Vacuum chamber design rules

small gap: reduced number of At of coils, pole radius reduced, orbits close to outer edge; large gap: large space: injection, extraction, probes, easier vacuum pumping

Zaremba, Magnets for Cyclotrons (2005) — p. 22

Tabletop: Frames the central tradeoff for a next machine: shrinking the reference machine's gap raises B for the same 538 turns, but everything (dee aperture, ion source, probe) must still fit and pump through it.

The total required aperture size is the sum of the good field region, the vacuum chamber thickness (0.3-2 mm) and a margin for installation and alignment (0-5 mm).

Zickler, Basic Design and Engineering of Normal-Conducting, Iron-Dominated Electromagnets — arXiv:1103.1119 (2010) — p. 14

Tabletop: Explains why the pole gap must exceed the chamber's internal height by a centimetre or so; useful when trading gap (and hence amp-turns) against chamber wall thickness.

top and bottom of the vacuum chamber should be thin, circular steel plates ... to decrease the magnetic gap as much as possible. To prevent field bypassing, the tank wall must be non-magnetic, preferably brass

Wouters, General Recommendations for Design of Small Cyclotrons — UCRL-476 (1949) — p. 5

Tabletop: Directly applicable chamber architecture for a small machine; every millimeter of chamber wall inside the gap costs ampere-turns.

it is advantageous to keep the gap between the magnet poles small. This tight spacing made the design of the vacuum chamber more difficult, but it was essential.

Baumgartner, The Cyclotron Kids' 2 MeV Proton Cyclotron — Cyclotrons 2013, WE1PB05 (2013) — p. 1-2

Tabletop: The central trade for a next machine: every millimeter of gap saved is field (and energy ~B^2) for free.

As the vacuum chamber radius is 2.2 cm larger than the radius of the magnet poles, these lids act as wide pole faces that draw the magnetic field lines out to larger radii.

Morrow, Focusing in the Houghton College Cyclotron — Houghton College thesis (2015) — p. 54-55

Tabletop: The highest-leverage cheap upgrade in this batch: swapping aluminium chamber lids for steel roughly doubles theoretical proton energy on a machine essentially identical to the reference machine's.

small gap: reduced number of At of coils, pole radius reduced, orbits close to outer edge, no space, very sensitive to errors : vertical losses. large gap: large space: injection, extraction, probes, easier vacuum pumping, lower field

Beeckman, Cyclotron Magnets — ECPM37 lecture, Groningen (2009) — p. 65

Tabletop: Frames the central decision for a next machine (the reference machine's chamber must fit in the gap) with the actual list of consequences on both sides.

The gap between chamber lids was chosen to be 5 in., leaving 1 1/4-in. clearance between D's and lids ... resulting in a D-voltage limit of about 70 kv due to breakdown.

Livingston & Blewett, Particle Accelerators (1962) — p. 175

Tabletop: At 1.3 kV the builder has enormous margin; for a next machine at several kV, ~50 kV/in of clearance in vacuum with rounded edges is a comfortable design gradient.

dust should be controlled and all grease removed (even fingerprints), and under no circumstances should steel wool or coarse abrasives be used in cleaning.

Livingston & Blewett, Particle Accelerators (1962) — p. 189

Tabletop: After every chamber opening, budget an hour of gradually raised dee voltage for conditioning before expecting stable beam.

The dees, made of thin sheet copper, arc 22.5 cm in diameter, 2.4 cm high, and they are separated by a gap of 1.5 cm.

McGuire, The Iowa State University 1.5 MeV Undergraduate Cyclotron (1961) — p. 7

Tabletop: A directly copyable dee geometry for an 8-10 inch pole; note the dees must be water cooled once RF power reaches ~kW.

the top and bottom of the chamber to bow in, which affected the capacitance of the dee and reduced the maximum voltage that the dee could withstand before flashing over.

Baumgartner, The Cyclotron Kids' 2 MeV Proton Cyclotron — Cyclotrons 2013, WE1PB05 (2013) — p. 2

Tabletop: Directly relevant to any thin-lid chamber on a next machine squeezed into a small magnet gap: plan support posts (clear of the beam spiral) from the start.

The selected value of 100 kv peak voltage requires about 1.5-in. clearance from dee-to-liner ... Since the magnetic gap is so precious ... this minimum value is taken for design.

Livingston & Howard (eds.), The Oak Ridge Relativistic Isochronous Cyclotron — ORNL-2648, OSTI 4275955 (1958) — p. 100

Tabletop: Scales directly: the reference machine's 1.3 kV needs well under a millimeter electrically, so their clearances are set by beam aperture and tolerance, but a 20-50 kV dee on a next machine should keep several millimeters to grounded surfaces.

This design strategy made it impossible to fix a single component of the apparatus, such as the insulation, without replacing the entire piece.

Haas, Characterizing the Performance of the Houghton College Cyclotron — Houghton College thesis (2009) — p. 47

Tabletop: A next machine should assume sparks WILL damage insulators eventually; screw-together modularity turns a total rebuild into a one-part swap.

