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To improve the magnetic noise projections from the far-field, we designed and built a new coil to inject magnetic fields. Since the injections are done at a distance of around 20 meters, it was hard to see the injected lines in B1 with the old coil, which was designed for a different purpose. Coil diameter, number of turns and wire diameter were optimized (within practical limits) to get the strongest magnetic fields around 100 Hz, given the specifications of the used audio amplifier. The old coil has an average radius of 10 cm, about 10000 turns and weighed about 40 kg, while the new one has a radius of 50 cm, 50 turns and (very conveniently) weighs just a few kg. The large number of turns is partly compensated by the bigger area of the coil, but the biggest improvement comes from the much lower self inductance, which limits the current at high frequencies (7 mH instead of 13 H). See fig 1 and 2 for some pictures.

Fig 3 shows a comparison between the two coils of the line amplitudes measured with the big magnetometers, we gain a factor of 50 at 200 Hz. We should be able to increase the field a bit more by using a shorter and thicker wire to connect the coil and by driving the amplifier to its limits.

On friday morning, we used the new coil to perform a far-field injection from the balcony on top of the DAQ room. Instead of a stand-alone signal generator, we now use a DAC channel and the calibration server for the optical calibrator to generate the lines. This allows scripted injections that are perfectly synchronous with the DAQ system, which should help with retrieving weak lines at high frequencies. Fig 4 shows the SNR for the lines retrieved from the magnetometers and B1. Using slightly longer and stronger injections we can hopefully go up to 200 Hz.

Fig 5 shows the obtained transfer function, which is pretty similar to the old one. Fig 6, finally, shows a provisional noise projection using clean data from the AC OFF test. The projected seems slightly higher than before, so probably the magnetic background is higher. Between 20 and 30 Hz, the projection is about a factor 3 below the sensitivity, which would mean it is contributing a little bit. Note that the error in the projections might be as large as factor of 2. The injections should be repeated in the central, MC and end buildings (which should hopefully not contribute after the magnet change).

Thanks to M. Bazzi and E. Campagna for help with constructing the coil.