In vacuum or background gas?

background gas

For short pulses acting on the target we have plasma plume expanding into vacuum or ambient gas. There is no stationary plasma length since plasma is expanding.

In stationary case situation is different. The plasma expands as long as there is positive energy balance at its front. So, along the paralel laser beam plasma moves in the direction of the laser with velocity proportional to the laser intensity minus energy losses.

When the laser beam is divergent the plasma eventually stops. In general the position of plasma front is given by locally (at given point) energy balance -- the power of emitted radiation and the power absorbed from the laser beam. Thermal conductivity are less important. Some considerations of stability lead to election of proper point of equilibrium. 

Such topics were examined some 40 years ago. Please refer to my conference paper:

https://www.researchgate.net/publication/252503285_Continuous_Optical_Discharge-Spherical_Symmetric_Approach

I have no other data at hand. The cone of focused beam can be considered also as spherically symmetric (part of sphere) with the focus at the center.

Conference Paper Continuous Optical Discharge-Spherical Symmetric Approach

Ali, so there are several factors involved in terms of laser intensity, material, gas pressure etc. Here are a few references to expansion into a gas.  This is studied in Pulsed Laser Deposition to understand the layer growth characteristics.

Check the following list:

http://jetp.ac.ru/cgi-bin/e/index?a=s&auid=637720

See the work with number 4 and 9.

Dear Ali,

Normally if a plasma is generated by focusing a pulsed laser at the surface of any sample, the laser produced plasma plume will be generated and it will expand perpendicular to the surface of the sample. The plasma expansion depends on the environment in which the plasma is generated. The plasma will expand more if it is produced in vacuum. However, if it is generated  in air, then it will not expand much but the plasma temperature will be gigh. The best way to monitor the length of the plasma  expansion is to register it using a CCD.

with best wishes,

Aslam