Magnetic reconnection driven by Gekko XII lasers with a Helmholtz capacitor-coiltargetX. 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 Articles you may be interested in Density-shear instability in electron magneto-hydrodynamics Phys. Plasmas 21, 052110 (2014); 10.1063/1.4879810 Bulk ion acceleration and particle heating during magnetic reconnection in a laboratory plasmaa) Phys. Plasmas 21, 055706 (2014); 10.1063/1.4874331 Two-fluid simulations of driven reconnection in the mega-ampere spherical tokamak Phys. Plasmas 20, 122302 (2013); 10.1063/1.4830104 Three dimensional density cavities in guide field collisionless magnetic reconnection Phys. Plasmas 19, 032119 (2012); 10.1063/1.3697976 Generation of episodic magnetically driven plasma jets in a radial foil Z-pinch Phys. Plasmas 17, 112708 (2010); 10.1063/1.3504221

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Magnetic reconnection driven by Gekko XII lasers with a Helmholtzcapacitor-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, China2University of Chinese Academy of Sciences, Beijing 100049, China3Department of Astronomy, Beijing Normal University, Beijing 100875, China4IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China5Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan6Beijing 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 mag-

netic field were measured to be approximately 1018 cm�3 and 60 T by using Nomarski interferome-

try 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 recon-

nection and reveals its potential in astrophysical plasma applications. VC 2016 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4944928]

I. INTRODUCTION

Producing strong magnetic fields in a laboratory by

using lasers is advantageous for doing research in various

fields of physical science such as materials science,1 atomic

and molecular physics,2 plasma and beam physics,3 and

astrophysics.4–7 One example of its use is simulating plasma

phenomena such as plasma magnetic reconnection (MR).8–13

MR, a topological rearrangement of magnetic field lines, is a

fundamental physics process that occurs in many plasma

phenomena such as solar flares, Earth’s magnetosphere, star

formation, gamma ray bursts, and laboratory-produced

fusion plasma.14–17 The most important feature of MR is that

it can efficiently convert stored magnetic energy into kinetic

energy by accelerating and heating plasma particles, leading

to a new equilibrium configuration at a lower magnetic

energy. MR has been observed in detail in experiments such

as disruptions of tokamak discharges, and the relaxation

processes in reversed field pinch and spheromak plas-

mas.16,18,19 Moreover, laser-driven MR experiments, mostly

done by inducing a self-generated magnetic field on the sur-

face of the laser target through the Biermann battery effect,

have been widely performed in recent years.8–13 These

experiments have revealed many notable features consistent

with MR, such as magnetic field annihilation,9,13 jet forma-

tion,8,10–12 electron energization,12 and the stretched current

sheet.13 However, because high-energy-density laser plas-

mas have complex three-dimensional effects, laser-driven

MR still has issues, including the production of high-speed

jets, the competition of thermal-plasma collisions, and the

reconnection dissipation. The required conditions for laser-

driven MR are an explicitly controlled magnetic field and a

low-beta plasma environment, meaning that the magnetic

pressure is dominant.

Producing a magnetic field by focusing laser beams on a

planar foil target has many generation mechanisms,20 and

compared with this magnetic field, the magnetic field pro-

duced by a coil is simple and explicit. Recently, Fujioka

et al.21 reported that a B-field near 1.5 kT can be produced

by driving a 1-kJ laser into a millimeter-scale capacitor-coil

target, as first proposed by Daido et al.22 Building on this

work, here we produced magnetic reconnection by using

double-turn Helmholtz capacitor-coil targets, making our

experiment quite different from earlier ones.8–13

Here, we use a Helmholtz capacitor-coil target consisting

of two Ni disks connected by two Ni wires bent into circles.

Adding photoionized and Joule-heated plasma near the two

wires moving toward each other, together with the magnetic

fields, could induce magnetic reconnection. Optical probing

revealed an accumulated plasma plume near the magnetic

reconnection outflow. Contrasting the most recent laser-

driven MR experiment, which produced a high plasma beta

value, our design produced a plasma beta value of much lower

than one, which might be similar to the environments of some

astrophysics plasmas such as the magnetosphere.

