Although the noise level was improv

Although the noise level was improved by introducing SAS, the detector sensitivity of TAMA is still limited by technical noises rather than fundamental noises such as thermal noise and quantum noise. The main contribution in the 100-500 Hz band comes from upconversion, which is induced by feedback forces applied on the test masses in the low frequency band (< 1 Hz). The noise is expected to be related to acoustic noise emission from Nd-Fe-B magnets attached to the mirrors, due to discontinuous jumps of the magnetization (Barkhausen effect [66]). The alignment control noise limits the sensitivity below 100 Hz and further reduction of control bandwidth is required to expand the detection band of gravitational waves down to ∼10 Hz. Experience in TAMA indicates that it is essential to reduce low frequency fluctuation of the mirror displacements and angles with appropriate control loops. Otherwise, the attenuation performance of SAS is spoiled by control-noise couplings and upconversion noise.