Unable to release Tempdb free space

Hi All,

My tempdb drive size is 50 GB. Now it is occupied 43 GB. when i checked sp_spaceused, the result as below.

database_name | database_size | unallocate space

tempdb               | 43161.94 MB   | 42953.48 MB

reserverd |  data | index_size | unused

1560 KB   | 688 KB | 712 KB     | 160 KB

Image may be NSFW.
Clik here to view.

when is tried to shrink, it is not releasing any space. there is no any active transaction.

can any one let me know why i am unable to release the space and also let me know how to release the space.?

Thanks & Regards

Krish