II. EXPERIMENT SETUP

Our experiments were performed at the Gekko-XII Laser

Facility. Figure 1 shows the experimental setup and target

configuration. The target, made of Ni, consisted of two disksa)Electronic addresses: jyzhong@bnu.edu.cn and gzhao@bao.ac.cn

1070-664X/2016/23(3)/032125/5/$30.00 VC 2016 AIP Publishing LLC23, 032125-1

PHYSICS OF PLASMAS 23, 032125 (2016)

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connected by two U-turn coils. The distance between the two

disks was 600 lm, and the thickness was 100 lm. Both the

thickness and width of the U-turn coils were 100 lm, the dis-

tance between the connecting coils was 600 lm, and the

length of the straight-line portion of the U-turn coils was 900

lm. We used laser beams to ablate the rear disk of the target

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 pro-

duced 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 opti-

cal probing, including shadowgraphy and interferometry,

with a 10-ns probe laser beam (kL¼ 0.532 lm). The B-field

was measured by assessing the Faraday rotation of the polar-

ization 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 V¼ 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

the two perpendicular components of the probe beam inten-

sity, separated by a Wollaston prism and recorded by a time-

resolved optical streak camera. The MR occurred between

the two coils, and the self-emission of the reconnection out-

flow was also imaged by an intensified charge-coupled de-

vice (ICCD) camera.

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 h¼L�V�B, where L is the

length of the optical path in a birefringent medium:

L¼ 0.5 mm in our experiment. Figure 2(a) shows the streak

image used to measure the magnetic field and gives the evolu-

tion of the horizontal IH and vertical IV components of the

probe beam, from which we could determine the intensity ra-

tio as IH/(IHþ IV). Figure 2(b) shows the variation of the in-

tensity ratio.

First, the large variation, as noted by the dashed box in

Fig. 2(b), was caused by the main lasers and the probe light

overlapping with the main lasers during 7.0–7.5 ns. Then, af-

ter the lasers irradiated the target, the intensity ratio varied

twice, as noted by the black and red triangles in Fig. 2(b),

which we believe, were caused by Faraday rotation. The two

rotations occurred are probably because the circuit currents

in the two U-turn coils were not synchronized, so both of the

rotations were caused by the hot-electron current produced

by the electrical potential of the two disks. Alternatively, the

first rotation was caused by the synchronized circuit currents

in the two coils, and the second rotation was caused by the

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

front disk. Shadowgraphy and interferometry assess the plasma evolution by

using a long-pulse (10 ns) probe beam (shown as a green line). MR occurs

between the two coils and is also detected by visible self-emission cameras.

The inset shows the topology of the magnetic reconnection in the Helmholtz

coil target.

FIG. 2. (a) Shadow image from the optical streak camera, showing the evo-

lution of the horizontal component IH and vertical component IV of the probe

beam. (b) The variation of the probe beam’s intensity ratio IH/(IHþ IV). The

large variation, shown by the dashed box, was caused by the main lasers and

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-

rent along the coils, and the second rotation, noted by the red triangle, is

caused by the reconnection electric field accelerating the electrons.

032125-2 Pei et al. Phys. Plasmas 23, 032125 (2016)

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reconnection electric field accelerating the electrons, which

we will explain later.

Using the first variation of the ratio and the relationship

between the ratio and the angle of the polarization plane IH/

(IHþ IV)¼ cos h/(cos hþ sin h), we determined the rotation

angle h: h¼ 1.67�. In addition, even though the optical probe

first passed through some plasma, the density of the plasma

was too low to affect the probe’s rotation. Thus, the rotation

of the optical probe within the plasma can be neglected.

Using h¼L�V�B, we calculated the B-field where the

TGG is placed as 0.29 T.

Using the value of the B-field at 3.6 mm from the middle

plane of the two coils, we easily obtained the B-field distri-

bution of the double-turn coils by using Radia code.23 This

code can simulate the initial shape of the double-turn coils

with the same dimensions and material as the actual target.

Figure 3(a) shows the B-field distribution computed in the

X-Y plane of the double-turn coils. The simulated magnetic

field lines at the middle plane of the coils (Z¼ 0) have oppo-

site directions, and along the coil direction, the B-field is

zero. The calculated B-field at 0.25 mm from the coil center

reaches as high as 60 T.

In Figure 2, the magnetic field first develops at �1.5 ns

after the lasers fire, which agrees with a previous experiment21

that used only one coil. In the present case, after the magnetic

field was produced first, the plasmas from the ionization of

the two coils move towards the reconnection region, which is

300 lm from the coils. As mentioned before, the second

Faraday rotation may have been caused by the electrons’

directional movement accelerated by the reconnection electric

field. After magnetic reconnection occurred, the magnetic

field changed quickly and induced the electric field,24,25 which

was perpendicular to the magnetic reconnection plane and

exactly along the U-turn coil, as shown in Figure 3(b). The

electrons accelerated by the electric field moved along the

U-turn direction and then induced the magnetic field, which

caused Faraday rotation again. The inflow plasma velocity is

assumed from interferometry.