Ceramic spacers hold the Dee and Dummy Dee apart with a gap of 0.635 cm. The entire Dee electrode assembly is supported by three KF-16 electrical feedthroughs through ports at 120 degrees from each other.

Yuly, The Houghton College Cyclotron: a Tool for Educating Undergraduates — Cyclotrons 2013, WE1PB01 (2013) — p. 2

Tabletop: Direct fabrication template; the single-Dee-plus-dummy topology halves the RF feedthrough problem versus two live Dees.

To make room for a filament and to avoid another electrical discharge from the filament to the lid, a 0.3 cm deep circular depression was milled out of the bottom of the upper lid.

Haas, Characterizing the Performance of the Houghton College Cyclotron — Houghton College thesis (2009) — p. 50

Tabletop: In the reference machine's tight pole-gap geometry, check every HV-to-ground clearance near the median plane; milling relief pockets is cheaper than chasing sparks later.

The outgassing rate of unbaked, uncleaned stainless steel is of order 10-5 Pa-m/s after 10 h of pumping... reduced by a factor of 10-100... to be suitable for high vacuum

O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed. (2003) — p. 308

Tabletop: Multiply the next machine's internal area by 1e-5 Pa-m/s and divide by delivered pumping speed to predict the 10-hour base pressure before drilling a single hole.

the molecular conductance per unit area of any structure in molecular flow has a maximum value [11.6 L/(s-cm2) for air at 22C]

O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed. (2003) — p. 48-50

Tabletop: Sets the ceiling on what the SI100 can actually pump through the chamber port: a 4-inch (81 cm2) opening passes at most ~940 L/s, and a baffled elbow far less - size the pump port as large and short as possible.

A virtual leak is a volume of trapped atmospheric gas that leaks into the vacuum vessel through holes or cracks that do not go all the way through the vessel wall. [Examples:] Unvented Screw, Two Welds in Series, Unvented Double O-rings

Bertolini, Accelerator Vacuum and Mechanical Engineering — USPAS course, UCRL-MI-201847 (2004) — p. 59-63

Tabletop: Every internal socket-head screw in the next machine (dee supports, ion source mounts) needs a vent hole, a slotted thread, or a vented washer; a slot machined in the O-ring groove floor serves the same purpose.

Steel supporting rods allow thin top and bottom plates to minimize thickness

Baumgartner & Heuer, The Cyclotron Kids 14-Inch Accelerator (2010) — p. 6-7

Tabletop: Every millimeter of lid steel is a millimeter of magnet gap; internal posts (placed outside the beam plane) let the builder close the next machine's gap without a lid that dishes under vacuum.

A 2.54 cm thick ring with an outer radius of 9.9 cm and an inner radius of 8.5 cm was milled from 6061 T6 aluminium ... Ports were made in the chamber wall using ten KF-16 flanges, which were secured using Hysol Loctite 1C vacuum epoxy

Yuly, The Houghton College Cyclotron: a Tool for Educating Undergraduates — Cyclotrons 2013, WE1PB01 (2013) — p. 2

Tabletop: A complete, copyable chamber spec for an 8-inch-pole machine, including the epoxy-in-flange trick that avoids welding.

after the welding, the bottom plate contracted so much that it bent the whole frame out of shape... seal the bottom plate against the frame using a flat Viton ring.

Baumgartner, The Cyclotron Kids' 2 MeV Proton Cyclotron — Cyclotrons 2013, WE1PB05 (2013) — p. 2-3

Tabletop: A fabrication trap the builder can skip entirely: o-ring/flat-gasket both lids, weld nothing flat and thin.

A typical figure, used in the MIT cyclotron, is a dR of 0.15R. The deflector gap is usually tapered ... Spacings as small as 1/8 in. can be used at the entry slit, opening to 1/2 in. or greater at the exit.

Livingston & Blewett, Particle Accelerators (1962) — p. 180-181

Tabletop: Scaled to 160 keV the same geometry needs only ~500-900 V on the deflector - an easy supply; keep the entry slit no wider than the turn separation.

304L SS, most commonly used in vacuum, a little more expensive... Joint design is critical from vacuum, metallurgical and distortion standpoints. Cleanliness is essential.

Bertolini, Accelerator Vacuum and Mechanical Engineering — USPAS course, UCRL-MI-201847 (2004) — p. 355-360

Tabletop: For a next machine's chamber welds specify 304L filler and pipe where possible; carbide precipitation in plain 304 welds is a known source of micro-leak porosity.

a lid with radius 10 centimeters and thickness of 3.5 millimeters would undergo less than 1 mm of deflection when covering a chamber with internal pressure of 1e-3 Torr

Dewan, Design and Construction of a Cyclotron Capable of Accelerating Protons to 2 MeV — MIT thesis (2007) — p. 12

Tabletop: Gives the builder the actual formula for trading a next machine's lid thickness against magnet gap - a few mm of 7075 plate suffices at 8-12 inch chamber diameter if the edge is well supported.

all cyclotron components must be made of magnetically transparent materials such as aluminum, copper, or brass

Dewan, Design and Construction of a Cyclotron Capable of Accelerating Protons to 2 MeV — MIT thesis (2007) — p. 9

Tabletop: Standard but easily violated rule: screws, feedthrough bodies, and detector hardware inside the reference machine's gap should be checked with a hand magnet before installation.

the VTO box and obround are mounted on a motor-driven carriage which operates on a rail system. This permits the removal of the dee heads... to facilitate maintenance.

