1.Introduction

Laser plasma propulsion is a new type of propulsion technology developed with the aim of small satellites performing new tasks at lower cost than those conventional satellites[1–6].During laser plasma propulsion,the laser irradiation of a target surface yields strong plasma ejection and a thrust force is induced in the opposite direction.In this case,the properties of the propellant(metals,polymers and liquids)have a remarkable effect on the thrust performance[7–13].Liquid propellants have high thrust and high coupling coefficient and are therefore considered very promising materials.Nevertheless,for all liquid propellants,serious sputtering behavior is observed in ablation,and a consumed volume percent is up to 95%.Almost all of the liquids are unionized and ejected with the plasma generation,which results in an extremely low specific impulse only tens of seconds[8,9].Many methods have been employed in an attempt to increase the specific impulse as well as the coupling coefficient.For example,the correlation between the viscosity of a liquid and the propulsion Efficiency has been determined[14,15].Moreover,the carbon powder(the diameter between 100 and 130nm)has been proposed as a doping material to increase the liquid viscosity so as to reduce the splashing[12,16].Considering both the high doping and the high absorption ability of carbon,carbon-doped glycerol has recently been proposed[12].

PFS投加量为1000 mg/L，设定交流变频磁场强度12 mT、磁化频率130 Hz，探究磁化时间对出水余铁及亚铁含量和pH值的影响，结果如图8所示。由图8可知，当磁化时间为5 min时，废水处理效果最佳；继续延长磁化时间，出水的余铁和亚铁含量呈增加趋势。因此，选择磁化时间为5 min较适宜。

In this study,liquid glycerol doped with carbon powder was used as a propellant for ablation and a nanosecond pulse laser with a wavelength of 1064nm was used as the ablation source.The coupling coefficient,specific impulse as well as the droplet distribution after ablation were measured.Furthermore,the ablation characteristics are discussed based on the volume consumption and droplet distribution.

综上所述，随着科学技术不断的深入与发展，先进的科技在农业生产领域得到极大的应用，在一定程度上推动着农业的发展。信息技术在高粱种植当中应用，不但可以利用信息技术进行高粱种植，而且还可以利用其进行病虫害的预防与治理。在高粱的种植过程当中，要做好高粱各个时期的管理，为高粱生长提供良好的环境。病虫害对于高粱的生长具有重要的影响，并且极大的降低了高粱的产量。基于此情况下，高粱种植人员需要做好病虫害的防治管理，在此过程当中，主要通过农业防治、生物防治、物理防治等手段防治高粱病虫害，降低病虫害所带来的影响，进而提升高粱的整体产量。

2.Experiments

A 3 × 8 × 5mm3aluminum cuboid container was used in the experiment.The liquid glycerol was placed in a cuboid cavity with a depth of 2.5 mm and a diameter of 1.5 mm.The pulsed laser used in the experiment is a Q-switched Nd:YAG laser.The width of the pulse was 10 ns and the wavelength was 1064 nm.The size of the laser beam is about 8mm and divergence is less 1mrad.The laser was directly focused to the cavity bottom by a focal lens(f=200 mm,Φ=50 mm).A schematic of the target velocity measurement and laser focus into cavity shown in figure 1.

Figure 1.A schematic diagram of an experimental device for measuring two beams of target speed.The illustration is a schematic diagram of laser pulse focusing to the cavity bottom.

Therefore,the higher the laser intensity,the less the volume was consumed.This is because the doped-carbon inhibits laser beam focus on the glycerol interior.And this inhibition leads to variations of the laser intensity,which yield in turn different coupling coefficients and specific impulses.As the plasma is induced on the glycerol surface,the laser intensity is lower than that of induced in the interior.A lower mass ejection results in a low coupling coefficient.

Typical images of the droplet distribution on the glass layer are shown in figure 6.For pure glycerol,the pulse can be directly focused on the cavity bottom.However,for 1%carbon doping,the carbon inhibits the pulse penetration and the laser is induced on the glycerol surface.Under this case,the laser spot is larger about 1mm,and the intensity accordingly become slower about 1.7 × 109W cm−2.It can be seen that the distribution is correlated with both the laser intensity and the doped-carbon.

3.Results and discussions

For example,the average size and distribution area of the pure glycerol shown in figures 6(a)–(f)are both smaller than those of the carbon-doped glycerol,shown in(A)–(F).Moreover,compared with that of its non-doped counterpart,the size of the carbon-doped glycerol is more sensitive to the laser intensity.This is especially evident in figure 6(c),which reveals that the droplets begin to agglomerate into a large droplet,and the size of some droplets is approximately 0.6mm.Droplet agglomeration is attributed to carbon doping,which increases the viscosity of the glycerol.To lower this viscosity,some water was added to the carbon-doped glycerol during the experiment.The results shown that with increasing water volume,the average size and distribution area approach those of the pure glycerol.Furthermore,the doped-carbon causes the laser ablation position vary from the glycerol interior to the surface,resulting in plasma induced on the surface and a nonuniform droplet distribution.

