The plasma produced by focusing a passively Q-switched 30 J ruby laser beam is studied as regards its suitability for minimum ignition energy measurement. By comparison with electric spark discharges, it would appear to offer shorter times and smaller volumes of the initiating plasma, as well as freedom from energy losses to electrodes and elsewhere within the circuit. The growth of the initiating plasma as well as that of the flame kernel are studied as a function of pressure and composition, the former using an optical delay line with a schlieren system and the latter by high-speed schlieren streak photography. It is found that the difficulties of the method are associated with the finite duration of the laser pulse. In the time between the onset of breakdown and the end of the pulse the plasma front facing the incident beam absorbs the incoming energy, leading to the production of an extended and unsymmetrical initiating source, an absorption threshold which may already be in excess of the minimum ignition energy, and the formation of a blast wave which may be powerful enough to initiate a detonation rather than a deflagration. For a pulse of approx. 20 nano second half width these effects become serious above about half an atmosphere for stoichiometric methane-air mixtures. For lower pressures, or near limit mixtures, the promise of the method is borne out and both ignition energies and quenching distances fall below those determined by the electric spark method. For higher pressures and faster reacting mixtures it will be necessary to decrease the duration of the laser pulse and the size of the plasma.

A preliminary investigation of the use of focused laser beams for minimum ignition energy studies

The use of calorimeters for measuring laser output energies and powers is broadly reviewed. Instruments are categorized as disc, cone, hollow-sphere, bolometer, volume-absorption, and partial absorption types.

http://iopscience.iop.org/article/10.1088/0022-3735/6/2/001/pdf

Calorimetric measurements of laser energy and power

Laser techniques in combustion research

Experimental observations are given of the shape and structure of laser-induced sparks formed in molecular and atomic gases. Absolute measurements of the laser energy absorbed during the breakdown process are presented.

Laser-induced multiple breakdown in gases

The design, construction, and operational characteristics of a new type of power meter for high average power cw lasers is described. This multikilowatt calorimeter unit employs a novel high‐velocity spinning water film to provide complete absorption of the incident laser radiation, at either high‐ or low‐power levels, while still retaining good dynamic response and sensitivity. The design is sufficiently simple such that it can be easily fabricated from readily available workshop materials. Utilization of a suitably selected recirculating absorbing dye allows the power meter to be used with a number of different cw lasers.

Spinning water film power meter for high‐power cw lasers

A new type of absorption calorimeter is described which is capable of measuring absolute energies over a wide range of laser outputs with an accuracy to better than 5%. It consists essentially of a spherical cavity which absorbs laser radiation focused into it by means of a lens. Experimental results are given for ruby laser pulses together with estimates of the errors involved using this technique.

https://iopscience.iop.org/article/10.1088/0022-3735/2/2/433/pdf

J R Wilson

Papers

A spherical cavity calorimeter for measuring absolute laser energies