Heat load measurements for the PIP-II pHB650 cryomodule

Phase-3 testing of the pHB650 cryomodule at the PIP-II Injector Test Facility was conducted to evaluate the effectiveness of heat load mitigations performed after earlier phases of testing and to continue pinpointing any sources of unexpectedly high heat loads.. The programme measured HTTS, LTTS, and 2 K isothermal/non-isothermal loads under “standard”, “linac”, and “simulated dynamic” operating modes, recording data both inside the cryomodule and across the bayonet can circuits. Thermal-acoustic oscillations were eliminated by replacing the original G10 cooldown-valve stem with a stainless-steel stem fitted with wipers. A newly developed Python script automated acquisition of ACNET data, performed real-time heat-load calculations, and generated plots and tables that were posted to the electronic logbook within minutes, vastly reducing manual effort and accelerating feedback between SRF and cryogenics teams. Analysis showed that JT heat-exchanger effectiveness and temperature stratification in the two-phase and relief piping strongly influence the observed loads and helped isolate sources of excess heat. The campaign demonstrates that rigorous pre-test planning, real-time diagnostics, and automated reporting can improve both accuracy and efficiency, providing a template for future PIP-II cryomodule tests and for implementing targeted heat-load mitigations.

💡 Research Summary

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The paper presents the results of Phase‑3 heat‑load testing of the pHB650 cryomodule at the PIP‑II Injector Test Facility (ITF). The pHB650 is a 650 MHz high‑beta superconducting RF (SRF) cavity module that must dissipate the dynamic heat generated during high‑power accelerator operation. Earlier Phase‑1 and Phase‑2 tests revealed heat‑load values exceeding the design specifications, especially in the high‑temperature (HTTS) and low‑temperature (LTTS) two‑phase circuits and the 2 K superfluid helium circuit. The excess was attributed to thermal‑acoustic oscillations (TAO) in the cooldown valve stem, temperature stratification in the two‑phase and relief piping, and sub‑optimal performance of the Joule‑Thomson (JT) heat exchanger.

Test configuration and operating modes
Three operating conditions were defined: “Standard”, representing nominal design parameters; “Linac”, reproducing the conditions expected during actual accelerator operation (higher RF power, higher mass‑flow rates, and elevated pressures); and “Simulated Dynamic”, which imposed rapid changes in heater power to emulate transient beam loading. For each mode, temperature and pressure data were recorded at multiple points inside the cryomodule and along the bayonet‑can circuits, covering the HTTS, LTTS, and 2 K isothermal and non‑isothermal loads.

Mitigation of thermal‑acoustic oscillations
During Phase‑1 and Phase‑2, the cooldown valve stem made of G10 composite exhibited TAO, causing periodic temperature spikes and large uncertainties in the calculated heat loads. The authors replaced the G10 stem with a stainless‑steel stem equipped with wipers that damp the oscillations. Post‑replacement measurements showed a complete suppression of the temperature spikes, especially in the 2 K circuit, leading to a more stable temperature profile and reduced heat‑load fluctuations.

Automation of data acquisition and analysis
A Python‑based script was developed to interface with the ACNET control system, acquire real‑time temperature and pressure data, perform on‑the‑fly heat‑load calculations, and generate plots and tables. The script automatically posts the results to the electronic logbook within a few minutes, dramatically reducing the manual effort previously required for data handling. This automation shortened the feedback loop between the SRF and cryogenics teams, increased the overall efficiency of the test campaign by more than 90 %, and allowed rapid identification of anomalous behavior.

Measured heat loads
Table 1 of the paper lists the design values (HTTS ≈ 144 W, LTTS ≈ 9.6 W, 2 K ≈ 14.2 W). Under Standard conditions with the original G10 valve stem, the measured loads were HTTS ≈ 239 W, LTTS ≈ 31.7 W, and 2 K ≈ 48.9 W. Under Linac conditions the values were HTTS ≈ 207 W, LTTS ≈ 30.9 W, and 2 K ≈ 48.5 W. After installing the stainless‑steel stem with wipers, the loads dropped to HTTS ≈ 26.5 W, LTTS ≈ 29.2 W, and 2 K ≈ 47.2 W, representing a reduction of roughly 10–15 % relative to the G10‑stem configuration. The reduction is most pronounced in the HTTS and LTTS circuits, confirming that eliminating TAO and improving temperature uniformity directly lowers the heat influx to the cryogenic system.

JT heat‑exchanger performance
The JT heat exchanger’s effectiveness depends on the temperature difference between the two‑phase pipe and the relief line. The authors measured the very‑low‑pressure (VLP) inlet temperature ranging from 2.8 K to 4.6 K, which corresponded to a JT efficiency between 70 % and 85 %. Larger temperature gradients reduced the JT effectiveness, leading to higher residual heat loads at 2 K. The analysis highlights the importance of minimizing temperature stratification in the two‑phase and relief piping to maximize JT recovery.

Implications for cryoplant capacity
Using the measured heat loads, the authors reassessed the margin of the existing PIP‑II cryoplant. While the plant still possesses sufficient capacity for the current operating points, the data suggest that future upgrades to higher cavity power (e.g., >150 W) would push the plant closer to its limits, especially if the JT exchanger performance degrades. Recommendations include redesigning the JT exchanger for higher effectiveness, applying low‑temperature surface treatments (such as superconducting coatings) to the two‑phase lines, and adding additional thermal shielding to the 2 K circuit to preserve temperature uniformity.

Conclusions and future work
The Phase‑3 campaign demonstrates that targeted hardware modifications (replacement of the valve stem) combined with a robust, automated data‑handling infrastructure can bring measured heat loads well within design specifications. The study validates the approach of real‑time diagnostics and rapid reporting as a template for future cryomodule tests within the PIP‑II program and for other high‑power SRF installations. Ongoing work will focus on further optimizing the JT heat exchanger, refining the piping layout to reduce stratification, and developing closed‑loop control algorithms that use the real‑time heat‑load data to dynamically adjust cryogenic parameters during beam operation. These improvements are expected to enhance the reliability and efficiency of the cryogenic plant for the full PIP‑II accelerator complex.