State-of-the-art devices for compact light sources Finn O’Shea RadiaBeam Technologies October 15, 2015 Cryogenic Short Period Undulators

Why short periods? Why cryogenics? A brief update of CPMUs being produced today. RadiaBeam’s CPMU Design philosophy Project goals Textured Dysprosium (or not) Why FACET? F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Outline

The obvious answer: The wavelength of the radiation produced gets shorter with period length. Either: Shorter accelerator to reach given wavelength, Longer wavelength reach of existing accelerator. Opens up option for doing beam gymnastics at lower energy. But K is going to get smaller as the period length shrinks Direct reduction in K Indirect loss of field strength from the smaller magnets dB r /dT ~ -0.1%/C Cooling allows you to get back 10+% of the field strength. Why short periods? Why Cryo? F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14,

The not so obvious answer Aggressive designs have working point deep in the 2 nd quadrant, increasing the risk of demagnetization Demagnetization happens when the working point goes below the knee: Cryogenics moves the knee left. dH cj /dT ~ -0.5%/C H cj can get 4-6x larger with typical designs This leads to reduced risk of demagnetization Local energy deposition raises the temperature of the grain. Makes shorter periods even possible. Why short periods? Why Cryo? F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14,

Vacuumschmeltze has made its PrFeB material a standard product. Neomax has made a PrFeB version that can be baked (lower Br, though). ESRF has built several NdFeB undulators and is now planning on PrFeB devices. HZB is building two PrFeB undulators; one for Hamburg, one for their ring. NSLS-II has built a few prototypes. Spring-8 has built an NdFeB undulator SOLEIL has built and installed a PrFeB undulator, building one more Danfysik built an NdFeB undulator for Diamond. Most of these undulators are λ u >= 14 mm RadiaBeam is working on building a 7 mm period undulator F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Current Work on CPMU

The original goal was to make an undulator to take advantage of textured dysprosium (TxDy). When you get to short periods (<1 cm) you have to very carefully trade off: Field strength Coercivity Pole saturation TxDy is meant to help with these problems by saturating at ~3 T. Dy is ferromagnetic below about 90K, so it is a great candidate for PrFeB undulators. F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, RBT Project Origin CoFe

Single crystal Dy works great It is also really expensive and can’t be produced in sizes large enough for undulators. RadiaBeam developed a process to induce “texture,” i.e. orient the axes of multiple grains to increase the permeability of the material. The results have been mixed, consistency is not undulator ready yet. F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Textured Dysprosium

Another of the difficulties with short period undulators is the tolerances. The tolerances scale roughly with the period length (or the gap which scales with the period length) This leads to expensive micromachining to hit the desired tolerances and extensive sorting of the magnets which generally can’t meet the specification absolutely. We took a different approach: modular construction. Also, secondary processing: install poles and then trim them down in-situ to get to the very tight tolerance on pole height. Risk: non uniform temperature F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Undulator Design

F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14,

Timeline required us to start undulator design during TxDy development, so in some sense we were aiming at a moving target. Ultimately we decided not to use TxDy in the undulator. Fall back to CoFe, but keep the same design to proof that. CoFe is really, really saturated because of its high permeability as compared to the TxDy 7 mm period, K ~ 0.8 at 80 K, gap = 2 mm F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Undulator Changes CoFe

F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Manufacturing Pictures

Compact x-ray light sources (λ<1 nm) will have to be based on advanced accelerators because E~1 GeV is practically required when λ u = 1 cm – short period. Undulators perform really well at synchrotrons because the beam is generally really stable. At the moment, advanced sources are not all that stable. Thus, we want a proof of principle experiment: How stable can we get the undulator radiation? How long will the undulator last? There are going to be miss-fires! New ideas like Trojan Horse allow us to work in regimes that synchrotrons (mostly) can’t with really short pulses. The requirements are co-aligned with colliders: charge, rep rate, stability. An undulator could be used as a diagnostic too: energy spread, emittance, etc. F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Why FACET?

Shorter period undulator can make good use of limited space or budgets by reducing required beam energy or increasing wavelength reach. But they require careful balancing of competing factors: pole saturation, magnet working point and field strength. RadiaBeam is working on a new material that should help with all three. In the meantime, we are building a 7 mm period undulator modularly in an attempt to control cost of construction. F. O'Shea - FACET-II Science Opportunities Workshop - Oct 14, Summary