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Figure 3: The CAPP experimental hall allows multiple dark matter search experiments to be installed and operate at the same time. The laboratory includes seven low vibration pads for parallel experimental setups, as well as several refrigerators and superconducting magnets.
sensitive result in the mass range to date, that there is no axion dark matter or axion-like particle within that range. A key component of the laboratory’s most
recently implemented data acquisition system is a Spectrum Instrumentation PCIe Digitizer card, model M4i.4470-x8. The scientists chose this unit as it can sample the incoming signals on two channels simultaneously, at rates up to 180 MS/s and with 16-bit vertical resolution. Importantly, the collected data can be streamed to a computer, over the PCIe bus, at a transfer speed in excess of 3 GB/s. This means that all of the data can be acquired and sent to the host computer with none of it being lost. A block diagram of the typical experimental setup is shown in figure 2. Dr. ByeongRok Ko, research fellow at CAPP,
explains, “Our aim was to improve the figure of merit in axion haloscope searches, i.e., the scanning rate. First, we realised a practical DAQ efficiency of greater than 99 per cent for a single channel, where the DAQ process includes the online Fast Fourier Transforms (FFTs). Then, using an IQ mixer and two parallel DAQ channels, we implemented software-based image rejection without losing the DAQ efficiency. This is more than twice the efficiency of earlier setups, which were based on using a conventional spectrum analyser.” Image rejection is important as axion
haloscope searches generally employ a heterodyne receiver which introduces unwanted image backgrounds. As such, image rejection becomes essential for maintaining a high scanning rate. In this case, the fast DAQ system provides an image rejection of about 35 dB, over a frequency range from 600 to 2200 MHz. “There are two important features of the Spectrum digitizer card that enable this which is why we chose it,” says Dr. Ko. “First, the on-
Instrumentation Monthly January 2023
Figure 2: In axion search, the signals from the haloscope are amplified before being acquired by the DAQ.
board memory of 2 GSamples, which can be used as a buffer, and second, a FIFO transfer mode over the PCI Express x8 Gen2 interface, that allows the continuous data streaming at speeds of greater than 3 GB/s.” Another challenge for the DAQ system is the
post processing of the data. This includes unit conversion, online FFT, averaging, and writing the power spectra to disk storage. The online FFT dominates the post processing time. In most cases, axion haloscope experiments require data at different resonance frequencies because the axion mass is unknown. Also, for various reasons, the data at each resonance frequency can be divided into several subsets at different timestamps. In such cases, the post processing can be performed in parallel while the next data is being acquired. Fortunately, the Spectrum Instrumentation digitizers come with drivers that support a wide range of programming languages, one of which is Python. Python’s multiprocessing module is ideal for this type of application and was subsequently used to demonstrate the above scenario. Yannis K. Semertzidis, the director of CAPP
and also a professor of KAIST (Korea Advanced Institute of Science & Technology), explains, “This experiment is not a 100-meter sprint but the first goal in a marathon run. We learned by doing it, and we tested new concepts to be used at higher-level systems in the future.” The team proved they can reach much better sensitivity than all other experiments previously done over the target frequency range. The plan now is to scale up their experiments with larger systems. To accelerate the search process, the team at
CAPP now runs multiple experiments using a variety of system designs. This allows the targeting of different axion mass ranges in parallel. Figure 3 shows a panoramic view of the CAPP experimental hall with its multiple experimental setups. Interested readers can follow the experimental progress on the CAPP website at
capp.ibs.re.kr/html/capp_en/
Spectrum Instrumentation
www.spectrum-instrumentation.com
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