Performance of high power lasers for rock excavation

Daichi Sugimoto, Proc. SPIE 3887-03 Advanced High-Power Lasers and Applications, Nov. 1999, Osaka Japan

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Performance of high power lasers for rock excavation Daichi Sugimoto¨A, Hayato TanakaB, Masamori Endo A, Shuzaburo TakedaB, Kenzo NanriA, and Tomoo FujiokaA A

Department of Physics, Tokai University, 1117 Kita-Kaname, Hiratsuka City,259-1292, Japan. B Department of Electro-Photo Optics, Tokai University ABSTRACT

Rock excavation experiment with a 10kW–class CO2 laser was demonstrated as a basic study for field application of high power lasers. Sample rocks used in this experiment as a workpiece were tuff breccia and granite. Effect of assist gases on the excavation rate was surveyed. Oxygen, nitrogen, and air were examined and found not to be useful. It was because the gas flow could not blow the molten rocks off, but only helps to cool it down in case the hole reaches certain depth. Excavation rate on both rocks for a various output powers was measured to determine thermal constants inherent to each rock. It was found that the excavation rate resulted in slower, as the hole becomes deeper, because of the deterioration in evacuation efficiency of the molten rock. Thermal parameters of the both rocks were derived from the experimental results. Using simplified thermal balance model, it was estimated that a 50 kW–class mobile laser system has a potential to outperforms the conventional mechanical excavation technique. Keywords: rock excavation, high power laser, civil engineering, laser drilling, CO2 laser

1. INTRODUCTION It is still vivid in our memory that in 1997, there was a disastrous tunnel–collapse accident at Toyohama, Hokkaido.1 It took for a long time (four days) to prepare many holes to dynamite huge rock, because it was a quite difficult area to perform the operations. Even now, there are more than one hundred of hazardous areas remaining in Hokkaido, where huge rocks are in a critical condition to be collapsed. Therefore, removing these rocks safely and efficiently is an urgent task. However, there has been no appropriate method of drilling or excavating these rocks for the use of dynamites or some static fracture materials under such a hazardous circumstances, since the shocks and vibrations of an ordinary cutting/drilling techniques may cause a second accident during the operation. High power laser devices have been identified as a potentially suitable tool for civil engineering processes such as cutting, drilling and welding. The advantages of using lasers over the conventional techniques are: (1) treatment is noiseless (2) no mechanical vibrations and no repulsive forces is generated due to the thermal–energy based material removal, (3) no land reaction wall is necessary (4) capability of increased tool–to–workpiece (standoff) distance because of a non–contact processing, (5) low production of secondary waste, and (6) easy integration into robotics systems for remote operation by virtue of a lightweight laser head. Because of these advantages, the high power laser system seems to be the only solution, when it comes to a civil engineering in difficult areas. However, only a few papers2,3 on the laser based rock excavation, which has been done in the past has been published in comparison with the studies on the metal processing. 4-6 One of the reasons may be the lack of capability for existing high power lasers to be integrated into a mobile system. Several research groups have developed chemical oxygen–iodine laser (COIL),7-11 whose unique pumping mechanism is suitable for a mobile system and the practical use of mobile–COIL system is now on the horizon. Therefore, our motivation is focusing onto the mobile laser system for field applications or civil engineering with the COIL in mind. We choose, however, a commercial high power CO2 laser for the first stage of the experiment, because of its availability. ¨

Correspondence: Email: [email protected]; Telephone: +81–463–58–1211 Ext. 3721; Fax: +81–463–50–2013.


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The objectives of this study is to identify rock excavation capability of the high power lasers and to analyze the interaction between rocks and the laser beam using a simplified thermal balance model, which allows one to estimate the laser power required for practical applications.

