Lasers in Eng., Vol. 17, pp. 311328 Reprints available directly from the publisher Photocopying permitted by license only

c 2007 Old City Publishing, Inc. Published by license under the OCP Science imprint, a member of the Old City Publishing Group

Energy Analysis of Solar vs Laser LightningN. K HAN , Z. S ALEEM , A. WAHID AND N. A BASDepartment of Electrical Engineering, COMSATS Institute of Information Technology H-8/1, Johr Campus, Islamabad, Pakistan

Ultrashort pulsed lasers and triggered lightning are very often considered potential future energy sources for electricity generation. This work is focussed on the energy engineering behind this most fascinating dilemma. The available ultrashort pulsed lasers are reviewed and the prospects of Attosecond to zeptosecond pulse generation barriers are discussed to investigate the potential solutions. Natural and rocket or laser triggered lightning are reviewed regarding pulse durations, inter-pulse periods, frequency of occurrence and number of pulses per ash in the light of available data. Laser pulses and lightning ashes of energy are evaluated critically in the more familiar kWh units and the prospects of causing very high power or energy pulsed events in highly repetitive (continuous) manner is analysed to develop sustainable ultimate energy sources. An attempt was made also to demonstrate that the available natural solar energy on one half of Earths surface is several billions times more than the present human survival needs. The possibilities of ultrashort pulsed lasers to record live movies of Attosecond to Zeptosecond short lived events and trigger Exa to Zettawatt threshold chemical, natural nuclear or articial processes are discussed.Keywords: Ultrashort pulsed laser, Lightning, Articial lightning, Femto-chemistry.

1 INTRODUCTION Available lasers can produce ultrashort pulses of the order of a few femto-seconds. The pulse duration can be shortened further using pulse compression techniques [1]. As the pulse duration decreases its pulsed peak power increases. The energy content can be amplied using the standard chirped pulse amplication techniques [2]. It is just possible to Corresponding

author: E-mail: nasrullahk@yahoo.com

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produce a few joules femtosecond laser pulses using the available technology. Successful operation has been demonstrated for attosecond lasers [3] and the theoretical basis for zepto and yoctosecond lasers has been reported by Kozlowska and Kozlowski [4]. It is expected to generate zeptosecond laser pulses routinely by 2012 and down to a few yoctosecond laser pulses by 2017. It might take even longer to generate the subyocto-second laser pulses due to material limitations. CW lasers can be produced from X-rays to FIR range, but Q-Switched (ns) or mode-locked (ps) lasers only from UV to FIR. However, passively mode-locked (fs) lasers may fall within the UUV and UV range. Harmonic mode-locking and SC lasers (fs to as) may fall in spectral range of UUV to X-rays. Light modulated lasers (as to zs) may fall in the range of (X-rays). Nuclear pumped lasers (zs to ys) may fall in the spectral range of X-rays to G-rays. The pulse shortening history, from the invention of the laser in 1960 up to 2017 is shown in Figure 1 [5]. Beyond Planks time limit (1043 ) with which many do not agree [6], we redene the concept of a Bangosecond1 (1045 ) [7] laser pulse, capable of transforming matter into energy. It may fall somewhere beyond G-rays. The theoretical considerations, in the light of intensive research, may result in the current hypothetical views at least to have produced subzepto-second laser pulses at the end of the next decade, 2017. Femto-second pulses can now be produced routinely, but it is another problem to measure accurately the true pulse duration. There are no photo-detectors or electronics fast enough to measure directly events on the Attosecond timescale. The fastest available electronic devices have a time resolution of about few hundreds of fs which cannot measure pulses shorter than 1fs [89]. One must use the Attosecond pulse to measure itself, using SPIDER [10], FROG [11] and THz Streak Camera [12]. Measurement techniques need to be developed further to measure natural suyocto-second events such as the mean lifetime of the top quark reported to be 0.4 yoctoseconds [13]. In reality, time, space, and energy all become intertwined at too short time intervals (Around Planks time) over too short distances (1035 to 1045 ), for which the energy becomes very large and matter-energy transformation might take place. According to Zewail [14] there are many natural movements, which take place in an extremely short duration i.e. a light sensitive pigment in the eye (retina) undergoes a chemical reaction in about 200 femto-seconds and double benzene rings in stilbene are bound by covalent bonds, which break in 300 femto-seconds on zapping with laser light. Similarly, the NaI bond breaks in about 8 femto-seconds. In order to see molecular motion at speeds of 10001 Term Bangosecond is used due to lack of any prex for 1045 second pulse in SI units. Word Bangosecond is consistent with big bang theory. Proposed unit for above pulse in SI system of units may be rimtosecond. Authors are writing an endorsement, with real examples, for need of prexes for 1099 to 10+99 quantities of space, time, energy and matter.

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FIGURE 1 Maximum intensity history of ultrashort pulse lasers (Redrawn to update [5]).

m/s (3600 km/hr) at a resolution of 0.1A, one needs femto-second pulses i.e. ( t)= S/c=0.1 1010 /100 = 1014 . Any object moving at 1000 km/s can be imaged at a 0.1Aresolution using Attosecond pulses only. Pulses of the order of 250 Attoseconds have been created and used to study the motion of electrons around the nucleus of a Neon atom. Further scientic research is required to develophighlyrepetitiveultrashortpulsedlasers.Electro-absorption(EA)gated CW and gain switched distributed feedback (DFB) laser diodes can produce the desired repetition rate of pulses 1520ps, which can be reduced further to the femto-second range after chirp compensation and adiabatic compression techniques. Simple mode-locking can generate a xed repetition rate, fs to 10ps transform limited pulses. Harmonic mode-locked EDF and SC lasers can produce a tuneable repetition rate ps to fs transform limited pulses from UUV to FIR. Available low energy repetitively pulsed lasers are shown in Table 1. Q switched solid state lasers typically operate from below 1 Hz to about 100 kHz. Mode-locked solid state lasers emit with pulse repetition rates between 50 MHz and a few GHz, but in extreme cases it is possible to reach 10 GHz. Gain switched lasers can provide repetition rates from

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K HAN et al. TABLE 1 Available repetition rates of pulsed lasers [After Ref.9} Modulator Q. Switching Gain Switching Modelocking H Modelocking Pulse duration Nanoseconds Picosecond Femtoseconds Attoseconds BW MHz GHz THz PHz Pulse Rep Rate 1Hz to several KHz 1Hz to several MHz 10 MHz to 100 GHz 10 GHz to 100 THz

below 1 Hz to many MHz. Attosecond pulse trains can be generated with repetition rates of several hundreds of MHz. Pulse shortening limiting factors may include gain narrowing, group velocity dispersion and etalon effects. Collision pulse mode-locking, double saturable absorber and Kerr lensing may produce direct femto-second pulses. In order to obtain a high repetition frequency, it is better to have a short cavity length. According to Wada [15] a passive short cavity has led to high repetition rates (480 GHz) 2ps pulses lasers. The spread of frequencies is inversely related to the temporal length of the pulse as illustrated in Figure 2. For example, a visible pulse, which is approximately ve femto-seconds long requires a range of optical frequencies of the order of 200 1012 Hz or alternatively a small rainbow of light spanning from 500nm to 750nm. Multiple passive grating Bragg bre lasers or dynamic grating multi-wavelength dye lasers or higher order harmonic generation are suitable options to produce the shortest possible pulses [7]. Similarly, well known optical parametric oscillators or implied super-continuum (SC) lasers have a wide spectral range and therefore they can produce subfemto-second pulses [16]. Multiple harmonic generation [17] with a mode-locking technique

FIGURE 2 Irradiance and spectrum of CW, long and ultrashort pulse lasers.

