When it is built, the Next Linear Collider (NLC) will be ten times the size of SLAC, and its beam at the point of collision will be on the scale of nanometers. Because the beams of electrons and positrons will be so small, tiny movements such as ground vibrations may upset them and cause them not to collide. The research done this summer is one of several projects that may give a solution to the problem of ground vibrations and the NLC. The idea is to build a accelerometer so that one can measure movements in the ground, correct for them, and keep the beams in collision.
This accelerometer would need an inertial mass in it that remains fixed for high frequencies (5-1000 Hz). If the accelerometer moves with respect to the inertial mass, it will only be due to vibrations. Then these vibrations can be measured and corrected for to keep the beams in collision. The intertial mass would be held by a soft spring. Thus the mass would remain fixed in space for high frequencies. Low frequencies (those below 5 Hz) would be detected using the beams themselves.
The structure of this vibration detector would be a double capacitor (see schematic diagram). In sending a positive sine wave through one capacitor and a negative sine wave through the other, the amplitude of the output wave will be proportional to position. If the middle plate, which is the fixed mass, does not move, then the output is zero (the sum of a positive and negative sine wave). If the mass moves up, the top capacitor gains in capacitance and the lower loses some; this means the positive sine wave will be larger in amplitude than the negative sine, and the sum of the wave amplitudes will be positive. So the output amplitude from the accelerometer would be proportional to the position of the inertial mass.
The accelerometer would sit on top of the final magnets in the collider. These magnets are responsible for keeping the beams in contact. Once the intertial mass movement is measured, the magnets would be pushed up or down using a computer feedback system.
Although accelerometer are commercially available, they cannot be used for this project. The accelerometers that are sensitive enough to detect motion on the scale of nanometers are too big and do not work in magnetic fields. Plus, they are too expensive to be used in large quantities.
The Summer Project
The research done this summer involved working on the design and testing of a prototype accelerometer that would sense the magnet motion of the NLC. Most of the tests carried out involved testing the electronics that would be used in constructing this motion sensor system, and determining whether or not the electronic equipment available was sensitive enough to allow for detection of nanometer motion.
A test capacitor was also built (see photo). This is a double capacitor with brass plates and non-conductive spring made of delrin. In this test capacitor, the gap between the bottom plates is fixed. The top gap can be changed by applying a force with a micrometer. This capacitor is a tool to test the electronics involved.
The end result of the research was that the electronic noise in the system with the test capacitor in place was less than .5 nanometers. This is a promising result because it means nanometer motion can be detected by the available electronics.
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Schematic Diagram of Circuit