First RF beam detector test at SY dump

K. Yonehara

(3/29/18)

Fermilab MI accelerator and NuMI target serve the most powerful neutrino beam to study the beyond-standard model. The available beam power will be ramped up to a multi-MW as a long term plan. A radiation hard beam detector is required to diagnose the quality of neutrino beam. We propose a conceptually new beam detector which is based on a gas-filled RF resonator. A plasma in the cavity is formed by interacting an incident beam with gas and shifts the gas permittivity. The shift is proportional to the beam intensity and measurable by observing the loading of RF power into the gas plasma, which is called a plasma loading. The RF detector mitigates several issues related with high radioactive environments. A cable configuration of the RF detector will be simple to mitigate a radiation damage problem. A gain of the output signal of RF detector can be calibrated by measuring RF parameter when the beam is turned off. It permits to measure the absolute beam intensity.

In order to verify the proof-of-principle of the RF detector, we carried out the first beam test at the SY beam line. Because the beam was extracted from the MI ring with the single-turn extraction mode, the beam was commissioned at the SY beam line on 2/7. The physics run was carried out on 2/8, 2/9, and 2/13 (Figure 1). We totally utilized the beam line less than 12 hours. The beam pulse length was 1.6 s and the beam intensity was typically ~2e12 ppp.

Figure 1: Log of single-turn extraction beam delivered to the SY beam line.

A prototype 2.4 GHz RF detector was fabricated and mounted in front of a SEM near the SY beam dump (Figure 2). The RF cavity is a pillbox shape and made of stainless steel. It has a beam window at the axial center on upstream and downstream RF plates, which is a 1 mm-thick SS. There are three RF interface ports on the cavity; one is for tuning the cavity quality (Q) factor (a loading loop), other is to pick-up RF signal in the cavity (a pickup loop), and the last is to feed RF power to the cavity (an input loop). There are two gas lines accessing from a sidewall; one is a gas inlet and other is a spare. The ambient air was used for the first beam test.

Figure 2: Prototype 2.4 GHz RF detector assembled in front of a SEM on SY dump beam line.

Figure 3 shows a circuit diagram of the RF system. A network analyzer was used as a RF source. A 2-volt rectangular pulse is fed into a mixer to form a pulsed RF signal, or a 2-volt DC is applied to the mixer to generate a continuum RF signal. A RF amplifier was used to increase a RF input power to 30 dBm (1-Watt). The power was decreased in a 160-m-long 3/8’’ heliax cable by 20 dB by reaching the RF detector then stored in the cavity. The stored power was detected by the pickup loop and sent to a real-time-peak (RTP) power probe through another 160-m-long cable.

Figure 3: Circuit diagram of the RF system.

Figure 4 shows a throughput power of the cavity as a function of the driving RF frequency. The observed Q factor was 80, which is consistent with the measurement by using a network analyzer. During the beam test, the cavity was operated at two different driving frequencies, one was 2.34916 GHz (about 15 MHz off from the resonance) and other was 2.36460 GHz (near resonance).

Figure 4: Observed throughput power as a function of the driving frequency.

Figure 5 shows the output signal from the cavity when the beam is turned on. The driving frequency is 2.34916 GHz. The output signal (baseline) was -16.5 dBm with no beam. It was oscillated when the beam was turned on. The number of oscillation peaks is 50-60 in 1.6 s, which is not matched to the number of beam bunches per pulse (82 bunches per pulse). A moving average of the signal (blue curve) suggests that the oscillation signal is beating. The number of beatings is 8-12 per pulse. A simple wakefield calculation suggests that the oscillation is not caused by the wakefield. Indeed, the observed oscillation amplitude has no correlation with the beam intensity (Figure 6) while the number of oscillation peaks seems to be changed by the beam intensity. It suggests that a micro-watt scale local field is induced around the pickup loop when the beam is passing through the cavity. Anyway, the beam signal is a major noise in the present system. We discuss how to eliminate the oscillation later.

The output signal always shows a sharp downward peak immediately after the beam is turned off. The stored RF power is recovered exponentially to the baseline. This is the plasma loading. When the beam is stopped, no more ion pair is produced and remnants are recombined in the recovery process. Figure 7 shows the observed recovery signal and an exponential curve. For convenience, we define the recovery time from the time at the lowest voltage point after the beam is turned off to the time at the output signal reaching to 99 % of the baseline. The definition will be changed in future when we study a specific plasma process. Because the time resolution of data acquisition is 6 ns in this measurement, the observed recovery time has a +/- 3 ns error.

Figure 5: Raw RF signal (Orange), moving average (Blue), baseline RF (Red). Green bar shows when the beam is turned on. RF recovery is shown as a green empty box which is explained in the document.

Figure 6: Beam noise on the RF pickup signal. The RF input source was turned off. The size of beam noise seems to be independent from the beam intensity while the oscillation frequency is dependent on the beam intensity.

Figure 7: Observed RF recovery signal at the driving frequency 2.36460 GHz. Blue is a raw data and orange is an exponential curve which is adjusted by eye. Solid red line is the observed baseline RF power while dashed red line is a 98 % of the RF power from the baseline.

Figure 8 is the measured RF recovery time as a function of the beam intensity. The recovery time is proportional to the beam intensity. The measurement validates the plasma loading theory. The theory also predicts that the cavity Q factor changes the beam sensitivity: Higher Q makes higher beam sensitivity. The measurement validates the prediction. When the cavity is operated near resonance (red point), it is high sensitive to the beam. On the other hand, the cavity is operated at 3 dB off from the resonance (blue point) it is low sensitive to the beam. A low-Q cavity is more effective when the RF detector measures higher beam intensity.

We observed an unexpected beam noise (beam-induced oscillation). It prevents to measure the transient plasma loading when the beam is turned on. In order to avoid the oscillation measurement, we plan to increase the input RF power. The input RF power will be increased by one or two orders of magnitude by using a second RF amplifier. We also plan to use a bandpass filter to cut a local electromagnetic excitation around the RF pickup loop on the output signal.

Once the oscillation is removed, we precisely investigate a plasma process in various conditions. The plasma loading in a pure gas, e.g. N2, He, and dry air doped gas will be measured. Because the plasma dynamics in those species are well understood, the absolute beam intensity can be measured in the RF detector. Investigating radiation damage at the RF coupling will be a key to design a practical detector. We design a waveguide to excite the RF detector (Figure 9). In order to complete those items, we need another beam time at the SY beam. We anticipate the beam time after the summer shutdown 2018.

Figure 8: Observed recovery time as a function of a beam intensity. A dashed line is a linear fitting of the measurement. A red point is taken in the maximum Q factor cavity while a blue one is a low Q factor one.

Figure 9: Front view of a gas-filled RF beam detector for NuMI demonstration test.

Appendix

Measured points at the RF power recovery are shown in the appendix. Data were taken at the maximum Q factor mode, low Q factor mode, and low input RF power mode at the low Q factor. The low input RF power mode is not included in Figure 8.

Maximum Q factor mode:

Low Q factor mode:

Low input RF power mode:

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