Oak Ridge Spallation Neutron Source superconducting rf linac availability performance and demonstration of operation restoration with superconducting rf cavity off

Recent advances in superconducting rf (SRF) technology allow for increasingly more reliable and higher power hadron linacs. They hold enormous potential for economic, scientific, medical, environmental, and national security fields. Moving this technology from one of a kind laboratory demonstration devices to industrial scale applications imposes strict requirements on the linac performance in terms of its stability, reliability, and availability. The Spallation Neutron Source (SNS) is currently the highest average power superconducting proton linac in the world that has been routinely operated since 2006. We present statistical data on the SNS SRF linac reliability obtained from its practical operational experience during 2020. We analyze the frequency and duration of SRF cavity trips and identify their causes. These data will show that SRF cavity trips are the most common source of single-point failure within the linac. In an industrial environment, where redundancy is a necessity to prevent interruption of facility operation, the linac beam can be quickly restored by redistributing the failed cavity function to downstream energy reserve SRF cavities. We present a practical demonstration of this approach. We intentionally turn off one of the cavities and recover the beam by readjusting the phases of downstream cavities to maintain the same linac beam energy output. We discuss limitations of this approach at SNS and how they can be overcome.

Figure 1

Layout of the Spallation Neutron Source facility.

Figure 2

Typical distributions of the SCL cavity energy gains (left-hand-side vertical axis) and synchronous phases (right-hand-side vertical axis).

Figure 3

Time evolution of the accumulated energy (right-hand side vertical axis) and instant power on the target (left-hand side vertical axis).

Figure 4

Weekly neutron beam availability at the SNS during FY20.

Figure 5

Break down of the SCL downtime in FY20 by the subsystem of its origin.

Figure 6

Distribution of SRF trips in FY20 by their duration.

Figure 7

Average time between SRF trips (left-hand side vertical axis) and average downtime per trip (right-hand side vertical scale) in the indicated trip duration categories in FY20.

Figure 8

Numbers of trips of the individual SRF cavities in FY20.

Figure 9

Example of an SRF cavity phase calibration. The beam energy measured downstream of the cavity is plotted versus the phase setting of the input rf signal.

Figure 10

Beam phase with respect to a reference rf signal measured in HEBT as a function of the injection turn number into the accumulator ring. The disabled SRF cavities are kept in resonance with the beam. Each point is the beam phase averaged over a time period corresponding to a single injection turn.

Figure 11

Horizontal closed orbit distortions measured in the accumulator ring when the beam is injected with the frequency of the disabled SCL cavities shifted by 7 kHz (red) and 20 kHz (blue).

Figure 12

Beam loss (shown in blue on the left-hand-side vertical axis) as a function of the frequency shift of the disabled SRF cavity. The time to move the cavity frequency (shown in red on the right-hand-side vertical axis) as a function of the amount of the frequency shift.

Figure 13

Change in the control system input rf phase settings relative to those in regular operation for the SRF cavities downstream of the disabled cavity.

Figure 14

Beam loss monitor patterns (BLMPs) during regular operation (top) and after retuning with the SRF cavity off (middle). The bottom graph shows the difference of the two patterns.