First Leg of the Beamline

last modified: Sept. 20 2007

Here we take the beam from the Ion source and focus it, steer it, shape it, determine its profile, and then either deflect it or guide it into a Faraday cup. Here's the current setup in the Lab (right to left: source, first Einzel lens, ExB filter, second Einzel lens, steerers, alignment bellows, beam defining slits, beam profile monitor, retractable Faraday-cup, chamber for spherical deflector, another beam profile monitor, and a static Faraday-cup.) Below the image, there's a more detailed view on the single elements (and for our reference, here's a link to the parts lists in case we want to make some more).

Top View
Side View

Miniature Einzel Lens

Our Einzel lens sits right after the exit of the Wien Filter that cleans up our beam from whatever other ions/electrons are extracted from the plasma in the source. To catch the beam at a small diameter, we need to put the Einzel Lens very close to the Wien Filter exit aperture. Following advice from Michael Rappaport, we have used ceramic balls as spacers between the three elements. A 45 mil (1.15mm) diameter wire is TIG welded to the top of the central lens element. Next time we should use a smaller diameter wire: the welder likes thicker wires because they don't just evaporate when he does the weld, we like a thicker diameter because it gives the wire stability, but we will still have to bend it. In the DIN100 Tee we used we had only little access to bend the wire at the right position, and it was a bit of a fight to make the connection to the electrical feedthrough nice and sound. The lens has now been conditioned to 25kV, which should be more than enough for everything we plan to do with it. Note that the only flange that can be opened without damaging anything is the downstream-flange of the tee (because of the electrical connection)! Here are some pics:

The assembly:

Miniature Einzel Lens after assembly

Mounted on the Wien Filter exit:

Miniature Einzel Lens mounted

Inside the tee with the electrical connection:

Miniature Einzel Lens electrical connection

XY Deflector

The steerers a scaled down version of a design we were given by Aaron Covington (many thanks again!). They fit into one 6" tee; according to our simulations they will deflect our beam nicely with a few 100V defletion voltages. We have two pairs of horizontal and vertical deflection electrodes, allowing accurate placing of the beam rather than just introducing an angle.

Right after mounting:

Closer view:


We have bought movable slits from National Electrostatics Corp. (NEC) that allow reading the current of the four jaws.

Beam Profile Monitor

We have bought a BMP 80 beam profile monitor from National Electrostatics Corp. (NEC) that allows us to determine the shape of the beam. With the beam travelling in direction z, a wire with the shape of a helix is rotated with the rotation axis at 45° to the x-axis (and/or y-axis). Because of the wire's helical shape and the inclination of the rotational axis, the wire crosses the beam once in the horizontal and once in the vertical direction on a 360° rotation. Secondary electrons emitted when beam is hitting the wire are collected by a cylindrical collector around the wire and give a current reading that allows determining the beam intensity as the wire moves through the beam, hence we can determine the beam profile. Sadly enough, NEC was not willing to make a dual wire scanner as they invented and use it at MIRF.

90° Deflector

We deflect the beam by 90° with a spherical deflector such that no UV photons from the ion source can reach the interaction region of the experiment. We decided to use a spherical deflector because (in a perfect world) it doesn't screw up our beam shape and needs only low voltages to operate as compared to other options like cylindrical deflectors or quadrupole deflectors. Again, the design was originally inspired by a design from Aaron Covington that he gave us, but he used spherical plates that we found would be too hard to obtain/machine. So we changed the design considerably. There are two alignment holes in the outer deflection electrode. Machining of the spherical surfaces was done at the Nevis machine shop with a CNC mill. Here is the complete assembly:

bender completed

Faraday Cup

We have a Faraday Cup following the design of Thomas et al. (Nucl. Instrum. Meth. A 536 p.11-21, 2005) that should allow for accurate current readings at low repeller voltages due to nifty design. We have also made a modification to the design to have a shorter, retractable cup with the same entrance opening.

retractable cup