NEW APPLICATIONS OF HIGH-REPETITION-RATE PULSE-PERIODIC LASERS IN THE ARCTIC

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1 NEW APPLICATIONS OF HIGH-REPETITION-RATE PULSE-PERIODIC LASERS IN THE ARCTIC V.V. Apollonov A.M. Prokhorov General Physics Institute, RAS, Moscow, Russia ABSTRACT This paper is dedicated to the jubilee of Prof. Oleg Figovsky The paper discusses the application of high-power lasers to break the ice cover cover (up to several meters in height) around such engineering structures as oil platforms and icebreakers. The most effective in this case is the joint application of high-power lasers and icebreakers in the cracking of ice covering the ocean. The most dangerous hazards resulting from drifting ice can be easily overcome with the help of high-power lasers. It is obvious today that lasers on offshore oil and gas platforms in the Far North will be in high demand and can greatly facilitate the safety of the personnel of such rigs Keywords: Arctic Ocean, Icebreakers. High-power lasers, Laser-assisted ice breaking INTRODUCTION Works in the shelf zone of the Arctic Ocean will soon require the use of innovative, highperformance hardware equipped with high-power lasers. In this connection, the government of the Russian Federation intends to perform a wide range of activities associated with the active development of oil and gas deposits on the shelf in the Russian Arctic. These activities will involve the mastering of the technologies based on applications of high-power lasers that will reduce ice loads on engineering structures on the continental shelf and will provide an increased speed of ships and tankers following the Northern Sea Route. It is expected that high-power lasers can break the ice cover (up to several meters in height) around such engineering structures as oil platforms and icebreakers. The most effective in this case is the joint application of high-power lasers and icebreakers in the cracking of ice covering the ocean. For the first time, the feasibility of laser-assisted ice breaking in the interests of the icebreaker fleet was reported by Clark et al. at the conference of the National Bureau of Standards (Colorado, USA) [1]. This technology at present is being actively developed in Canada, USA, Norway, Sweden, South Korea, and Russia. It is expected that due to the positive test results that nobody questions, our country will start full-scale production of icebreakers equipped with high-power laser systems. It is known that experiments on the use of lasers in the Arctic have been conducted for many years and most new tried and proven technologies are already considered successful. A laser installed on an icebreaker must comply with the requirements of the Russian Maritime Register of Shipping. In particular, such a laser should be electromagnetically compatible with onboard equipment and comply with fire and radiation safety regulations. The weight of the laser system must be no more than 5 tons, and the time of continuous operation must amount to several tens of thousands of hours. The most dangerous hazards resulting from drifting ice can be easily overcome with the help of high-power lasers. It is obvious today that lasers on offshore oil and gas platforms in the Far North will be in high demand and can greatly facilitate the safety of the personnel of such rigs. 133

2 WATER AND ITS STATES Journal Scientific Israel- Technological Advantages" Ice is one of the most enigmatic and simultaneously usual states of water. Water has a molecular weight equal to about 18.02, and may exist in liquid, gaseous, or solid states. Water is characterized by high boiling (+100 C) and melting (0 C) points, high values of heat of phase transitions [melting, vaporization (9.70 at 100 C, kcal/mol), sublimation]. Water has an anomalously high specific heat capacity, which is of great importance in the nature: at night and during the transition from summer to winter water cools slowly, and in the afternoon or during the transition from winter to summer it slowly heats, thus moderating Earth s climate. Water also has an unusual property of expanding when it is cooled down to its solid form; as a result, the density of ice is lower than that of water, which is not typical of other substances in the transition from the liquid to the solid state. This property of water creates a lot of problems for engineering structures that are in the aquatic environment. Among other anomalies of water one should mention the high surface tension and dielectric constant and a large thermal conductivity. The thermal conductivity of water is higher than that other liquids and the thermal conductivity of ice is markedly higher than that of other non-metallic solids. The thermal conductivity of ice at 0 C is about four times higher than that water at the same temperature, i.e. ice conducts heat much faster than immobilized (not flowing) water in tissues. If at the same time we take into account that the thermal diffusivity of ice is much higher than that of water, it becomes clear why biological tissues freeze faster than become thawed. This property is fully applicable to the technologies of engineering construction under conditions associated with prolonged low temperatures. The analysis of the well known phase diagram of water states is important in the development of technological regimes. For example, if ice is heated at a pressure less than kpa (4.58 mm Hg), it directly evaporates. This property is the basis for the development of the drying techniques for object and food preservation by freezing. One of the characteristics of water that distinguishes it from other substances is the reduction of the melting point of ice with increasing pressure, which is important when working at considerable depths of sea areas. Tables 1 and 2 show the basic thermal and mechanical properties of ice in a wide range of negative temperatures that are important for assessing the effectiveness of laser applications for its destruction by melting and sublimation (evaporation). Table 1. Thermophysical properties of ice. t, 0 С с i, kj(kg K) ρ i, λ i, kg/m 3 W/(mK) а i 10 6, m 2 /s Table 2 lists the physical data required for the analysis of various mechanisms of ice destruction. It should be noted that at low temperatures in the Arctic and Antarctic, purely mechanical destruction of ice is possible by using nonuniformly distributed laser radiation or electrical discharges, similarly to the destruction of rocks due to thermomechanical stresses in mining or restoration of defective concrete structures in civil engineering. 134

