Shielding studies for SRF cavities at Fermilab Camille Ginsburg and Igor Rakhno Fermilab, Batavia, Illinois 60510, USA 1SATIF-10, CERNJune 2-4, 2010

Outline Introduction Source term issues: – Field-induced dark current in SRF cavities – Trajectory analysis – Fowler-Nordheim model – Intensity of the predicted dark current (normalization) Vertical Test Facility at Fermilab Conclusions 2SATIF-10, CERNJune 2-4, 2010

Introduction Major components of an SRF test facility: (i) SRF cavities; (ii) cryogenic equipment; (iii) internal shielding; (iv) external shielding. There is no incoming beam. The tests are designed to ensure that, at a given accelerating gradient, the cavities do not lose the stored RF energy to dark current. June 2-4, 2010SATIF-10, CERN3

Introduction The major hazard during SRF cavity tests is due to γ-radiation produced by field-emitted electrons. Neutron component, generated by the gammas, is not negligible (≈10%) for accelerating gradients of MeV/m. The field-induced emission is generally the result of dust contamination in the cavity. Model numerical predictions lack accuracy, that’s why we have to use a semi-empirical approach. 4SATIF-10, CERNJune 2-4, 2010

Field-induced dark current Experimental observations Locations of the field-induced emission are random. The emission can happen anywhere − mostly around irises (locations with the highest surface curvature and, correspondingly, the highest local electric field.) Tiny dust contaminations are believed to increase the local electric field by an enhancement factor of 100 (!) and more. However, for a given SRF in a test session, the emission usually does not occur at several sites. It usually happens at a single site and lasts until a significant amount of RF energy stored in the cavity is pumped into the generated dark current (≈1 mA). 5SATIF-10, CERNJune 2-4, 2010

Field-induced dark current blue – cavity surface, pink – electric field by POISSON SUPERFISH code 6SATIF-10, CERNJune 2-4, 2010 Iris

Field-induced dark current blue – cavity surface, pink – electric field by POISSON SUPERFISH code 7SATIF-10, CERNJune 2-4, 2010

Trajectory analysis 8SATIF-10, CERNJune 2-4, 2010 Trajectories of the field-emitted electrons were analyzed in order to understand the phase-space distribution of the generated dark current. FISHPACT code was used for the purpose of generating a discrete representation of the distribution. For every single iris, 5 emission sites were studied: the iris itself and 4 sites in its vicinity (both sides). Emission sites near flanges were taken into account as well. The surface electric field has the right sign to facilitate the electron quantum tunneling under the surface potential barrier for 50% of the test time. The corresponding RF phase intervals were divided into 18 equal bins, so that we built 17 sample trajectories for every emission site. Not all of the trajectories are created equal ! The phase-space coordinates (r, E, Ω) of the locations where the trajectories hit the inner cavity surface were recorded for subsequent modeling with MARS15 code.

Trajectory analysis 9SATIF-10, CERNJune 2-4, 2010

Trajectory analysis The highest energies could be achieved for electrons generated around the flanges. However, probabilities of these trajectories are very different depending of RF phase (i.e. prompt surface electric field). Relative probabilities of the trajectories are determined by means of the Fowler-Nordheim model. June 2-4, 2010SATIF-10, CERN10

Fowler-Nordheim model of field emission Fowler and Nordheim (1928) provided the first quantum mechanical description of this phenomenon as a tunneling: where j is density of the generated electron current, Ε is the local electric field, A and B are slowly varying functions of E, and φ is work function of the emitting material. From practical standpoint, however, the expression is not satisfactory. The field-induced emission was observed at much smaller fields (≈0.01) than those compatible with the FN expression. June 2-4, 2010SATIF-10, CERN11

Fowler-Nordheim model of field emission In order to better fit experimental results, the following expression was found to be more adequate: The two additional parameters, S and β, are effective emitting area factor and local field enhancement factor. Microscopic defects of the emitting surface can explain existence of the enhancement factor, β. Local field around such a defect can be much higher than predicted field at an ideal surface. The parameters, S and β, are the major unknowns. In our simplified discrete model, β is assumed to be 100 for every single emission site. The parameter S is also assumed to be same for every site, so that it is eliminated by the normalization procedure. June 2-4, 2010SATIF-10, CERN12

Normalization of the predicted dark current June 2-4, 2010SATIF-10, CERN13

Normalization of the predicted dark current June 2-4, 2010SATIF-10, CERN14 The Test Facility at Fermilab is similar to the Tesla Test Facility at DESY. That’s why we use their measured dose rates to perform a normalization, other things being equal. For 90% of the measurements the dose rate at a given location under the external shielding in the DESY/TTF did not exceed 5 rem/hr. We assume that the total dark current generated in an SRF cavity corresponds to the dose rate of 5 rem/hr at a similar location in the Test Facility at Fermilab → a conservative approach. It is an ‘average’ description. In real life, if an SRF cavity generates a lot of dark current in a test (high dose outside shielding) → another cleaning. All the emission sites around the irises are assumed to be equal regarding probability to generate dark current. We can not do better.

Vertical Test Facility at Fermilab The Test Facility at Fermilab is in operation since The vertical test cryostat VTS1 was designed to test single 1.3 GHz 9-cell cavities at accelerating gradients of MV/m. Two additional cryostats with common design, VTS2&3, are being procured, and are sized such that six 9-cell cavities can be installed per cryostat. June 2-4, 2010SATIF-10, CERN15

Vertical Test Facility at Fermilab June 2-4, 2010SATIF-10, CERN16

Vertical Test Facility at Fermilab June 2-4, 2010SATIF-10, CERN17 The SRF cavities are submerged in superfluid helium and the operations are performed at 2K. Test throughput will be gained through common cool-down and warm-up time, with cavities tested sequentially. Needless to say, space for shielding, either internal or external to the cryostat, is limited. Measurements at VTS1 revealed that total cool-down time is about 180 minutes, and the internal shielding is responsible for about 1/3 of that. Our goal was shielding optimization, removal some internal shielding and adding external one. All the six lead blocks (inherited from DESY/TTF) above the cavities, were removed. The estimated total cool-down time for VTS2&3 is about 240 minutes, and the lead blocks would increase that by about 30%.

Vertical Test Facility at Fermilab June 2-4, 2010SATIF-10, CERN18

Vertical Test Facility at Fermilab For VTS1 the predicted dose rate under the external shielding is ≈ 250 mrem/hr. June 2-4, 2010SATIF-10, CERN19

Conclusions June 2-4, 2010SATIF-10, CERN20 The described approach provides for a description of spatial, angular and energy distributions of field-emitted electrons in SRF cavities. The normalization procedure is conservative. On the average, it overestimates the predicted dose rate by a factor of more than 2. The described approach to shielding design for test facilities is justified by the possibility of extra cleaning procedures for tested SRF cavities.