It is common to see discussions forums talk about plasma weapons. We have seen the light sabre in Star Trek movies . The Members of forums muse about the possibility of a plasma sword. In the gaming world plasma rifles are already common place. But are such weapons just figments of fertile imaginations or is there a real possibility that such weapons could be created and if so how might they be brought into reality. Remember that the flip open video phone seemed very far fetched when I first saw it on Star Trek but now such a device would not even get a turn of the head in a shopping mall. Maybe, the beaming up of the video phone user by Scotty might raise a few eyebrows.
Recently I saw a post on Physics Forums that asked about a Taser like plasma weapon using a Tesla Coil to create an electrical discharge. The author saw a problem in that we yet can not direct the discharge at a specific target, at least not at a significant range. To that end he proposed using an IR laser. The laser would in his opinion turn the air along its path into a plasma. Now, the air plasma is an electrical conductor and the electrical discharge from the capacitor bank will naturally favor this path over anything else. That’s how he intended to direct the discharge toward a specific target.
Set Phasers to stun
So in short, the device will fire a laser into a target (in this case a person) and the air along the laser path will turn into plasma. A short time later, the tesla coil will release a discharge and this discharge will flow through the plasmasized air and into the target and through the target into the ground, which of course will theoretically cause an electrical shock to the target person. Unfortunately, the forum killed the question on the grounds it all sounded a bit dangerous, particularly in a forum frequented by smart students. A plasma rifle is fine as long as there is nor real likelihood of it every being built. The weapon could be put on stun with a low electric current or lethal with a high electrical current.
Is a Phaser weapon possible?
It did get me to thinking about would his idea work. Well getting a laser, even an IR to high enough powers to ionise the air would require extremely high power levels. Hardly, portable and could you really create a plasma channel for the discharge to travel down. Well, there has been some success in directing lightning by the use of lasers. The paper below claims to model a 70m channel of plasma using a 50 ps, 20mJ laser pulse. That is a Gigawatt pulse but with a one shot pulse followed by a discharge voltage the idea, at least in principle, would work.
Plasma channel formed by ultraviolet laser pulses at 193 nm in air Yuanyuan Ma, Xin Lu, Tingting Xi, Qihuang Gong, Jie Zhang Chinese Optics Letters , Vol.07 , Issue 09 , PP.865 (2009)
Abstract The propagation of picosecond deep ultraviolet laser pulse at wavelength of 193 nm in air is numerically investigated. Long plasma channel can be formed due to the competition between Kerr self-focusing and ionization induced defocusing. The plasma channel with electron density of above 10^13 cm-3 can be formed over 70 m by 50-ps, 20-mJ laser pulses. The fluctuation of laser intensity and electron density inside ultraviolet (UV) plasma channel is significantly lower than that of infrared pulse. The linear absorption of UV laser by air is considered in the simulation and it is shown that the linear absorption is important for the limit of the length of plasma channel.
Let us make some basic estimates. The ionization potentials of oxygen and nitrogen are 12 eV and 16 eV, and their fractional contents in the common air are 21% and 78%, respectively. Thus, a complete single-electron ionization of a 1 m—long, 100 lm—diameter air filament will require the expenditure of about 0.5 J of energy from the heater pulse. Commercial Q-switched Nd:YAG lasers are available with pulse energies of up to 50 J and custom laser systems may deliver yet much higher pulse energies. Thus, from the energy standpoint, the generation of a completely ionized, 100 um-thick plasma channel or an array of several such channels, with the length of the order of tens of meters, is feasible. See for example Pavel Polynkin and Jerome V. Moloney , APPLIED PHYSICS LETTERS 99, 151103 (2011).
A number of recent studies of plasmas at atmospheric pressures have detected plasma jets consisting of high velocity bullets. The plasma densities are such that the bullets would cause very little damage to anyone hit by them, but one could imagine scaling up the power and distance into something more lethal. See for example – Experimental investigations of plasma bullets, Journal of Physics D: Applied Physics, Volume 42, Number 5. N Mericam-Bourdet et al 2009 J. Phys. D: Appl. Phys. 42 055207
The Plasma sword presents a problem in how do we confine the plasma to the region of interest. I have not figured that one out yet. However, I think there is enough evidence to say that an effective plasma weapon could be created with today’s technology. The plasma teaser being the most interesting, maybe we could call it a crowd pl-easer.