3 Table 2. Physical properties of ice. Property Values Note Heat capacity, cal/(g C) 0.51 (0 C) Heat capacity significantly Heat of melting, cal/g decreases with decreasing temperature. Heat of evaporation, cal/g 677 Thermal expansion coefficient, 1/ C (0 C) Thermal conductivity, cal/(cm s C) Specific electric conductivity, Ohm 1 cm (0 C) Activation energy 11 kcal/mol Surface conductivity, Ohm (11 C) Activation energy 32 kcal/mol Young's modulus, dyne/cm (-5 C) Resistance, MN/m 2 : to crushing to rapture to slicing Average effective viscosity, P Exponent of the power law of flow 3 Activation energy under deformation and mechanical relaxation, kcal/mol LASER-ASSISTED ICE BREAKING Polycrystalline ice 11,44 21,3 Increases linearly by kcal/(mol C) from 0 to K Currently, many technological processes make use of high-average-power lasers (100 W and higher), which operate in two modes, i.e., CW and pulse-periodic (P-P) regimes with a low pulse repetition rate (from a few to hundreds of Hz) and a pulse duration in the range of tens to hundreds of microseconds and milliseconds. In most cases, interaction of laser radiation with matter is accompanied by a purely thermal mechanism, because advantage is taken of the ability of a laser source to deliver a sufficient amount of energy to a small surface area of the processed material. Recently, fiber lasers have become very popular in various applications due to a number of very important advantages. However, in the case of fiber lasers, the thermal mechanism caused by the continuous mode of laser radiation and leading to the formation of a significant amount of the liquid phase, it is not efficient. High-repetition-rate, high-average-power, P-P laser systems operating in the Q-switched regime, which provides a pulse duration in a periodic sequence from a few to hundreds of nanoseconds, allow one to implement a fundamentally different mechanism of radiation matter interaction, i.e., sublimation of a substance to be treated, which provides a local release of energy not only in space but also in time [2 4]. It should be noted that in this case, the substance is evaporated explosively and locally, without an intermediate liquid phase. This mechanism can significantly extend the range of technological applications of laser sources. However, the high-repetition-rate regime of interaction has not yet found use in actual practice because of the high complexity of its implementation at high average powers required in many technological processes. In the early 2000s, we proposed for the first time and fabricated a highrepetition-rate, P-P laser having a power of 10 kw. The peak power of radiation generated by our laser was by several orders of magnitude greater than the average power of the laser source, i.e., the power that is typical of CW operation. The possibility of varying the pulse repetition rate and, hence, the value of the peak power has allowed us to control the interaction of radiation with matter over a wide range of parameters. 135

4 Let us compare the two regimes (CW and P-P) from the point of view of their efficiency of ice cutting, i.e., possible thickness and cutting speed for a given power of the laser source. It is known that in the case of CW radiation, the threshold power density of evaporation of ice is defined as: where A is the absorption coefficient at T ev = 100 C, k is the thermal conductivity, and r 0 is the radius of the laser beam cross section in the focal plane. The resulting laser power is given by the expression: where S is the area of the laser spot. For a repetitively pulsed laser with the same parameters and the corresponding thermal regime, the threshold power density can be expressed as: (1) (2) where is the thermal diffusivity. The pulsed power and average power can be expressed through the expression: where is the pulse repetition rate, and is the pulse energy. Thus, on the basis of the above-said, we can generally conclude that (3) (4) for a 1-kHz laser pulse frequency and a duration of 50 ns and higher. For the parameters mentioned this ratio is equal to Thus, for the repetition frequency of 1 khz and pulse duration of 50 ns, i.e., the regime, which is sufficient for addressing most technological challenges, the ratio of the average repetitively pulsed power to the cw power turns out to be within a few hundredths. In other words, this means that cutting of ice having the same thickness and length requires the use of a laser whose power is 30 times less. In this case, we do not take into account the dynamics of the interaction of radiation with the products in the cutting zone, which in the case of cw operation is very power consuming. Consider the process dynamics in terms of the explosive steam generation rate and minimization of the process of the liquid phase formation in the cutting zone in the case of high-repetition-rate, repetitively pulsed regime. The amount of the liquid phase (water in this case) is proportional to, which corresponds to some zone heating by an individual laser pulse. The shorter the pulse duration at a fixed thermal diffusivity, the smaller the amount of water that prevents the penetration of laser radiation to the cutting zone. At the same time, the main fraction of the laser pulse energy transforms ice into steam, which, being explosively generated in the interaction region, carries away a small quantity of water that is simultaneously formed with the steam. The rate of the liquid phase removal from the interaction region (drilling and cutting) during the cutting of ice by the steam pressure is proportional to 1/τ. The both factors of the process obviously indicate the need to decrease the laser pulse duration. For the high-repetition-rate, P-P regime, the pulse duration should be as short as possible, based on the requirements to the magnitude of the average power and the off-duty ratio of the pulse sequence generated by the laser. Let us now compare the two generation (CW and P-P) regimes from the point of view of the requirements to the accuracy of the scanning system used for cutting. It is obvious that to maintain (5) 136

