Study of dark current phenomena in a superconducting accelerating cavity at the S-DALINAC

Radiation measurement in the environment of FLASH using passive dosimeters

“…The corresponding neutron effective dose equivalents were, on the other hand, 0.12 and 0.25 mSv h −1 at the gradients 22.5 MV m −1 and 30 MV m −1 , respectively. The sharp jump of gamma dose rate with increased gradient is elucidated with the Fowler Nordheim field emission theory [11,12]. The gamma dose rate (19 mGy h −1 ) at a gradient of 30 MV m −1 found to be 76 times higher than the corresponding neutron dose equivalent rate (0.25 mSv h −1 ).…”

“…The high gradient (∼25 MV m −1 ) applied across the superconducting, high purity niobium cavities of the accelerator modules driving the FLASH causes a significant level of field emission electrons [11]. These field emission electrons are accelerated within the accelerator modules, hitting the internal wall surface resulting in the production of a strong radiation field predominantly made of gamma rays (bremsstrahlung) and photoneutrons.…”

Electron emission contributions to dark current and its relation to microscopic field enhancement and heating in accelerator structures

“…It is undesirable, not only because it limits operational performance [3][4][5][6][7][8][9][10], but because it also plays a role in breakdown [11][12][13][14][15][16][17][18]. Though much is known about the relationship between dark current (thermal-field emission), local heating of asperities, field enhancement, and their failure mechanisms, good theoretical models are hampered by a lack of emission models that are correct in the parameter regime of both high fields and temperatures, flexible field enhancement models, and the relationship of temperature and field enhancement to resistive (i.e., heating due to electron-phonon scattering) and Nottingham (i.e., excess energy given up by an electron at the Fermi level scattering to occupy a state below the Fermi level vacated by a field emitted electron) heating for conditions that occur.…”

“…Such approaches are appropriate to the phases of dark current modeling [9,15,16,20,21]) prior to large temperature excursions, but their applicability afterwards is undermined by (i) when temperatures at the emission site approach the melting point of the metals (emission is thermal field rather than FN); (ii) when migration of material results in nanoprotrusions that dynamically grow but whose size precludes their adequate consideration by numerical means; and (iii) when the only acknowledgment of geometry is through the positing of a field enhancement ''beta factor'' without considering its companion effect on the ''notional'' emission area [22]. That such considerations matter is supported by treatments in which the thermal-field emission for multidimensional structures is numerically found [23,24] and which capture effects absent in the analytical approaches.…”