Electron Focusing Lens

For a Modulated X-ray Source

Background

The MXS has been worked on by Olin students before, check out their work here.

The MXS

MXS stands for Modulated X-ray Source, a device which Goddard Space Flight Center (GSFC) uses for various applications including instrument calibration and X-ray communication. The MXS uses ultraviolet light to produce electrons which are accelerated by a high voltage int a target to produce X-rays. This system can pulse the X-ray output as fast as the driver can modulate the ultraviolet LED with no moving parts, which makes the MXS perfect for spaceflight.

The MXS in block diagram form.

One downside of the system is the severe reduction (On the order of 10^-8) in the number of X-rays released relative to the number of incedent photons from the UV LED. To counter this reduction, an electron multiplier was inserted into the system. The electron multiplier consists of a set of six twisting so that electrons travelling through it experience a number of collisions. The interaction can be simplified as follows: Each time an electron hits the wall of a tube it produces two electrons which travel further into the multiplier, each having interactions that double their number as well. The gain (electrons output for every electron that enters) of this multiplier is approximately 10^8, so given the doubling process that the electrons experience we can assume that there are roughly n interactions needed to reach the gain of 10^8. 2^n = 10^8, which means n is approximately 28. If these 28 interactions are evenly distributed down the length of the multiplier, and we assume the electric field (caused by holding the entrance of the multiplier at -3kV and the exit at 0V) is also evenly distributed, the energy difference between each step is 3kV/28 = 107. This means that the electrons leaving the multiplier have an average energy of approximately 107 eV.

The Electron Multiplier relative to other components of the MXS.

The shape of the tubes within the electron multiplier ensure a high amount of collisions, but they also cause the electrons to leave the multiplier in six distinct beams. This can be seen when looking at the target of the MXS, where a "spiral galaxy" shape- the same shape as the exit of the electron multiplier- is burned on after long use.


The exit of the electron multiplier in comparison to the "burn" mark left on the
MXS target (above) and an X-ray "pinhole" image of the X-ray source. (below)

Focusing the Beam

In order to improve the characteristics of the exiting X-ray beam the beam of electrons from the electron multiplier must be focused to a single point that is as small as possible. Our team is developing a type of electrostatic lens, the Einzel lens, to insert into the MXS system that will turn the six distinct beams into one small one. An Einzel lens consists of three plates or rings held at different potentials that can focus a beam of electrons to a single point without changing the energy of the beam. By tightening the focus of the electron beam, the resulting X-ray's leaving the MXS should also have a tighter focus providing for more efficient data transfer.

A simplified diagram of an Einzel lens.