5 stable conditions for cutting ice (stable cutting quality on curvilinear segments, i.e., when turning the laser cutter on the surface of the ice field) it is necessary to provide a constant power flux density. With respect to the average power of the high-repetition-rate, P-P laser, expressed as:, this condition is It follows that in terms of energy, the repetitively pulsed regime of laser operation is more suitable for cutting ice than the CW regime. This means that stable conditions imply constancy of the laser power and the beam scanning velocity. This is particularly important for the cutting of ice along a complex trajectory. In this case, it is impossible to maintain the same velocity at linear sections and sharp turns of the trajectory. If is not constant and varies according to a certain law, then in the case of a CW radiation source it is impossible to satisfy the condition at any laser power. It should be also noted that in the most general case, the physical processes proceeding during the high-repetition-rate, P-P cutting of materials are very similar to those during drilling in various engineering products. The cut is produced by a set of individual holes when the laser beam moves along the surface of the original object. DATA ANALYSIS AND COMPARISON WITH THE EXPERIMENT Clark et al. [1] presented data of ice cutting experiments for a CW CO 2 laser of 50 and 4500 W. The cited figure of merit for ice cutting, measured in cm 2 /(s kw), is the area of the cut, which differs in magnitude for the two power levels presented. These figures of merit were as follows: 8 cm 2 /(s kw) and 3 cm 2 /(s kw), respectively. Taking them as a basis, we assume the first number to be optimistic and the second pessimistic. In the case of a CW CO 2 laser with an average power of 100 kw, we obtain for an 8-cm deep cut in ice the rate of about 1 m/s (optimistic estimate) and 38 cm/s (pessimistic estimate). In the case of a high-repetition-rate, P-P CO 2 laser with an average power of 100 kw, we obtain the following estimates for the same 8-cm deep cut in ice: the optimistic estimate amounts to 30 m/s and the pessimistic estimate 11.4 m/s. Based on the data obtained, it is important to pay more attention to the distribution of the laser power along an ice field in front of the icebreaker and to the geometry of the ship bow. Because laser radiation, cracking ice for the subsequent effective navigation through ice-covered waters, can be divided, using an optical system, into 2, 3 or more beams, the action of which must be accompanied by appropriate efforts of a specially designed bow of the ship or a set of metal structures, located on its bow. The above estimates clearly indicate that a 100-kW onboard laser system is sufficient to provide a large number of beams to crack ice and to meet many challenges that we will have to face during the development of the wealth of the Arctic Ocean. CONCLUSIONS Proceeding from the foregoing, one can conclude that for many processes requiring the processing and cutting of different materials (in particular, ice), the high-repetition-rate, P-P regime of laser operation seems much more efficient than the conventional CW regime. Fabrication of high-power laser systems and their application to solve the stated problems will be an important national economic challenge that requires the full attention and support of the State. (6) (7) 137

6 REFERENCES 1. A.F. Clark, J.C. Moulder, R.P. Reed (1973). Applied Optics, 12 (6), V.V. Apollonov, V.V. Kiyko, V.I. Kislov, A.G. Suzdal tsev (2003). Quantum Electronics, 33 (9), V.V. Apollonov (2010). 'Impulsar': New Application for High Power High Repetition Rate Pulse-Periodic Lasers, Laser Pulse Phenomena and Applications, Dr. F. J. Duarte (Ed.), ISBN: , InTech, DOI: / Available from: P-P lasers. 4. V.V. Apollonov (2014). High Power P-P lasers (New York: NOVA Science Publishers Inc.). 138

PAPER No.4: Environmental Chemistry MODULE No.5 : Properties of Water and hydrologic cycle

PAPER No.4: Environmental Chemistry MODULE No.5 : Properties of Water and hydrologic cycle Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 4: Environmental Chemistry 5: Properties of Water and Hydrologic Cycle CHE_P4_M5 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction