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Planar Undulator - Thermal Requirements and Heat Loads. Superconducting Undulator Workshop, Rutherford Appleton Laboratory 28 th April 2014. T Bradshaw On behalf of the SCU group. Magnet Operating Point. “Load line” is for illustration only – slightly curved in reality.

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Planar undulator thermal requirements and heat loads

Planar Undulator -Thermal Requirements and Heat Loads

Superconducting Undulator Workshop, Rutherford Appleton Laboratory 28th April 2014

T Bradshaw

On behalf of the SCU group

Planar undulator thermal requirements and heat loads

Magnet Operating Point

“Load line” is for illustration only – slightly curved in reality

Wire Cu:Sc ratio is 0.575:1

Need temperatures below 4K to give adequate margin for the conductor

Estimations give low minimum quench energies for the magnet ~µJ which is worrying as this effects the magnet stability

Note that load line is not linear as shown





Top Magnet



Bore tube

Bot Magnet


The thermal resistances at the end of the bore tube are the bellows assemblies



  • Cryocoolers at ends of magnet will reduce load on 1.8K stage

  • Beam load 30-40W ?

Beam tube cooled by two dedicated cryocoolers

Wakefield heating uncertain (see later)

Current leads






Upper Magnet (1.8K)


Lower Magnet (1.8K)

Heat load summary

Heat load summary



Magnet 93mW





Beam tube

The 1.8K system has a heat lift capacity of 200mW – size should be adequate.

Beam tube could be up to 30-40W worst case (see later)

Breakdown of 2K load – total 93mW ignoring any joint heating

Cryocooler operating points

Cryocooler operating points

Coolers on beam tube

Cooler on turret

...these are approximate positions

There was a study on the use of cheaper 408Ds with lower cooling power – this solution was not feasible as beam tube temperature ended up too high

Wakefield heating

Wakefield Heating

What we were originally working to in terms of heat loads:

Image Current Heating in Diamond.doc - Duncan Scott estimates – what we originally worked to.

We have been following COLDDIAG – instrument on Diamond looking specifically at wakefield heating.

From Robert Voutta’s presentation at meeting on 16th January 2013

Wakefield heating1

Wakefield Heating

From Robert Voutta’s presentation at meeting on 16th January 2013

Note that these loads are over a 490mm length

Wakefield heating2

Wakefield Heating

Numbers are scary:

Wiggler I15 getting 20W over full length (1820mm)

Wiggler I12 getting 10W over full length (1640mm)

Shamelessly taken from TD-ID-REP-081 by Ed Rial

Looked at behaviour of cryocoolers and derived heat load from load map and helium pressure control

Note that they have been able to reduce the spread by plotting against a parameter derived from bunch lengths etc… - this is for illustration

Note that DLS wish to increase current from 300mA

Also separation in wigglers is higher than the planar

Wakefield heating3

Wakefield Heating

  • What do we do?

  • Suspicion is that most of the heating is not wakefield heating – it is due to small cavities that are absorbing rf

    • Need to ensure that the beam tube is as clean as possible

    • Roughness ≈ skin depth – a few microns

    • Transitions – Steps less than 100 microns, minimise gaps

  • We don’t have a good handle on the size of the heating

    • - have made provision for extra cryocoolers – likely discontinuities are at the ends of the beam tube which is where we have situated the cryocoolers

  • - have a good margin of safety on the cooling power

  • Need to keep following the COLDDiag and DLS measurements:

  • Basically proportional to resistivity and inversely proportional to gap – make an attempt to scale from other results …..

    Wakefield heating4

    Wakefield Heating

    The Cryogenic Permanent Magnet Undulator (CPMU) results were taken with beam tube at 147K (TDI-ID-REP-084). If we assume that the RRR of the copper used was about 100 and the wakefield heat is proportional to resistivity then the heating effect should be reduced by a factor of about 40. This seems to give anomalous results.

    The “adjusted for SCU” column assumes a 1/gap dependancy and for the CPMU a proportionality to resistivity. The SCU parameters are gap = 5.2mm and length = 2m.

    Ignore adjusted

    Assumes copper with an RRR=100

    Wakefield heating5

    Wakefield Heating

    Different materials and different RRRs will give very different beam heating – if we are understanding the numbers…..

    Probably will end up gold plating – only require a few microns which is the skin depth at these frequencies

    Turret assembly

    Turret Assembly

    System is a continuous flow cryostat with a flow of ~10mg/s

    Turret details

    Turret details

    Aim is to test the turret assembly for cooling power and operation – there are some wrinkles that we need to understand.

    We are also testing the current leads, thermometry and thermal balances.

    Turret details1

    Turret details

    Turret in preparation

    Cryostat heat loads

    Cryostat Heat Loads

    • Cryostat uses HTS leads to limit load on 4K stage

    • Beam tube is assumed at 12-16K and a load of 40W from beam heating

    • Using 3 x Sumitomo RDK-415 coolers. One on turret and two on the beam tube

    Cryostat tests

    Cryostat Tests

    Struggling to get the correct mass flow – looking at needle valve and trap as flow restrictors



    • Conductor requirements are for ~2K.

    • MQE for the conductor is a bit worrying.

    • Using a continuous flow cryostat to get ~200mW (require ~100mW).

    • Not sure of beam/wakefield heating – allowed for worst case. Will probably need plating.

    • Turret works – but flow needs looking at.

    • Looking at thermal aspects of bath to magnet interface – test programme.

    Planar undulator thermal requirements and heat loads


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