In our experiment, we measured the evolution of the

plasma around the coils by using interferometry, changing

the optical probing beam’s delay time to 1, 2, or 3 ns. At 1

and 2 ns, the plasma around the coils was almost not pro-

duced. At 3 ns, the fringes shifted obviously, as shown in

Figure 4(a). Based on the displacement distance of these

fringes, �300 lm, and the time interval of 1 ns, we deduced

the velocity of the inflows to be �300 km/s. The magnetic

reconnection will occur after 1 ns, which is consistent with

the experimental result shown in Figure 2. The second rota-

tion component is the first time an induced electric field has

been measured in a laser-driven magnetic reconnection

experiment.

Figure 4(a) shows an optical Nomarski interferometry

image, taken at 3 ns after laser irradiation. The coils of the tar-

get become expanded because of the X-ray radiation from the

disks and the Joule effect; the probe beam could not pass

through them, but the fringes outside the coils shifted dramati-

cally, which indicates a change in density. The electron density

outside the coils, ne, can be inferred from the interferograms.

To extract the phase data from the interferograms, we used the

Interferometric Data Evaluation Algorithms (IDEA) suite of

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 inter-

ferogram; the electron density outside the coils (noted by the

yellow arrow) is on the order of 1018cm�3. 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 experi-

ments. 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 mag-

netic 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

and released to accelerate and heat the plasma particles.

Because the reconnection outflows in the coil can interact

FIG. 3. (a) 3D magnetostatic model in the X–Y plane (Z¼ 0), simulated by

Radia code. (b) Diagram of the reconnection electric field along the U-turn

current direction.

032125-3 Pei et al. Phys. Plasmas 23, 032125 (2016)

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with the plasma from the disks, it produces an outflow with

much greater density and energy. Moreover, the reconnection

outflows will accumulate gradually inside the coils and even-

tually form the structure shown in Figure 5. Additionally, we

estimate the plasma beta value near the coil as b¼ pe/(B2/

2l0)¼ nekTe/(B2/2l0), where the electron density is ne

¼ 1024 m�3, the Boltzmann constant is k¼ 1.38� 10�23J/K,

and generally the electron temperature from the coils is

<100 eV. Even if we choose an electron temperature of

Te¼ 100 eV¼ 1.16� 106 K and a B-field of 50 T, the esti-

mated plasma beta value is �0.016, and it will be even

smaller at a lower electron temperature. This low beta value

indicates that in such a plasma environment, the plasma mag-

netic pressure dominates the plasma thermal pressure, causing

a fast magnetic reconnection.

At the bottom of the coils, the magnetic reconnection

x-point structure should have occurred when viewed from

the Z–Y plane. However, we did not observe such a structure

because the electron density outside the coils was too low. In

a future experiment, we would add a foil away from the bot-

toms of the coils to produce additional magnetized low-

density plasmas.

IV. CONCLUSION

This is the first laboratory-scale experimental study of

magnetic reconnection with an explicitly controlled magnetic-

field environment produced by a double-turn Helmholtz

capacitor-coil target. Using the Faraday effect, we measured a

B-field of 60 T around the coil. We also measured in this

strong magnetic field an accumulated plasma plume, which

appeared near the magnetic reconnection outflow. The

observed Faraday rotation indirectly proves the existence of a

reconnection electric field in a low beta magnetic reconnection,

which suggests potential applications in astrophysics and

plasma physics, such as in studying reconnection in solar flares

and magnetotail.

ACKNOWLEDGMENTS

We are very grateful to the Gekko-XII laser operation

staff and the target preparation staff for their valuable help

and support. This work was supported by the National

Basic Research Program of China (973 program) (Grant

No. 2013CBA01500), the National Natural Science

Foundation of China (Grant Nos. 11273033, 11135012,

FIG. 4. (a) An interferogram taken at

3 ns after laser irradiation; (b) distribu-

tion 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.

032125-4 Pei et al. Phys. Plasmas 23, 032125 (2016)

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11220101002, and 11205015), and the Beijing Nova Program

(Z131109000413050). This work was also supported by the

Collaboration Research Program of the Institute of Laser

Engineering at Osaka University and by the Bilateral Program

(Grant No. 11311140164) for Supporting International Joint

Research between the National Natural Science Foundation of

China (NSFC) and the Japan Society for the Promotion of

Science (JSPS). F. L. Wang is supported by the Youth

Innovation Promotion Association CAS.

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