Figure 2.Target momentum induced by laser pulses at different incident laser intensity for ablation 1%carbon-doped glycerol.

Interestingly,it is noted that when the laser intensity increased from 0.9 × 109to 1.7 × 109W cm−2,the target momentum decreases from 6.5 to 3.3 gcms−1.The target momentum increases slightly only at intensity value over 1.7 × 109W cm−2.In order to investigate this phenomenon,the coupling coefficient and the specific impulse were measured as shown in figure 3.It is well-known that the coupling coefficient is defined as the ratio of the target momentum to the incident laser energy.And the specific impulse is the ratio of the target momentum to the consumed glycerol mass.In this experiment,different intensities were realized by changing the incident laser energy for the same focal area.It can be seen from figure 3 that the coupling coefficient accordingly decreases from 145 to 41 dyneW−1with the increasing intensity.For this case,it can be deduced from the definition of the coupling coefficient that the decrease of the momentum is attributed to the decrease of the coupling coefficient.

Figure 3.Coupling coefficient(Cm)and specific impulse(Isp)generation at different intensities with 1%carbon-doped glycerol.

As the figure 3 shows that laser intensity has an obvious impact on the specific impulse and only a few seconds of the specific impulse are observed.Furthermore,the relationship of the coupling coefficient and the specific impulse is in disagreement with the inverse relationship between them[19].With increasing laser intensity,they have a similar tendency.This is because,for liquid ablation,the glycerol ejection mainly determine the target momentum rather than the induced plasma.In laser ablation,especial for liquid propellant,the target momentum or thrust force depends largely on the ejection of the unionized droplets.The effect of other phenomena such as boiling and plasma itself is minor.

3.2 化学防治 梨树芽萌动时，全树喷1次具有铲除作用的10～20倍液尿素，杀死在芽鳞内越冬的菌丝，对减少初侵染源有明显效果。

Carbon at high wavelengths of 1064nm showed high absorption[22].At medium laser intensity,plasma shields more laser energy interaction with glycerol.At higher intensity,this plasma shielding effect increases due to the ionized ambient gas localized in the front of the plume[7].Thus,the target momentum and coupling coefficient both decrease.The shielding effect is related to the duration of the laser pulse[23],i.e.,the shielding effect becomes more intense with the pulse duration increasing.For the nanosecond laser pulse used in the current experiment,the plasma is induced by the pulse front edge,and the tail part of the pulse will interact with the plasma.This prevents direct interaction between the energy and the glycerol,resulting in low glycerol consumption.Based on the definition of the specific impulse,it is expected that a high specific impulse is obtained for a low mass at a given target momentum.Therefore,a high intensity and a long laser pulse are anticipated for a high specific impulse.

Figure 4.Ablation Efficiency generation at different intensities with 1%carbon-doped glycerol.

Figure 5.Consumption mass of glycerol ablated by laser pulse with different laser intensities with 1%carbon content.The inset is the percent consumed from the whole glycerol volume.

As shown in figure 1,the target velocity was the ratio of the distance between the two beams(Dl)to the time width(Dt)between the two signals.Under this case,the distanceDl was easier to be controlled than previous devices[17,18],in which two parallel beams and one probe beam were used.

Figure 6.Distribution of the droplets for pure glycerol(a)–(f)and carbon-doped glycerol with 1%content(A)–(F)ablation by laser intensity of 0.9 × 109W cm−2(a/A),1.0 × 109W cm−2(b/B),1.28 × 109W cm−2(c/C),1.52 × 109W cm−2(d/D),1.7 × 109W cm−2(E/e),1.96 × 109W cm−2(f/F).

It is found that some carbon-doped glycerol still remains in the cavity after ablation,and the residual volume increases with laser intensity increasing.Figure 5 shows that the consumption volume and the relative percentage(shown in the inset)both decrease with increasing laser intensity.In fact,the percentage of 85%and 72%occurrence at intensities of 0.9 × 109and 1.0 × 109W cm−2,indicate that a Significant fraction of the volume is consumed at low intensity.

