Characterization of the radiation tolerance of cryogenic diodes for the High Luminosity LHC inner triplet circuit

Cryogenic bypass diodes are part of the baseline powering layout for the circuits of the new ${\mathrm{Nb}}_3\mathrm{Sn}$ based final focus magnets of the high luminosity Large Hadron Collider. They will protect the magnets against excessive transient voltages during a nonuniform quenching process. The diodes are located inside an extension to the magnet cryostat, operated in superfluid helium and exposed to ionizing radiation. Therefore, the radiation tolerance of different types of diodes has been tested at cryogenic temperatures in CERN’s CHARM irradiation test facility during its 2018 run. The forward bias characteristics, the turn-on voltage and the reverse blocking voltage of each diode were measured weekly at 4.2 K and 77 K, as a function of the accumulated radiation dose. The diodes were submitted to a total dose close to 12 kGy and a 1 MeV neutron equivalent fluence of $2.2\times {10}^{14}{\mathrm{cm}}^{-2}$. After the end of the irradiation program the annealing behavior of the diodes was tested by increasing the temperature slowly to 293 K. This paper describes the experimental setup, the measurement procedure and the analysis of the measurements performed during the irradiation program as well as the results of the annealing study.

Figure 1

Circuit layout of the HL-LHC inner triplets for IP1 and IP5. The drawing shows the quadrupoles Q1, Q2a, Q2b, Q3, the main, and trim power converters with their crowbars, the Coupling Loss Induced Quench protection systems (C) [10] and the cold diodes [11]. Image adapted by courtesy of CERN’s Magnet Circuit Forum (MCF).

Figure 2

Inner part of the experimental cryostat for testing of the radiation tolerance of cold diodes for the HL-LHC triplets. The setup is based on a cryocooler and houses two stacks of 4 diodes each. Diodes D1-D4 are mounted on the first stage plate (77 K), diodes D5-D8 on the second stage plate (4.2 K).

Figure 3

Measured relative increase of the forward voltage $U_f$ in diode D1 (top) and diode D4 (bottom, as in [21]) as function of ${\mathrm{\Phi }}_{\text{neq}}$, for currents from 1 kA to 18 kA.

Figure 4

Relative change of the forward voltage $U_f$ versus ${\mathrm{\Phi }}_{\text{neq}}$ at 18 kA for the diodes D1, D2, D3, and D4. The last measurement points of each diode indicate the value of $U_f$ after a thermal cycle to room temperature. The dotted lines indicate the equivalent ${\mathrm{\Phi }}_{\text{neq}}$ levels of $U_f$ after the thermal cycle. The error-bars correspond to a 25% uncertainty in ${\mathrm{\Phi }}_{\text{neq}}$ and an error of $+8\%$ in the forward voltage due to self-heating. Image adapted from [21].

Figure 5

Dynamic series resistance $R_d$ of diodes D1, D2, and D4 as a function of the current after the end of the irradiation program. The series resistance was calculated from the gradient of the measured forward characteristics curves. The reference values were derived from the CHARM calibration run before irradiation and from a lab setup.

Figure 6

Schockley series resistance extracted from the forward characteristics of the diodes, as a function of ${\mathrm{\Phi }}_{\text{neq}}$. A fit using a functional dependence in the form of Eq. (2) was performed. The fit parameters for D1 are $K_{R,h}=9.8\times {10}^{-19}$ and $K_{R,e}=2.7\times {10}^{-19}$, for D4 the fit yields $K_{R,h}=7.7\times {10}^{-21}$ and $K_{R,e}=1.2\times {10}^{-18}$ in the units of $\mathrm{\Omega }{\mathrm{cm}}^2$ respectively.

Figure 7

Schockley equation fit on the full dataset $U_f(I_f,{\mathrm{\Phi }}_{\text{neq}},T=77\mathrm{K})$ for diodes D1 and D4.

Figure 8

Schockley equation fit on the full datasets $U_f(I_f,{\mathrm{\Phi }}_{\text{neq}},T=77\mathrm{K})$ for diodes D1, D2, and D4, based on the exponential model. The forward voltage at a current of 18 kA was extrapolated up to a ${\mathrm{\Phi }}_{\text{neq}}=1\times {10}^{15}{\mathrm{cm}}^{-2}$ assuming the linear and exponential model.

Figure 9

Top: Turn-on voltage $U_{\text{to}}$ of D1, D2, D3, and D4 as a function of ${\mathrm{\Phi }}_{\text{neq}}$ measured at a current of $I=0.2$ A at 77 K [21]. Bottom: Turn-on voltage $U_{\text{to}}$ of D5, D6, D7, and D8 as a function of ${\mathrm{\Phi }}_{\text{neq}}$ measured at a current of $I=0.2$ A at 4.2 K. The last measurement points of each diode indicate the value of $U_{\text{to}}$ after a thermal cycle to room temperature. The dotted lines indicate the equivalent ${\mathrm{\Phi }}_{\text{neq}}$ levels for these annealed $U_{\text{to}}$ values.

Figure 10

Reverse blocking voltage $U_{\text{rev}}$ at a reverse current of $I_{\text{rev}}=1\mathrm{mA}$, as a function of ${\mathrm{\Phi }}_{\text{neq}}$ [21].

Figure 11

Relative increase of the diodes’ forward voltages at the end of the irradiation program (solid lines) and after the thermal cycle (dotted lines), as a function of the current. The measurements were performed at 77 K. D3 was omitted in this plot for better visibility, however it behaves very similar to D4.

Figure 12

$U_{\text{to}}$ of diode D5 from 3.9 K to room temperature and zoom between 0 and 10 K. The orange reference indicated the $U_{\text{to}}$ value at the beginning of the irradiation program [21].

Figure 13

$U_f$ of diode D4 from 65 K to room temperature for different current levels.

Figure 14

Leakage current of D3 as a function of the reverse bias voltage at different temperature levels after the irradiation program. The measurements were performed during a thermal cycle at the end of the irradiation program. For reference the leakage current of the same diode at room temperature before the irradiation program is shown (red).