 The unit is the hertz direction of that flow, usually expressed in watt per (Hz). square metre (W m-2). However it is also often quoted in mW cm-2 The use of hybrids such as Wcm -2 are best 11 Peak pulse power density avoided except where really necessary, since they can In pulsed systems such as radar equipment the term cause confusion. Here the shorter form in common use 'peak pulse power' is used when what is actually meant power density' is used hereafter because of the frequent is the r.m.s. power in the pulse (see Figure 2.5-7). occurrence in text. All references to power density, This should not be confused with instantaneous peak electric field and magnetic field are to r.m.s. values, power. unless otherwise stated, in common with the practice in RF safety standards. 12 Pulse duty factor 5 Energy density Where tp is the pulse duration in seconds and n is the This is, strictly, related to volume (J m -Superscript(3) but is almost pulse repetition rate in Hz, then the duty factor DF = tpn and has a value less than 1. universally used in radiation protection work as the product of power density and time and expressed either For example, if tp==2 us and n = 500 Hz, then: in units of watt-hour per square metre (Whm-2) or joule DF = 500 x 2 x 10 6 = 0.001 per square metre (J m-2). 1 J = 1 Ws. It is sometimes used to express a total energy limit, for example, 'not Many people find it easier to work with the reciprocal, more than 5 Whm-2 in a six minute period'. in this case 1/0.001 = 1000. In terms of the energy in a volume, e.g. J cm the The relationship between peak pulse power density definition relates to the energy in a minute volume di- (Spk) and the mean power density (Smean) in a pulsed vided by that volume. With a power density of 10 W m-2 system is: the energy in a cubic centimetre of air is 0.033 picojoules. Spk = Smean/DF 6 Electric field strength (E) at a point A vector quantity defined as the force acting on a unit or, if using the reciprocal of DF, specific absorbtion rate positive charge at that