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can produce even shorter pulses. The above two methods increase a good chance of direct Attosecond laser pulses [1819]. 1.1 Solar Vs Pulsed Laser Power Calculations The power density (watts/cm2 ) of a normal 257.0mJ, 1.0 fs laser pulse may be approximated by (P= E/ t). The laser pulse power is relatively low in the unfocussed mode but extremely high in the focussed mode. The power density of an unfocussed laser may be 3 1012 watts/cm2 or 3 1016 watts/m2 and that of the focussed laser of order of 3 1021 watts/m2 . The peak pulse powers of ys, as and fs lasers with 43J energy may be approximated by: 1ys, 43J peak pulse power 1as, 43J peak pulse power 1fs, 43J peak pulse power E/ t = 43/1 1024 = 4.3 1025 Watts E/ t = 43/1 1018 = 4.3 1019 Watts E/ t = 43/11015 = 4.3 1016 Watts

If every human (out of 6.6 billions population) uses 1kW of power daily then the average power used by 6.6 billions people on the Earths surface is 1kW 6.6 109 = 66 1011 watts. This implies the total consumption of 6.6 billions people is about (0.66 1013 /43 1015 = 0.15 102 ) i.e. 0.1% of a 1fs laser power. The sun shines on the Earths surface @1.3 103 W/cm2 = 1.3 107 W/m2 Earth radius is 6.4 106 m Earths surface area 4r2 = 4(6.4 106 )2 = 5 1014 m2 Half of Earths surface area 2.5 1014 m2 = 2.5 1018 cm2 Total sun power on half earth 1.3 107 2.5 1018 = 4.3 1025 Watts This means the total sun power delivered to half of the earth surface at an instant is just equal to the peak power of 1.0ys laser pulse. The kWh energy generated by a high repetition rate quasi-cw laser with a pulse energy of 43J and pulse duration of 1ys or 1fs in 12 Hrs (12 60 60 = 43200 s) may be estimated by For as lasers: 4.3 1025 43200 = 5.15 1023 kWh For fs lasers: 4.3 1016 43200 = 5.15 1014 kWh If we assume a pulse train of zero inter-pulse duration as shown in Figure 3, then the kWh energy of the pulsed laser would be half of the upper values. This may be illustrated for Gaussian pulses using the equal area criterion by superposition of normal and inverted pulses. Practically, it is not possible to produce pulse trains without any inter-pulse period of 2nL/C. Theoretically, it may be as short as the pulse duration itself or somewhat longer. Solar energy received by Earth in 12 Hrs = 4.3 1025 12 = 5.16 1023 kWh. 6.6 billion People use energy @1kWh/person/12Hrs = 12 1 kWh 6.5 109 = 79.2 109 kWh.

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FIGURE 3 Superimposed laser(s) pulses may act as CW laser source.

This means the sun delivers 6.5 1012 times more energy everyday than all living species can use. This is an enormous amount of energy, which we must nd the wisdom of how to use it for our specic energy requirements. The sun energy is much more than enough for our needs. Technically there is not any impending energy crisis in near future. 2 LIGHTNING AND ENERGY PROSPECTS Lightning is a form of visible electric discharge (arc) between the rain clouds or clouds to Earth. How rain clouds become charged up to 10 kV/cm is not fully understood, but most rain clouds are negatively charged at the base and positively charged at the top. The negatively charged leaders, called stepped leader, zip downward in about 100 segments of 50m long jumps at a speed of about 10km/s bridging the clouds to earth. It may take 100s of ms to approach the ground but subsequent dart strokes are 10 times faster than the stepped leader. This initial phase involves a relatively small current (100s of A) and an invisible leader. When the stepped leader is quite close to the ground, a few positive streamers arise from grounded objects to the approaching leaders. The stepped leader and the positive streamers meet in the air to cause a short circuit, a return strike, with a

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FIGURE 4 A lightning ash consisting of four strikes lasting 567ms [Based on Ref.20].

