Technology Development for Mid-Frequency Gravitational-Wave Detector

该奖项支持低温梯度测量学的研究，并阐述了美国国家科学基金会“宇宙之窗”重大构想的优先领域。引力波的直接探测是天体物理学和基础物理学中的一个突破性研究领域，这要归功于长期的技术发展，从2015年开始，NSF的高级LIGO天文台成功地探测到了引力波。然而，先进的LIGO探测器只在大约20赫兹以上的频率下敏感，而将于本世纪30年代发射的LISA空间探测器将只覆盖0.1赫兹以下的频率。SOGRO是一种新的中频引力波(GW)探测器的设计，用于探索LIGO和LISA之间的频率范围。SOGRO采用磁悬浮测试质量，并利用超导体的奇异特性和增强材料在低温下的稳定性。与LIGO不同，SOGRO对来自天空任何地方的GWS同样敏感，并能够分辨来源方向和波的极化。本研究项目的目的是通过对现有的超导重力梯度仪(SGG)进行改造，展示SOGRO的关键技术。这项工作将测试探测器概念的技术可靠性，并帮助确定需要进一步研究的领域，以确定SOGRO的可行性，同时培训下一代科学家掌握精密超导测量技术。除了GW科学，改进的SGG还可以造福地球物理学，特别是新兴的重力辅助地震预警领域。通过探测地震早期地面破裂产生的瞬时重力信号(以光速传播)，SGGS可以减小预警系统的盲区大小并增加其提前时间，这应该会减少地震造成的损失。通过结合球面上六个对称分布的测试质量在球面上等距点的运动，SOGRO探测器设计具有球面GW天线的全张量特性。20世纪90年代初，基线为19厘米、机械悬挂测试质量(TMS)的三轴对角分量SGG在1 Hz时达到了5x10^(-13)Hz^(-1/2)的应变灵敏度水平，比其他梯度计(包括原子干涉仪和扭杆)的灵敏度高出三个数量级。SOGRO是张量SGG的一个大大放大的版本，基线为50米。为了达到GW信号的目标灵敏度，SOGRO必须满足几个苛刻的技术要求。其中两个是：(1)在0.1K的温度下，悬浮超导TMS必须具有低至0.01 Hz的差模频率和高达10^8的品质因数；(2)平台必须以这样的方式悬挂，以使探测器与地面角运动隔离到百万分之一。本研究项目的目的是用磁悬浮TMS和13.5厘米的基线(为NASA资助的项目建造)来修改现有的SGG，并展示(1)在T=4.2K时，差模频率低至0.3赫兹，Q高达3x10^5，以及(2)通过将平台作为钟摆悬挂，将地面角运动隔离10^3倍。这些功能的成功演示将使SOGRO的两项关键技术比全尺寸SOGRO GW探测器所需的水平提高两到三个数量级。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award supports research in cryogenic gradiometry and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. Direct detection of gravitational waves is a breakthrough research area in astrophysics and fundamental physics, thanks to long-term technology development which led to the successful detection of gravitational waves by NSF's Advanced LIGO observatories beginning in 2015. However, the Advanced LIGO detectors are only sensitive at frequencies above about 20 Hz, while the LISA space detector, to be launched in the 2030s, will only cover frequencies below 0.1 Hz. SOGRO is a design for a new mid-frequency gravitational-wave (GW) detector to explore the frequency range in between LIGO and LISA. SOGRO employs magnetically levitated test masses and utilizes the exotic properties of superconductors and enhanced stability of materials at cryogenic temperatures. Unlike LIGO, SOGRO would be equally sensitive to GWs coming from anywhere in the sky, and would be capable of resolving the source direction and wave polarization. The goal of the present research project is to demonstrate key technologies of SOGRO by modifying an existing superconducting gravity gradiometer (SGG). The work will test the technical soundness of the detector concept and help identify areas that will require further studies to establish the feasibility of SOGRO while training the next generation of scientists in precision superconducting measurement techniques. In addition to GW science, an improved SGG can benefit geophysics, especially the newly emerging field of gravity-aided earthquake early warning. By detecting transient gravity signals arising from ground rupture in the early stages of earthquakes, which travel at the speed of light, SGGs could reduce the size of the 'blind zone' of early warning systems and increase their lead times, which should reduce losses due to earthquake shaking.By combining the motions of six symmetrically distributed test masses at equidistant points on a sphere, the SOGRO detector design has the full-tensor characteristics of a spherical GW antenna. A three-axis diagonal-component SGG with a 19-cm baseline and mechanically suspended test masses (TMs) reached a strain sensitivity level of 5 x 10^(-13) Hz^(-1/2) at 1 Hz by the early 1990s, which is three orders of magnitude more sensitive than demonstrated to date by other gradiometers, including atom interferometers and torsion bars. SOGRO is a greatly enlarged version of the tensor SGG with a 50-m baseline. To reach its target sensitivity for GW signals, SOGRO must meet several demanding technical requirements. Two of them are: (1) the levitated superconducting TMs must have differential-mode frequency as low as 0.01 Hz and quality factor as high as 10^8 at the temperature of 0.1 K, and (2) the platform must be suspended in such a way to isolate the detector from the ground angular motion to one part in a million. The objective of the present research project is to modify an existing SGG with magnetically-suspended TMs and a 13.5-cm baseline (that was constructed for a NASA-funded project) and demonstrate (1) a differential mode frequency as low as 0.3 Hz with Q as high as 3x10^5 at T = 4.2 K, and (2) isolation of the ground angular motion by a factor of 10^3 by suspending the platform as a pendulum. Successful demonstration of these features would advance two key technologies of SOGRO to within two to three orders of magnitude from the levels required for a full-scale SOGRO GW detector.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.