How to output file to /dev/ram2?

Hi all,
I want to maintain a RamDisk file "ram_disk.gz", I
unzip it "ram_disk" fisrt, and then try to coyp it
to /dev/ram2 using command: dd if=ram_disk of=/dev/ram2,
but I got a error message:"dd: opening /dev/ram2: device
not configured", why? and how to resolve this problem?

Thanks　　　　　　

mke2fs /dev/ram 2048