how to render volume as image with alpha background? I have a problem when I render this (using f12 ) node tree on my object in blender cycle I get nothing I tried to render another things and I get a results but with this node tree in my screen shot I get nothing 
in render mode 


when i click f12


my node tree and my transparent option
The problem is due to the limitation of the image viewer (and the same problem occurs if you try and save your render as PNG) and is the same as that described in Rendering fire animations with transparent background. Change the viewer mode into RGB mode and you should see the results.

When rendering such an image (with combined alpha and emission) you should use a format that supports such images - such as EXR.