The regular ~1nm variations in ETM coating thickness cast a diffraction ring onto baffles that may produce excess noise through modulation of retro-reflected light (https://dcc.ligo.org/T1300354). A test of whether this scattering noise may limit our sensitivity could be made by shaking a beam tube baffle in the region where the maximum light power falls on the baffles, about 2375 m from the vertex. Because only one or a couple out of many baffles would be affected by shaking and because the LLO sensitivity is still about 3 orders of magnitude away from the goal at the most troubling beam tube resonance (~14 Hz), the increase in motion over normal would have to be at least 4 orders of magnitude for the present LLO sensitivity.

In order to see if I could increase motion of the beam tube by this much I made a pusher system and tried it out here at LHO. Figure 1 shows the voice coil shaker and the coupling rod that has interchangeable springs to optimize force. A universal joint is required to minimize non-axial forces on the voice coil plunger and the coupling rod. The system was powered by a 150 W inverter in an Uplander LT van. Figure 2 shows the relative positions of, at the left, the accelerometers, in the middle, the shaker, and, on the right, a beam tube enclosure door.

Figure 3 shows that the shaker increased the axial beam tube motion at the accelerometer by 4 orders of magnitude at 14 Hz. The noise floor for the shaking injection is higher than for non-shaking because I had to reduce the gain by 10. The injection is not as monochromatic as I would like. This version 2 is better than the first version, but there are still peaks injected at higher frequencies from slight rattling of bolts, springs and other components in the rod. Nevertheless, it looks like it could be used for a test as long as we are focusing on direct (vs. upconversion) coupling.