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A Grudiev presented preliminary results and conclusions of the tests performed at CERN to understand the impact of beam induced damage on the performance of the RFQ.

High voltage DC breakdown tests were conducted using a symmetric electrode setup of two large parallel plates with adjustable gap in between, to make the system more analogous to RF structures. DC pulses of 500ns to 100 us were used, with voltages up to 10kV and gaps width between 20 and 100um.

The surface electric field is inversely proportional to the gap and the scaling law found with voltage is like:

Enorm=(V/Vmax)*(dmax/d)^0.72

which allows referencing to the RFQ performance (78.3kV, Es=34MV/m).

The E-field levels reached in the Linac4 RFQ and in the DC setup turn out to be consistent and indicate that the E-field limit used in the RFQ design (34MV/m) cannot be increased withour coimpromising the breakdown rate.

Irradiation tests were also perdormed on a Cu-OFE sample at the Linac4 source test stand.

No evident correlation was found between breakdown localisation and irradiation spot of the beam footprint. Breakdowns are evenly distributed on the whole surface exposed to halo, proiving that small delocalisations caused by low irradiation doses (halo) can cause breakdowns.

DK asked if XPS measurements were conducted to assess the sample purity and if the effect of baking was considered . AG replied that XPS analysis didn't show anything significant. Also he believes baking is not likely to affect material delocalisations  in the same way as irradiation.

Comparing to non-irradiated samples there is no impact on the Efield limit but a factor 2 to 4 higher number of breakdowns during conditioning in the case of the irradiated sample.

Preliminary conclusion of these studies is that, assuming Cu-OFE as baseline material for the construction of RFQ3 (different materials are not so well known and may present machining problems, like Nb), the E-field limit stays comparable to the current RFQ value of 34 MV/m.

 

Minutes by GB, 04/06/2021.

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