Generating 64 bit random values using AES-CTR I need to generate 64 bit random values using AES-CTR.

Using fixed 64 bit plain text and sequential nonces can I assume the  higher order 64 bit output values of the AES-CTR to be random with a period of $2^{64}$?
AES-CTR can be used as a random number generator if you don't mind that it never repeats the same 128 bit value - but the chances of that happening are minimal with a true RNG anyway.

In that case there is no need to have any plaintext though: you'd simply create the key stream (the encrypted counter values). If you only have a higher level cipher at your disposal you can simply do this by encrypting zero bytes.

If your key (the seed) is random then you can leave the IV/nonce set to zero as well and just start the counter (encoded as 16 bytes) at all-zeros.



If you'd use only 64 bitsof the 128 bit key stream generated by encrypting the counter then you would not have a repeated block within $2^{64}$ blocks. Although AES-CTR does have a period of $2^{128}$ blocks there is no guarantee that a value of 64 bit isn't repeated earlier.

Of course the cipher still has a period (as in, a long repeated sequence) of $2^{128}$ blocks as the counter will restart at zero when that happens, inconsequential of how many bits are actually used of each block.