2. THEORY OF LASER EXCAVATION The nature of the laser rock excavation process is illustrated in Fig.1. Following assumptions have been made to derive a theoretical model for an interaction between a laser beam and material. (1) The laser beam is assumed to be cylindrical because large focal length may be justified in the field applications. (2) Part of the laser beam turns into heat due to absorption, and the material begins to melt down creating a so–called hotspot as the temperature reaches its vitrification temperature. (3) Then, part of the hotspot is eventually evacuated by the vaporization from the upper surface, and hotspot moves downward with a constant velocity vc. (4) Part of the thermal energy stored in the hotspot dissipates through its surface at a constant rate. (5) Because heat balance is time independent, the hotspot does not change its thickness Dh and diameter d1. (6) Thermal radiation from the material surfaces is negligibly small. (7) Heat conductivity, thermal diffusion rate, and some other coefficients are assumed to be temperature independent. (8) Temperature inside the hotspot is uniformly distributed and is assumed to be at the vitrification temperature point.

Heat conduction

Hot spot

Dh

v d

Heat conduction

Laser beam Evacuated vapor and molten rock Fig. 1 Schematic drawing of the simplified beam–rock interaction model.

Therefore, an equation for the heat balance laser beam and material interaction is given by,

d eP = p ( ) 2 v d C - pdsDTDh , 2 e absorption coefficient P C vd d s DT Dh

laser power [W] melting/vaporizing energy per unit volume [J/mm3] drilling speed [mm/sec] beam spot diameter [mm] heat transfer coefficient [W/mmK] thermal gradient adjacent to hot spot [K/mm] thickness of hot spot [mm].

(1)


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The first term on the right hand side represents the net energy consumed for the deformation of unit–volume–material per second. The second term on the right hand side indicates the heat loss due to the thermal conduction. Only the lateral surface of the hotspot contribute to the loss, because the heat released from the bottom surface effectively contribute to the material removal. In order to be able to derive the physical constants peculiar to a material, equation (1) should be rewritten as follows,

P = xdvd + h d x=pC/4e h=pDhsDTDh/e,

(2)

where, x and h are constants, depending only on the material properties and the wavelength of the laser. Because these constants can be determined from the experiment, one can readily estimate the drilling capability at an arbitrary laser power and a spot diameter.

3. EXPERIMENTAL SETUP Laser device employed in the experiment was SM series industrial discharge CO2 lasers (United Technologies). It utilizes a large–volume transverse gas flow system to obtain continuous operation at multi kW power levels. Maximum output power delivered to the workpiece in this experimental system was 13 kW. Optical resonator used for the laser system is positive branch unstable resonator. The magnification of the resonator is 4. Output coupler window for the discharge chamber is the aerodynamic window. The near field beam diameter is f50.8 mm.

Sample f=∞ f=∞

f=5m

Fig.2 Experimental setup.

Figure 2 shows a photo of the experimental setup. Three mirrors (two flat mirrors and one focusing mirror) are used to deliver the laser beam from the laser head to the sample rocks. Two flat mirrors were used to direct the vertically aligned laser beam to the focusing mirror, whose focal length is 5m. All these mirrors are made of copper with Au coat and are cooled with water to prevent thermal distortion. The focusing mirror faces a little upward direction so that the incident beam


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will have a small angle to the ground. Therefore, the effective evacuation of molten rock by the gravitational force is expected. Two types of rock were used as sample workpiece. One was a tuff breccia from Kinaushi Kamoenai, Hokkaido, Japan. Its primary components are quartz, orthoclase, carbonate, and some volcanic ejecta. Density, thermal conductivity, and compressive strength of the rock are 1.4~2.5 kgf/cm3, 1.1 W/m/K, 100~400 kgf/cm2, respectively. Rock collapsed at the Toyohama tunnel is classified into the similar species. This kind of rock is distributed over all the Hokkaido areas and considered to be most dangerous one. The other was granite. It was selected for comparative experiments. Its primary components are quartz, feldspar, and plagioclase. Density, thermal conductivity, and compressive strength of the rock are 2.5~2.7 kgf/cm3, 3 W/m/K, 800~2000 kgf/cm2, respectively. These rocks were supplied from the Hokkaido Development Agency.