During laser pulse interaction with target,target properties have a Significant effect on the thrust.Difference from the solid target,a high coupling coefficient is generally obtained due to the splashing behavior and most of the unionizedliquid ejection.Companying with the liquid splashing,a very low specific impulse is obtained.However,this value can reach a few hundred for ablation solid propellants[9].For low viscosity liquids,such as water,the coupling coefficient and specific impulse always present a proportional relationship[20].If the thrust source is only coming from the plasma expansion,an inverse relationship is presented between the coupling coefficient and the specific impulse.For example,during laser ablation of a solid target in a vacuum condition,specific impulse exhibits a strong inverse proportionality to the coupling coefficient[21].In current experiment,the ablation Efficiency at different laser intensity varies as shown in figure 4.It changes from 2.7%to 0.5%under the experimental intensities.And the thrust comes from both the plasma expansion and the ejection of the unionized droplets.

The ablation consumption induced by the laser pulse was directly measured by a balance with an accuracy of 0.01mg.Because the mass loss is very minor for every shot,every experimental data was averaged by 10 times.The carbon powder were directly added to the glycerol.Prior to the ablation,the composite liquid was dispersed by a magnetic stirrer for 10 min,the contents were 0.1,0.5,1,3 and 5wt%,respectively.In order to obtain the distribution,a glass layer of 1mm(thickness) × 40mm(diameter)from the target surface of 45 mm was placed as the reception screen.The size,morphology and distribution area of the droplets were obtained.

Figure 2 shows the target momentum induced by laser pulses at different incident laser intensities for ablation 1% carbondoped glycerol.

在DSP中实现H.264的编码器优化，优化过程主要为4步：算法优化、C代码优化、线性汇编的优化和CCS编译器选项优化[11]。优化过程如图7所示。

Figure 7.Coupling coefficient(Cm)and the generation of specific impulse(Isp)at different carbon contents.

Figure 7 shows the dependence of the specific impulse and coupling coefficient on the carbon contents.On one hand,both the specific impulse and coupling coefficient are related to the target momentum,so a similar tendency is presented.On the other hand,the carbon content has a strong effect on both of them.For example,the coupling coefficients are 374 and 52dyne W−1,the specific impulse are 10.7 and 1.5 s for pure glycerol and 5%carbon-doped glycerol respectively.

供试玉米新品种有科河699、大民3307，以先玉335作为本试验对照品种，参试品种均由民勤县红沙梁镇农技站提供。

4.Conclusions

The target momentum,coupling coefficient,specific impulse and droplet distribution were determined for nanosecond pulse laser ablating carbon-doped glycerol.The results showed that the effect plasma shielding had a Significant effect at high laser intensity.In this case,the laser energy interacted with the laser plasma rather than with the glycerol,resulting in low consumption of the glycerol.Moreover,the doped-carbon also reduced the splashing of the glycerol and gave rise to variations in the droplet size and distribution.The definition of specific impulse suggests that at a given target momentum,a high specific impulse and a low consumption may be anticipated.

This work was supported by National Natural Science Foundation of China(Grant Nos.10905049,51472224)and Fundamental Research Funds for the Central Universities(Grant Nos.53200859165 and 2562010050).

References

[1]Phipps C R et al 2012 Adv.Space Res.49 1283

[2]Phipps C R 2014 Acta Astronaut.93 418

[3]Ye J F et al 2011 Chin.Opt.4 319(in Chinese)

[4]Kudryashov S I et al 2011 Appl.Phys.Lett.98 254102

[5]Bulgakova N M et al 2011 Appl.Surf.Sci.257 10876

[6]Kudryashov S I,Lyon K and Allen S D 2006 J.Appl.Phys.100 124908

[7]Lorazo P,Lewis L J and Meunier M 2006 Phys.Rev.B 73 134108

[8]Zheng Z Y et al 2014 Appl.Phys.A 115 1439

[9]Zheng Z Y et al 2006 Appl.Phys.A 83 329

[10]Ahmad M R et al 2015 Laser Phys.Lett.12 056101

[11]Mazzi A,Gorrini F and Miotello A 2017 Appl.Surf.Sci.418 601

[12]Xue Y T et al 2014 High Power Laser Part.Beams 26 101020(in Chinese)

[13]Jamil Y et al 2013 Appl.Phys.A 110 207

[14]Zheng Z Y et al 2012 Chin.Phys.Lett.29 095202

[15]Fardel R et al 2009 Appl.Phys.A 94 657

[16]Zhang Y et al 2008 Appl.Phys.A 91 357

[17]Zheng Z Y et al 2006 Chin.Phys.15 580

[18]Choi S et al 2010 Appl.Phys.A 98 147

[19]Luke J R,Phipps C R and McDuff G G 2003 Appl.Phys.A 77 343

[20]Zheng Z Y et al 2016 Chin.Phys.B 25 045204

[21]Phipps C R et al 2004 Appl.Phys.A 79 1385

[22]Phipps C R Jr et al 1988 J.Appl.Phys.64 1083

[23]Phipps C R et al 1990 Laser Part.Beams 8 281