Tan without pan

it is not possible and also not logical.

PAN is the primary requirement of Income Tax. Its not logical because you want to apply for TAN , at the time of application you have to provide PAN. After deduction of TDS as and when you will upload E TDS return there also you have to mention your PAN. As when you will issue Form 16/16A/27D there also it is mandatory to mention PAN of deductor.

Govt.depts. like Railways , Telephones, Postal Dept. etc. etc.etc. will not have PAN but still they have to deduct tax . Though they do not have PAN they deduct, they file return (without mentioning PAN). 

so it means it is not necessary to have a pan no to apply for tan

Yes, it is not necessary