Plasma generated electromagnetic oscillations at the frequency an order ion-cyclotron have been investigated in the high energy content regimes of GDT operation. That waves are found to be an Alfvén ion-cyclotron instability. The microinstability threshold scaling law has been defined: with increase of the plasma column radius to Larmor radius ratio the threshold value of the diamagnetism in the midplane decreases. It qualitatively matches the theoretical calculations.