Technology Development for Mid-Frequency Gravitational-Wave Detector

该奖项支持低温梯度测量的研究，并解决了NSF“宇宙之窗”大创意的优先领域。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的高级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20 Hz以上的频率敏感，而将于21世纪30年代发射的丽莎空间探测器只覆盖0.1 Hz以下的频率。 SOGRO是一种新的中频引力波（GW）探测器，用于探索LIGO和丽莎之间的频率范围。 SOGRO采用磁悬浮测试质量，并利用超导体的奇异特性和低温下材料的增强稳定性。 与LIGO不同，SOGRO对来自天空任何地方的GW同样敏感，并且能够分辨源方向和波偏振。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的精确超导测量技术。 除了GW科学之外，改进的SGG还可以使地球物理学受益，特别是新兴的重力辅助地震预警领域。 通过检测地震早期地面破裂产生的以光速传播的瞬时重力信号，SGG可以缩小预警系统的“盲区”，增加预警系统的准备时间，从而减少地震震动造成的损失。通过将六个对称分布的测试质量在球体上等距点的运动结合起来，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 cm基线和机械悬挂试验质量（TM）的三轴对角分量SGG在1 Hz处达到了5 x 10^（-13）Hz^（-1/2）的应变灵敏度水平，这比迄今为止其他梯度仪（包括原子干涉仪和扭杆）所证明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必须满足几项苛刻的技术要求。 其中两个是：（1）悬浮的超导TM必须具有低至0.01 Hz的差模频率和在0.1 K温度下高达10^8的品质因数，（2）平台必须以这样的方式悬浮，以将探测器与地面角运动隔离到百万分之一。 本研究计划的目的是对现有的SGG进行磁悬浮TM和13.5 cm基线的改造（为NASA资助的项目建造），并证明（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。 这些功能的成功演示将使SOGRO的两项关键技术从全尺寸SOGRO GW探测器所需的水平提高到两到三个数量级。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.