same order. Figure 5 shows typical shadow images and self- emission images (450 nm) of the Helmholtz target experiments. At a delay of 10 ns, the coil becomes expanded, as shown in Figure 5(a), just like in the interferometry image in Figure 4(a). The outflows accumulated at the interior of the coils, which appears from the two orthogonal directions shown in Figures 4(a) and 4(b); in this region, there is also strong self-emission, as shown in Figure 5(c). When magnetic reconnection occurred, the outflows moved towards the interior of the coils, collided with each other, and produced the accumulated plasma plume at high temperature. While the outflow plasmas spread quickly outside the coil, it is dif- ficult to measure because its density becomes too low. The magnetic field near the coils has been estimated by simulations to be tens of Tesla. When a low-density plasma expands from the surfaces of the Ni coils, two bent magnetic fields move together with a surrounding plasma. By magnetic reconnection process, the stored magnetic energy dissipated FIG. 3. (a) 3D magnetostatic model in the X–Y plane (Z ¼ 0), simulated by Radia code. (b) Diagram of the reconnection electric field along the U-turn and released to accelerate and heat the plasma particles. current direction. Because the reconnection outflows in the coil can interact

FIG. 4. (a) An interferogram taken at 3 ns after laser irradiation; (b) distribution of the electron density outside the coils.

FIG. 5. (a) and (b) Shadow images taken from two orthogonal directions at a delay of 10 ns. (c) Self-emission image taken at a delay of 10 ns.

with the plasma from the disks, it produces an outflow with IV. CONCLUSION much greater density and energy. Moreover, the reconnection This is the first laboratory-scale experimental study of outflows will accumulate gradually inside the coils and even- magnetic reconnection with an explicitly controlled magnetic- tually form the structure shown in Figure 5. Additionally, we field environment produced by a double-turn Helmholtz estimate the plasma beta value near the coil as b ¼ p /(B2/ e capacitor-coil target. Using the Faraday effect, we measured a 2l ) ¼ n kT /(B2/2l ), where the electron density is n 0 e e 0 e B-field of 60 T around the coil. We also measured in this ¼ 1024 m3, the Boltzmann constant is k ¼ 1.38 1023J/K, strong magnetic field an accumulated plasma plume, which and generally the electron temperature from the coils is appeared near the magnetic reconnection outflow. The <100 eV. Even if we choose an electron temperature of observed Faraday rotation indirectly proves the existence of a T ¼ 100 eV ¼ 1.16 106 KandaB-field of 50 T, the esti- e reconnection electric field in a low beta magnetic reconnection, mated plasma beta value is 0.016, and it will be even which suggests potential applications in astrophysics and smaller at a lower electron temperature. This low beta value plasma physics, such as in studying reconnection in solar flares indicates that in such a plasma environment, the plasma mag- and magnetotail. netic pressure dominates the plasma thermal pressure, causing a fast magnetic reconnection. ACKNOWLEDGMENTS At the bottom of the coils, the magnetic reconnection x-point structure should have occurred when viewed from We are very grateful to the Gekko-XII laser operation the Z–Y plane. However, we did not observe such a structure staff and the target preparation staff for their valuable help because the electron density outside the coils was too low. In and support. This work was supported by the National a future experiment, we would add a foil away from the bot- Basic Research Program of China (973 program) (Grant toms of the coils to produce additional magnetized low- No. 2013CBA01500), the National Natural Science density plasmas. Foundation of China (Grant Nos. 11273033, 11135012,

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