Or use just the emission for the volume:
[![enter image description here][1]][1]


EDIT:
Just to give you an idea how it can look like with just emission and render really fast:
[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/rRrEr.jpg
  [2]: https://i.sstatic.net/1f1QX.jpg