4. RESULTS AND DISSCUSSIONS 4.1. Effects of assist gases It is well known that in a laser–based metal processing, an efficiency and a quality of the processing are remarkably enhanced with the aid of the assist gas.4-6 For the rock excavation experiment, however, laser irradiation conditions such as beam spot diameter, energy density, depth of the drilled hole, and viscosity of molten materials are much different from those of the metal processing. Therefore, restudying the effect of assist gases for the rock excavation becomes important. Dependence of the penetration depths upon the gas incident angle was surveyed. A schematic drawing of the nozzle conditions were shown in Fig.3. Two types of incident angles transverse flow (type-(a)) and coaxial flow (type-(b)) were examined as typical examples. Nitrogen was used as an assist gas. Results are shown in Table 1. On the contrary to our expectation no enhancement, but deterioration in the excavation rate was observed for both gas directions. Dependence of gas species was also studied. Three types of gases, O2, N2, and air, were examined. Gas flow rate, laser irradiation time, beam incident angle were fixed to be 100 L/sec, 30 sec, 20 degree, respectively. Penetration depths for different gas species were measured and converted to the excavation rate. The gas flow direction, namely the assist gas nozzle direction was set to be as the coaxial gas flow condition, simply because the better result than the transverse flow was obtained in the previous experiment. The excavation rates for each gas species are shown in Table 2. According to the results, no noticeable dependence on the gas species was observed and the best excavation rate was obtained in the absence of assist gas.

Type-(a)

Type-(b)

Molten rock

Laser beam

Fig. 3 Schematic drawing of nozzle conditions.

Table 1 Excavation rate for different gas flow directions

Gas condition Excavation rate[mm/s]

(a) 1.25

(b) 1.22

Non 1.46


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Table 2 Excavation rate for different assist gas species.

Gas species Penetration depth [mm] Excavation rate [mm/s]

N2 55 1.83

O2 55 1.83

Air 60 2.00

Non 70 2.33

Eventually, the assist gases were found to be not useful for large–bore rock excavation. The results may be explained as follows: as the hole becomes deeper, the gas flow could not blow the molten rocks, but only helps to cool it down. Therefore, we decided to conduct the succeeding experiment without a assist gas. However, it is still true that the laser beam is interfered with a plasma and molten materials generated by the laser irradiation. These must be removed somehow so as to improve the excavation efficiency. 4.2. Excavation rate as a function of a penetration depth Figure 4 shows the dependence of the excavation rate as a function of the penetration depth. The aim of the measurement is to understand the beam–rock interaction mechanism. Penetration depth for various laser irradiation times were measured to calculate the excavation rate as a function of a penetration depth. According to the data the excavation rate decreases as the penetration depth increases, and at the deeper penetration depth of more than 100 mm, excavation rate becomes independent of the depth of hole. Following hypothesis has been made to understand the phenomenon: the excavation rate was decreased, as the hole becomes deeper, due to an inefficient evacuation of the molten rocks. Such molten rocks remaining inside the hole absorb the incoming laser beams. In this case, thus, most of the laser energy may be dissipated in vaporizing the molten rock. Thus, larger energy would be necessary to excavate the equal amount of rock. In other words, the effective rock removal energy C [J/mm3] in eq.(2) has larger value when the penetration depth becomes deeper. This phenomenon must be taken into account in case where the laser excavation capability is being estimated, since several meters of penetration depth may be postulated in the practical field. In the Toyohama tunnel case, for instance, average depth of a hole excavated with a crawler drill was approximately 8 m. Consequently, vaporization–dominant value of the rock removal energy is considered to be adequate.

0.8 ]s 0.7 / m0.6 m [ et ar 0.5 n o it 0.4 av ac x 0.3 E 0.2

Tuff breccia

40

60

80

100 120 140 160

Penetration depth [mm]

1.8 ]s 1.6 / m m [ 1.4 et ar 1.2 n o tia 1.0 v ac 0.8 x E 0.6 20

Granite

40

60

80

100

120

140

Penetration depth [mm]

Fig.4 Excavation rate as a function of penetration depth. Two types of rocks tuff and granite were tested.

4.3. Derivation of the physical parameters The rock excavation rate for various output powers was measured to derive x and h in equation (2). Figure 5 shows the results of the experiment for tuff breccia and granite, respectively. The horizontal and the vertical axis indicate d/nd and P/d, respectively. The solid lines in the figures represent linear fitting of these plots. The both axes are chosen so that the slope and vertical–axis–intersection of the linear fitting correspond to the x and h, respectively. For tuff breccia, parameters x and h were found to be 8.94 and 21.1, respectively. For granite, x and h are 16.2 and 4.56.