Ramler & Parker, The Argonne 60-Inch Cyclotron — ANL-5907 (1959) — p. 6-7

Tabletop: At tabletop scale this means: chamber slides out of the gap, dee removable through a lid, pump cart disconnectable - the difference between a research tool and a sealed monument.

The slit is most functional if it is installed in the place of the largest radial size of the beam... The closer to the center the device is installed, the more efficient it is

Smirnov, The Cyclotron and Its Modeling — Phys. Part. Nuclei 52 (2021) — p. 56

Tabletop: Practical placement rules if the builder adds a beam-defining post or slit to clean up phase spread and improve turn separation at extraction radius.

The chamber parts into two halves in a vertical plane through the center of the magnet, permitting the removal of the chamber without disturbing the magnet pole tips.

Creutz, Design and Construction of Synchro-Cyclotron — NYO-780 (1950) — p. 15

Tabletop: Directly applicable packaging rule - make the next machine's chamber removable (or serviceable in place) without unbolting pole tips or shims.

Numerous holes are drilled in them to facilitate vacuum pumping.

Creutz, Design and Construction of Synchro-Cyclotron — NYO-780 (1950) — p. 19

Tabletop: Directly applicable - virtual leaks behind liners and skins are a classic small-chamber trap; drill the next machine's liners generously.

disclosed a re-entrant cavity resonator mode between the two pole pieces of the magnet with the vacuum tank walls as the return circuit resonant near the lower frequency limit. This was easily suppressed by strapping the pole pieces together.

MacKenzie et al., Design of the Radio-Frequency System for the 184-inch Cyclotron — UCRL-64 (1948) — p. 22

Tabletop: Directly applicable - the pole-chamber geometry of an 8-inch machine forms the same parasitic cavity; copper straps pole-to-pole (or liner-to-liner) are a one-hour fix worth doing preemptively.

At 20 megacycles the space between electrodes which will allow an electron to reach an energy around 30 volts in 5 cm. There are very few paths, along the magnetic field, in the neighborhood of the dee that are greater than this.

MacKenzie & Waithman, R.F. System for Frequency Modulated Cyclotron — MDDC-1045 (1946) — p. 12

Tabletop: Directly applicable geometry rule - at 9 MHz electron oscillation amplitudes are larger still, so keep open RF-exposed volumes and along-field gaps in the next machine's chamber small or shielded.

There was considerable sparking between the HV electrode and the .002 inch copper foil. The copper foil was warped badly ... The 0.002 inch copper foil supported in this manner is not suitable for this job.

Sewell, 184″ Cyclotron: Vertical D.C. Electrostatic Deflector — MDDC-1051 (1947) — p. 2

Tabletop: A next machine's septum should be thicker (>= 0.010 in), tensioned, or heat-sunk at both edges; conditioning sparks are inevitable and each one dumps its energy into the nearest thin edge.

Approximately 60 kv could be held on the high voltage electrode of this deflector.

Sewell, 184″ Cyclotron: Vertical D.C. Electrostatic Deflector — MDDC-1051 (1947) — p. 2

Tabletop: A next machine needs only a few kV/cm over a few cm of channel - an order of magnitude below what 1947 in-tank hardware sustained, so deflector voltage should not be the limiting risk; the series resistor for spark current limiting is worth copying.

a choice of radius, 11 in. or 20 in., is thus obtained by shifting the position of the dees and target. In either case the ion source position remains unchanged and the beam orbits remain centered

Howard (ed.), Electronuclear Research Division Semiannual, period ending 20 September 1953 — ORNL-1663 (1954) — p. 18

Tabletop: Variable energy WITHOUT retuning B or rf - since E ~ r^2 at fixed field/frequency, a repositionable target (or dee assembly) gives an educational machine two calibrated energies for the price of one; the invariants to protect are source position and magnetic centering, exactly as ORNL did.

the results indicated a force greater than 35 tons for the cyclotron magnet. For comparison the force of atmospheric pressure is 24 tons.

The University of Washington 60-Inch Cyclotron: Progress and Status Report of Design and Construction — AECU-1951, University of Washington (c. 1950) — p. 30

Tabletop: Any ferromagnetic chamber lid or pole-integrated cover on a tabletop machine sees magnetic clamping comparable to or exceeding vacuum load — check both cases (energized/de-energized) for deflection, and expect assembly/disassembly forces. Stray field at the RF tube/amplifier is likewise a real design input for a compact machine.

Aluminum was chosen over stainless steel because of its short half-life property.

The University of Washington 60-Inch Cyclotron: Progress and Status Report of Design and Construction — AECU-1951, University of Washington (c. 1950) — p. 46

Tabletop: DIRECT for any machine that will make neutrons: aluminum's activation products die in hours-days while stainless (Co-60 from cobalt traces) lives for years. Choose the beam-facing metal for the machine you hope it becomes, and TIG aluminum is proven UHV-adequate practice from 1951.