Cryogenic Permanent Magnet Undulators

Finn O’Shea March 27, 2013

HBEB 2013, Puerto Rico

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Outline1) Motivation for shorter period technology

As usual, it is money

2) Prototype undulator has been tested Results are ‘unsuprising’ and that is a good thing!

3) Improving performance of cryogenic undulators through the use of rare-earth element poles – the DPU project

Cryogenic compatible magnetic materials lead to improved performance

Can we do the same with the pole material?

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Motivationo Researchers from many branches of science are using modern light sources to do a lot of cutting edge research

o LCLS accepts ¼ of proposals for beam time

o These machines are big (km) and expensive ($1/2 billion)

o Linacs are the really expensive part

o NGLS is being built through a mountain – real estate is getting more expensive

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Where do short periods fit in? “Moore’s Law” of radiation brightness is doubling every

10 months since the 1960s – this has come from bigger, more expensive facilities

Increased brightness does not lead to increased access unless the facilities become more common

The path to cheaper access is to increase the amount of radiation produced by each electron Lots of beam lines on synchrotrons Multiple beam lines at FELs: LCLS-II, NGLS, SwissFEL Reduce the energy of the electrons required to produce

the desired wavelengths

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To make shorter periods… The magnetic material needs to be very radiation resistant

In vacuum undulators have higher exposure because of the smaller gap and no vacuum chamber wall for protection

Cryogenically cooled magnets show a modest increase in remanent field and a massive increase in coercivity They also show an increase in resistance to radiation induced

demagnetization Originally attributed to coercivity increase,

more likely due to increase in heat capacity decreasing the effects of local heating

Rad damage is not well understood: Radiation damage is reversible with

remagnetization no structure change

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Strategy to increase survivability1) Use material that has maximum remanent field

at room temperature Limitation is ability to assemble the undulator

2) Cool as much as possible to get the highest possible magnetic field and largest coercivity/heat capacity

The clear choice is PrFeB No SRT (NdFeB) High remanent field (SmCo)

Benabderrahmane, NIM A 669, 1 (2011).

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Example of performance change

LCLS Normal Energy = 13.6 GeV Charge = 250 pC Norm Emittance = 0.4 μm Saturation Length: 60 m Pulse Energy = 1.5 mJ Pulse Length = 100 fs B = 2 x 1033 ph/(s mm2

mrad2 0.1%)

LCLS Low Charge/CPMU

Energy = 4.5 GeV Charge = 250 fC Norm Emittance = 33 nm Saturation Length: 15 m Pulse Energy = 2.8 μJ Pulse Length = 0.5 fs B = 1.3 x 1036 ph/(s mm2

mrad2 0.1%)PRSTAB 070702 (2010)

CPMU9Testing of Cryogenic Permanent Magnet Undulator – 9 mm at the Next Linear Collider Test Accelerator

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Cryogenic Permanent Magnet Undulator – 9 mm period

• 9 mm period length

• 20 period prototype

• Compensated 1st integral

• Working temp down to 11K

• NLCTA experiment run at 43K

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CPMU - II Design process was iterative: use FEM and BIM

codes to determine magnetically safe assembly and operating conditions as material is characterized. Results in a 2D geometry with pieces that are

strategically chamfered to reduce reverse fields

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Measuring the field Field is measured at

cryogenic temps on a specially constructed measurement bench at HZ-Berlin Bpeak=1.15 T (K=0.97)

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Next Linear Collider Test AcceleratorFacilities at NLCTA made it an excellent place to test the undulator using a scaled experiment at optical frequencies.

• Radiation bandwidth measurement

•Energy modulation measurement

• Confirmation of microbunching

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Undulator Radiation Bandwidth

at NLCTA the bandwidth of the radiation should be dominated by the 5% bandwidth of the single electron radiation process

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Laser seeding 800 nm laser is used to

seed the FEL mechanism Laser is shorter than the

electron beam, they are about the same transverse width

Leads to very 3D process Energy modulation strength

is function of distance along beam and radius

Genesis predicts:K = 0.97K = 0.94

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Observation of microbunching Microbunching of

electron beam causes coherent emission of transition radiation

CTR signal shows up as near field structure when laser is turned on

Null tests showed that this is likely forward CTR from OTR3 rather than backward CTR from OTR4, which is the screen that is imaged

Laser off Laser on

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Summary of Results Bandwidth of the undulator radiation is

dominated by the 5% interference bandwidth – consistent with expectations

Energy modulation is consistent with expected value from iFEL interaction

CTR appears when modulation is turned on – microbunching is occuring although scattering in OTR3 is spoiling the measurement

Rare-earth polesDysprosium poles at RadiaBeam Technologies

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Why replace CoFe? Vanadium Permendur (49% Fe, 49% Co, 2% V) is an excellent

pole material Saturates at low applied field:

μi~104 and Hsat<<0.1 T Bsat = 2.35 T

The reason for replacing CoFe as the pole material of choice is to get higher saturation induction Because we need to cool a CPMU anyway, what gains can be

realized?

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Rare-earth elements Materials such as

dysprosium, gadolinium and holmium show large sat. ind. at cryogenic temperatures Dy -> 3.8 T (single crystal)

Single crystals are hard to grow and polycrystals are not useful

Secondary re-crystallization can be used to develop “texture”

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Secondary Re-crystallization Rolling then annealing exploits an energy

advantage that results in the growth of the grains in-plane

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Performance of textured Dy 100 μm thick foils

• Competitive with CoFe if the applied field is greater than ~0.10 T• Thinner foils should work even better • Non-linear nature and mixing of 1120 and 1010 in different samples could be a problem

CoFe

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Short test undulator Most poles are CoFe One pole pair is replaced with Dy laminated poles The field is measured while the undulator is

cooled BCoFe is compared to BDy

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Results

~3%

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Details Effect is reproduced in Radia with the magnetization

curves measured from one pole (destructive measurement via VSM)

Dy Pole position exposed it to larger applied fields than a typical pole

working point is above the Dy > CoFe point

Shows promise

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Summary Using shorter period undulators can:

Decrease the electron beam energy required to reach a given wavelength

Extend the reach of existing facilities This can lead to cost savings and a potential increase in

accessibility A prototype short period cryogenic undulator has been built

and tested in a successful scaled experiment at optical wavelengths

Rare-earth poles have the potential to outperform CoFe poles in undulators that can be cooled to the temperatures where they are ferromagnetic

Thank you!