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Several papers related to the rock excavation using CO2 laser have been reported in the past.2,3 The melting/vaporizing energies per unit volume C [J/m3] for those experiments were calculated so as to compare those to our results. For each case, volume of material removed by laser radiation was estimated geometrically from the focal length, beam diameter, cutting rate, defocusing length, and kerf depth. Only, heat conductive effect was assumed to be negligible. In a rock excavation experiment done by K. Fukui,3 C was approximately 9.36 for Kimachi sandstone. The value is very close to the tuff breccia case, since an ingredient and the generation process of the sandstone is similar to the tuff breccia. For Inada granite, C was estimated to be 14.8 [J/mm3], which was also very similar to our results. Generally, specific weight of the granite (2.7) is approximately 2 times higher than that of the tuff breccia (1.4), and hardness is higher as well. It means that the density of each rock is somehow reflected in a variation of C.

700

400

Tuff rock x=7.15 300 h=13.4 ]

600 ] m500 m / W [ 400 d / P 300

m m / 200 W [ d / P100

0 0

10

20 30 40 2 vd×d [mm /s] Fig.5

50

200 10

Granite x=16.2 h=4.56

15

20 25 30 2 vd×d [mm /s]

35

40

Determination of the thermal parameters for each rock

4.4. Estimation of the rock excavation ability of high power laser device In the case of tunnel collapse at Toyohama, it took about 96 hours to prepare the holes for the dynamites using a crawler drill. According to the investigative report,1 the average depth of the holes was approximately 7.87 m. Number of holes drilled per day was approximately 19. Then the total depth of the hole was 598 m. Therefore, the average excavation rate of the crawler drill in Toyohama case was calculated to be 1.73 mm/sec. On of the advantages of high power lasers is that the machine head requires no scaffolds, resulting in the reduction of the preparation time. Therefore, the duty cycle of laser excavation can reach nearly 100 %. Thus, the total operation term can be shorter, even though the instant excavation rate is inferior to the mechanical method. Assuming a 50 kW of output power, excavation rate of the laser can be estimated from the equation (2). The beam diameter must be larger than at least f50 mm, because the dynamites used at the Toyohama accident was “Akatsuki”, whose diameter was f50 mm. Substituting these figures into the equation (2), excavation rate of the 50 kW CO2 laser is given by

n d = ( P / d - h ) / x × d =2.76 mm/s for tuff breccia =1.23 mm/s for granite Therefore, the total operation time necessary for the 598 m drilling would be 60.2 hours for tuff breccia and 135 hours for granite. As the result, operation term for tuff breccia excavation can be reduced approximately by a factor of 2 using the high power laser system.

5. CONCLUSIONS Feasibility study of rock excavation with a high power laser device was conducted. Sample rocks used in this experiment as workpieces were tuff breccia and granite. Excavation rate on both rocks for a various output powers was


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measured to determine thermal constants inherent to each rock. It was found that the excavation rate resulted in slower, as the hole becomes deeper, because of the deterioration in evacuation efficiency of the molten rock. Effect of assist gases on the excavation rate was also surveyed. Oxygen, nitrogen, and air were examined and found not to be useful. It was because the gas flow could not blow the molten rocks off, but only helps to cool it down in case the hole reaches certain depth. Thermal parameters of the both rocks were derived from the experimental results. Using simplified thermal balance model, it was estimated that a better performance than a conventional excavation technique can be possible by virtue of a 50 kW–class mobile laser system. As far as demands are concerned, a suitable laser processing system for civil engineering should have (1) high power at least 10 kW, (2) good beam quality and near infrared wavelength for optical fiber delivery, (3) less dependence on the electric power supply for mobile laser systems carried by a truck or a helicopter. Among various high power lasers, chemical oxygen–iodine laser (COIL) is the only laser, which fulfills those requirements, because it is capable of producing a multi–megawatts of output power in CW operation mode8 with a good beam quality, and the output power can be delivered through silica optical fiber with an extremely low transmission loss under the favor of 1.315mm wavelength. Most importantly, the laser system happens to be independent from an electric power supply due to its unique pumping mechanism, i.e., an excited–state oxygen O2(1D), which is a power source of the COIL, is produced by a chemical reaction between an aqueous solution of H2O2 and a gaseous Cl2, and an iodine atom I(2P3/2) in a ground state is excited to the upper state I( 2P1/2) by the O2(1D) through a near resonant energy transfer. Therefore, no external power source in principle is required for the pumping process except for a vacuum pump driving. This is the most distinguished feature of the COIL, if a mobile laser system is concerned. Other high power lasers, on the other hand, such as CO2 or Nd:YAG laser requires a huge facility for the electric power supply, which makes mobile type system much difficult. As far as the wavelength is concerned, it was found experimentally that the rock excavation rate of 1.315mm was almost same as that of 10.6mm.14 Therefore, the application of this work to a mobile COIL is straightforward. In Tokai University Laser Research Group, conceptual design of a 30 kW–mobile COIL system has been already accomplished for D&D of nuclear facility.13 Therefore, 50 kW–mobile COIL system is not an impractical argument. However, operation period of the mobile COIL is now limited by quantity of its fuel. According to the latest estimation done by Endo, fuel required for the 2–hour operation with 30 kW output power device would be 1500L.13 Thus, a partial fuel recycle system for the long term operation may be the residual research task for rock excavation by COIL.

