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At ANSTO we have developed an Electron Cyclotron Resonance (ECR) ion source to investigate new concepts for mass spectrometers designed to measure isotopic ratios of elements such as carbon, nitrogen and oxygen. The low pressure ECR plasma presents particular challenges when used for mass spectrometry. The elements we are interested in measuring are typically present as residual gas in vacuum systems and hence we need to achieve ultra-high vacuum throughout our system. Also ECR plasmas generate highly reactive species of these elements which can then bond to internal surfaces. A number of measures have been taken to combat these difficulties. For example, we have shortened the plasma bottle length to minimise the surface area. In making this change we have also discovered that the useful plasma volume is much less than expected. Originally the source was designed with a mirror ratio of around 2.2. With the restricted bottle size, our effective mirror ratio is 1.7 and yet the performance of the source is unaffected. This and other design modifications will be discussed.
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