much higher current (10s of kA) follows. A lightning ash may consist of a single or multiple strikes separated by 2060ms or on average 30ms time intervals. Subsequent return strikes are initiated by the dart leaders, akin to the stepped leader as shown in Figure 4 [20]. The long duration (500ms) single strike (10100A) lightning ash may be more dangerous for utility TSS or MOV devices. With regard to test electrical power equipment against a lightning threat IEC 616431 uses 10/350s, VDE 0160 uses 100/1300 s, IEEE C62.41.1 uses 10/1000 s test waveforms and recently IEEE Std C62.41.2-2002 uses 100kHz Ring Wave and the 1.2/50 s-8/20 voltage current combination waves. The specic energy parameter (W/R) may be calculated using I2 dt [20]. Lightning phenomena occur under such diverse natural weather conditions it is hard to predict the times involved in 1 to 40 (Average 56) lightning ashes occurring in 12 s. Every return strike occurring after 15ms needs a fresh start leader. However, the channel looses its ionization in about 100ms. Most of the 80% multiple strike events follow the same channel or location but some subsequent strikes may be separated by 0.24.5 miles. Figure 4 is based on Umans explanation [20] for three strikes of lightning. It is usually applied to the most negative lightning ashes but does not t to some of the negative and all positive lightning ashes. The main strike travels at a speed of 2 4.9 107 m/s within a few tens of s but the peak current decays over several tens of s. Before 1994 the inter-strike time

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K HAN et al. TABLE 2 Typical CG lightning ash parameters [1-88] Names Parameters Step-length Steps interval Velocity Charge Voltage Velocity Charge Velocity di/dt t peak t halfpeak Peak current Charge Channel length Channel dia No. of strokes Stroke interval Flash duration Charge Magnetic eld Temperature K Temperature C Temperature F Length Electron () Super-cooling Negative Positive Negative Positive (rare) Can measure Occurrence Visible Sound Heat Radio waves Average Possible Based on Uman measurements 50 m 3-200 m 50 s 30-125s 150km/s 100-2600 km/s 5 C 3-20 C 1 106 V 1 106 -1 108 V 2000km/s 1000-21,000km/s 1 C 0.2-6 C 80,000km/s 20 160 106 m/s 80 106 kA/s 106 kA/s 2 s < 1 30 s 40 s 10-250 s -10-20kA -110kA 2.5 C 0.2-20 C 5km 2-14km 2 cm Size of thumb 3-4 1-40 40 ms 3-100 ms 0.2 s 0.01 - 2 s 25 C 3-90 C 1000 G 900-1100 G 30,000K 24,000-40,000K 10,000 C 8,000-16,000 C 18,000 F 15,000-60,000 F 5km ............. .......... ............. -10-20 C Negative C -120kA -20-120kA +300kA +30 - +300kA 1 105 1 108 5 108 3 1011 1 107 1 1013 6 km 1.2-12.5miles away 2 km 0-4.5 miles around 2 1-3 35 10-50 35 10-50 28 10-50 Typical Based on others 60-80 m 50-60 s 0.1C 10 C 1 108 -1 109 V 0.9-1c 2-3 C Decays in 100ms 300x106 kA/s(+ve) 1.82.5 s 30s -200kA 300 C (+ve) 1500-7000 feet 0.1 m 5-6 1-500 ms 1 500 ms 300 C (+ve) Hotter than sun 20,000 C 50,000 F 6 miles 1023 -1024 e/m2 10kV/cm 138 250 kWh 84000 kWh 100W bulb/0.25Yr 100Wbulb/100Yrs 3-4 miles 500 m Based on data http://FusEdWeb. llnl.gov/CPEP/

Stepped leader Dart leader

Return stroke

Lightning ash

Channel

Graupel Lightning Types Lightning Energy Lightning effects Total Energy dissipated 100kJ/m

was thought to be 315 ms but recently it was measured to be