ACKNOWLEDGEMENT Authors wish to thank Mr. K Nakai (Hokkaido Development Agency) for providing the sample rocks. Authors also would like to express their sincere thanks to Dr. K. Minamida, Dr. S. Yamaguchi and Mr. M. Oikawa (Nippon Steel Technoresearch Corp.) for their beneficial cooperation.

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Detailed information is found at: http://www.jiti.co.jp/graph/toku/toyohama/toyohama.htm B. R. Jurewicz, “Rock Excavation with Laser Assistance” Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 13, pp. 207-219, 1976. K. Fukui, T. Takatani, M. Sato, K. Nagai, and S. Okubo, Shigen–to–Sozai, 112, pp. 537-542, 1996 (in Japanese). G. Chryssolouris, Laser Machining, Springer-Verlag, New York, 1991. D. Belforte and M. Levitt, The Industrial Laser Annual Handbook, Pennwell Pub., Tulsa, Oklahoma, 1987. W. W. Duley, Laser Processing and Analysis of Materials, Plenum Press, New York, 1983. M. Endo, S. Nagatomo, S. Takeda, M. V. Zagidullin, V. D. Nikolaev, H. Fujii, F. Wani, D. Sugimoto, K. Sunako, K. Nanri, and T. Fujika, “High-Efficiency Operation of Chemical Oxygen-Iodine Laser using Nitrogen as Buffer Gas”, IEEE J. Quantum Electron., 34, pp. 393-398, 1998. J. L. Moler and S. Lamberson, “The Airborne Laser (ABL)-Legacy and a Future for High-Energy Lasers”, Proc. of SPIE, 3268, pp. 99-105, 1998. W. E. McDermott, N. R. Pchelkin, D. J. Benard, and R. R. Bousek, “An Electronic Transition Chemical Laser”, Appl. Phys. Lett., 32, p 469, 1978. M. V. Zagidullin, V. D. Nikoraev, M. I. Svistun, N. A. Khvatov, and N. I. Ufimtsev, “Highly efficient supersonic chemical oxygen-iodine laser with a chlorine flow rate of 10mmols-1”, Quantum Electroics, 27, pp. 195-199, 1997.


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D. Furman, B. D. Barmashenko, and S. Rosenwaks, “Parametric study of an efficient supersonic chemical oxygen-iodine laser/jet generator system operating without buffer gas”, IEEE J. Quantum Electron. 34, pp. 1068-1074, 1998. P. M. Noaker, “Laser delivers 7.36 kW via fiber”, Laser Focus World, 35, pp. 22-23, 1999. H. Tanaka, D. Sugimoto, M. Endo, S. Takeda, and T. Fujioka, “Chemical oxygen-iodine laser for decommissioning and dismantlement for nuclear facilities”, Proc. of SPIE, 3887, to be published. D. Sugimoto, M. Endo, K. Sunako, K. Nanri, and T. Fujioka, “Cutting and drilling of inorganic materials for civil engineering using a chemical oxygen–iodine laser”, The Review of Laser Engineering, 1999, to be published.