Technology Development for Mid-Frequency Gravitational-Wave Detector

该奖项支持低温梯度学的研究，并解决了NSF“宇宙之窗”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的一个突破性研究领域，这要归功于长期的技术发展，这使得NSF的先进LIGO天文台从2015年开始成功探测到引力波。然而，先进的LIGO探测器只对大约20赫兹以上的频率敏感，而将于2030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是一种新型中频引力波（GW）探测器，用于探测介于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在低温下增强的材料稳定性。与LIGO不同的是，SOGRO对来自天空中任何地方的gw同样敏感，并且能够分辨出源方向和波的偏振。本研究项目的目标是通过改造现有的超导重力梯度仪（SGG）来演示SOGRO的关键技术。这项工作将测试探测器概念的技术合理性，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进行精密超导测量技术。除了GW科学之外，改进的SGG还可以为地球物理学，特别是新兴的重力辅助地震预警领域带来好处。通过探测地震早期由地面破裂产生的瞬态重力信号，这些信号以光速传播，sgg可以减少早期预警系统“盲区”的大小，并增加预警时间，这应该可以减少地震造成的损失。通过结合六个对称分布的测试质量在球上等距点的运动，SOGRO探测器设计具有球形GW天线的全张量特性。到20世纪90年代初，具有19厘米基线和机械悬挂测试质量（TMs）的三轴对角线分量SGG在1 Hz时达到了5 x 10^(-13) Hz^（-1/2）的应变灵敏度水平，比迄今为止其他梯度仪（包括原子干涉仪和扭力棒）所证明的灵敏度提高了三个数量级。SOGRO是SGG张量的放大版，基线为50米。为了达到对GW信号的目标灵敏度，SOGRO必须满足几个苛刻的技术要求。其中两个要求是：(1)悬浮超导TMs在0.1 K温度下的差模频率必须低至0.01 Hz，质量因子必须高至10^8；(2)悬浮平台必须以使探测器与地面角运动隔离到百万分之一的方式悬浮。当前研究项目的目标是用磁悬浮TMs和13.5 cm基线（为nasa资助的项目建造）修改现有的SGG，并演示(1)在T = 4.2 K时，差模频率低至0.3 Hz， Q高达3 × 10^5；(2)通过将平台悬挂为钟摆，将地面角运动隔离10^3倍。这些特征的成功演示将使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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.