Experimental demonstration of particle acceleration with normal conducting accelerating structure at cryogenic temperature

In this paper, we present an experimental demonstration of the high-gradient operation of an X-band, 11.424 GHz, 20-cells linear accelerator (linac) operating at a liquid nitrogen temperature of 77 K. The tested linac was previously processed and tested at room temperature. Low-temperature operation increases the yield strength of the accelerator material and reduces surface resistance, hence a great reduction in cyclic fatigue could be achieved resulting in a large reduction in breakdown rates compared to room-temperature operation. Furthermore, temperature reduction increases the intrinsic quality factor of the accelerating cavities, and consequently, the shunt impedance leading to increased rf-to-beam efficiency and beam loading capabilities. We verified the enhanced accelerating parameters of the tested accelerator at cryogenic temperature using different measurements including electron beam acceleration up to a gradient of $150\mathrm{MV}/\mathrm{m}$, corresponding to a peak surface electric field of $375\mathrm{MV}/\mathrm{m}$. We also measured the breakdown rates in the tested structure showing a reduction of 2 orders of magnitude compared to their values at room temperature for the same accelerating gradient.

Figure 1

A plot of the surface resistance for copper with $\mathrm{RRR}=400$ versus temperature at 11.424 GHz. At low temperatures, the surface resistance, predicted by ASE, saturates to higher values than the ones calculated from the classical skin depth. Operating at 77 K results in a reduction in the surface resistance of a factor of 2.25 compared to 300 K.

Figure 2

(a) The experimental setup for accelerator testing at the XTA facility at SLAC. XTA is divided into two stations, referenced to as station 1 and station 2. The first station generates the electron beam used in the experiment, and the tested accelerator is installed in the second station. (b) A foam-insulated stainless-steel line delivers LN from Dewars outside the tunnel to the cryostat where the tested linac is setting inside, through a hole in the tunnel wall. A level detector is inserted inside the cryostat and provides a control signal to a solenoid that controls the flow of the LN in the feedline. A vent-out line transports the boiled LN outside the tunnel.

Figure 3

The reflection coefficient at the rf input to the accelerator at 300 and 77 K. As the accelerator structure cools down, it shrinks leading to a shift of the resonance frequency of 36 MHz. The tested linac is designed for critical coupling at 300 K, $|s_{11}|=-32\mathrm{dB}$ at resonance, and becomes overcoupled at 77 K, $|s_{11}|=-8\mathrm{dB}$ at resonance, because of the reduced surface resistance which increases the intrinsic quality factor of the accelerating cavity while the external one remains constant.

Figure 4

(a) The measured intrinsic quality factor of the accelerating structure versus the resonance frequency of the structure. The measured values are obtained using Q-circle fitting of the measured reflection coefficient at the rf input port. (b) We compared the measured values of the internal quality (black dots) with our simulations (dashed line). In our simulations, the temperature at each point is used to calculate the surface resistance using the theory of ASE and simulate the intrinsic quality factor.

Figure 5

(a) A picture of the installed cryostat in the XTA beam line and (b) the LN feeding system outside the tunnel.

Figure 6

The measured energy gain of an electron beam moving down the axis of the distributed-coupling linac for a compressed 200 ns square rf pulse with an input power of 13–33 MW, achieving an accelerating gradient of $100–150\mathrm{MV}/\mathrm{m}$ and a peak surface electric field of $250–375\mathrm{MV}/\mathrm{m}$. Blue dots and the dashed line are measured and simulated values, respectively. Simulations used the measured input rf pulse and the cavity model at 77 K.

Figure 7

(a) The processing history of the distributed-coupling linac at 300 and 77 K with the total number of pulses and breakdowns. The structure was tested using 400 ns stepped pulses with 200 ns flat gradient and 300 ns pulses with 100 ns flat gradient. (b) A zoom-in of the collected breakdown statistics at 300 and 77 K and different gradient levels. In the second plot, we discarded the periods of ramping up to the target gradient, following a breakdown event, in our calculations of the number of pulses and breakdowns. The BDR are collected for 400 ns stepped pulses with 200 ns flat gradient.

Figure 8

The measured forward and reflected pulses at the input of the accelerator structure at a gradient of 95 and $120\mathrm{MV}/\mathrm{m}$ for 300 and 77 K operation, respectively. The figure also shows the simulated gradient using the cavity model and the good agreement between measured and simulated reflected waveform.

Figure 9

The collected breakdown rates with the fitted slope of the distributed-coupling accelerator structure at 300 and 77 K. The breakdown rates at 300 K are obtained from our previous testing of the same accelerator structure at room temperature. For both operating temperatures, the data were collected for a 400 ns stepped pulse with a flat gradient of 200 ns. The results show a reduction by 2 orders of magnitude, $\times \frac{1}{100}$, in breakdown rates at the same gradient levels from 300 to 77 K operation.