Quadrupole Focusing

In Section 4.7, it was shown that the ion beam passing through the sector field exit slit (the magnet image point) should exhibit a crossover point in the dispersive direction (X) and an elliptical cross section in the non-dispersive direction (Y). The sector field has essentially removed the inherent cylindrical symmetry of the plasma extracted ion beam because of its ″handed-ness.″ The crossover and slight vertical divergence was fixed so the deceleration step could be carried out on an as-symmetric-as- possible, parallel beam having circular cross section and small waist. This was

accomplished using electrostatic quadrupole fields in the beam drift space between the exit slit and decelerator. A quadrupole field scheme was chosen because its two plane symmetry seemed better suited to fix the unequal divergences of the beam in the X and Y directions separately. Also, a quadrupole field provides much stronger focusing action than an axially symmetric lens of comparable length and field strength making the beamline shorter (Lawson, 1988).

A single quadrupole lens focuses in one plane and defocuses in the other.

However, two lenses arranged in a focusing-defocusing pair have an overall net focusing effect (Lawson op. cit.). Along this idea, a quadrupole doublet with eight independently adjustable lens elements was constructed for focus correction of the ion beam after the magnet exit slit. This system is shown in Fig. 4.15. It consists of eight rectangular aluminum plates held in place using a Delrin support structure that slips inside and keys to the ID of a custom 6-way cross (6″ CF flanges) attached after the magnet exit slit chamber. This chamber is also floating with the beamline and pumped by a 200 L/s Alcatel turbo-pump through another glass electrical break. The lenses were biased for Y- focus correction in the first of the quads and then X-correction in the second. The idea being that the first quad is used to over-correct the Y-divergence to account for the defocusing action in the Y-direction of the second quad. The two Y-plates in the first quad and the two X-plates of the second were independently adjustable with the remaining plates connected directly to the beamline floating potential.

Four separate floating (on the beamline) +4 kV DC supplies (Ultravolt, model 4A12P) serve as the individual quad plate biases. Line power for these supplies is provided by the same 20 kV isolation transformer mentioned earlier.

Finally, there was a 6″ CF gate valve at the quad chamber exit to isolate the front beamline from the scattering chamber when the plasma chamber must be taken apart and cleaned. This allows UHV to always be maintained in the scattering chamber and scattered product detector. Typical pressures in this chamber were 1-5•10-7 torr,

irrespective of plasma operating pressure. The huge cryopump on the previous stage as well as the Alcatel turbo provided significant reduction in the pressure load to the downstream portions of the beamline.

4.8.2 10° Ion Deflector Magnet

Collisions of energetic ions with background gas atoms can generate fast neutrals through charge exchange processes. In fact, fast neutral beams are produced using this method by shooting fast ions through a charge exchange cell containing a background gas in the mTorr range (Souda et al., 1995). This process of fast neutral generation, although much less significant in our system, could possibly influence scattering results.

Unfortunately, fast neutrals are very difficult to measure quantitatively, because they must be ionized and detected as charged species. The efficiency for ionization scales inversely with velocity because faster particles spend less time in the active ionization region, therefore, neutrals in the keV range are extremely hard to detect (Scoles, 1988). We have therefore taken the approach to rid the beam of fast neutrals on purpose, even if they may not exist to a significant degree in our system. This is simply done by

deflecting the ion beam 10° with a small magnetic sector field right before the decelerator entrance so that any fast neutrals in the upstream beam are not within line-of-sight of the target. The short flight distance through the decelerator to the sample is unimportant because this region is held at 1-5•10-8 torr.

A schematic diagram of the 10° magnet is given in Fig. 4.16. Since the field requirement was much less (600 gauss max for ~ 50″ radius), magnet cooling was not important. A C-frame choke of C1018 low carbon steel (2″ x 5″ rectangle) and 5″ OD bobbin support was used. The air gap was 2.5″ with 5″ square pole shoes which gave an EFF calculated virtual pole surface ~1.2″ past the physical pole boundary. The exciter coil was an 80 lb spool of #20 AWG magnet wire (~25,000 ft, estimated @ 12,000 turns) that was specially re-wound by REA Magnet Wire Company on an industry standard 12″

x 7″ reel to have both ends of the continuous wire length accessible. The center of the plastic reel was cut out using a CNC mill and simply slipped over the steel choke. Power for the coil was provided by a 0-600V, 0-1.6A current regulated DC supply from

Lambda-ENI. Since the beam deflection was only 10° and the deflection occurred much closer to the target, a hall-probe feedback system was not necessary to keep the beam from wandering on the sample surface. Performance of the magnet is shown in Fig. 4.17 for field measurements at mid-pole gap.

The 10° beamline chamber (see Fig. 4.18) in this region was made from a stiff hydroformed vacuum bellows (1.5″ OD x 12″ long) from Varian. It was held in place by bolting the 2.75″ CF flange ends to an aluminum frame structure machined to have a 10° misalignment at 50″ radius of curvature. As arcing was a problem, mica sheets were used between the floating chamber and grounded pole shoes of the magnet.