Observation of Beam Emittance Reduction due to Gas Sheet Injection for Beam Profile Measurement

In a high intensity ion accelerator, a non-destructive beam monitor is required to realize a continuous measurement of the beam for improving the beam quality in operation for users. We have developed a non-destructive 2-D beam profile monitor detecting photons produced by interaction between a beam and a sheet-shaped gas. Though the developed monitor is a non-destructive type, the injected gas sheet should induce scattering of the beam particles which makes the beam emittance large. We have measured a beam current and a phase space distribution of the J-PARC 3 MeV, 60 mA H− beam with change of the gas sheet flux. It was found that the beam current reduction was in linear relation and the root mean square emittance was constant or decreased by up to 4.5% in y-y′ plane and did not change in x-x′ plane against a rise in the gas sheet flux. These result indicate that the developed monitor can be utilized as a non-destructive one depending on the gas sheet flux condition.


INTRODUCTION
In a high intensity accelerator, a non-destructive or a minimal-destructive beam monitor is indispensable to prevent beam loss by continuous measurement.Non-destructive beam profile monitors based on beam-gas interaction are being developed in many institutes [1,2,3,4].We have developed a non-destructive 2-D profile monitor with a localized sheet-shaped gas, the gas sheet monitor, as shown in Fig. 1 [5,6,7].The gas sheet interacts with the ion beam and produces scintillation photons whose spatial distribution depends on the beam profile.The 2-D beam profile can be obtained by detecting the photons produced on the gas sheet plane.The gas sheet monitor is one of non-destructive monitors, and the non-destructiveness should be evaluated because the extra injected gas may induce beam scattering making the beam emittance large.On the other hand, since our monitor does not involve any external electric and/or magnetic fields, the produced plasma may partially or fully cancel the beam space-charge force and suppress space-charge-driven emittance growth; the emittance relatively becomes small [8].In this paper, we report the influences on the beam against the gas sheet injection from perspectives of beam current reduction and change of beam phase space distribution.

Gas Sheet Monitor
Figure 1 shows the developed gas sheet monitor system consisting of the gas sheet generator, the cover chamber with the slit and the turbomolecular pump (TMP) for cutting the tail part of the gas distribution, the main chamber with the TMP and the cryo pump, and the photon detector system.The gas sheet formation is based on the beaming effect of the gas molecular motion in vacuum engineering, and the gas sheet generator was designed as a rectangular conduit of 100 mm × 50 mm × 0.1 mm [5,9].In the main chamber, the injected gas sheet made of nitrogen interacts with the beam and emits photons by a process of the first negative bands (B → X): mainly 391 nm and 428 nm [10].The spatial distribution of the photons is detected with the detector system composed of the set of optical lenses, the image intensifier based on a multi-channel plate (Hamamatsu Photonics K. K., C9016-21 [11]), and the charge-coupled device camera (BITRAN Co., BU-66EM [12]).The image intensifier detectable at 300-700 nm increases the photon signal intensity by up to 10 4 times and change the signal into 545 nm lights.The CCD camera detects the output lights with 1920 × 1080 pixels of 5 µm sensor.The developed gas sheet monitor system.The gas sheet is formed by the generator and the slit, interacts with the beam in the main chamber, and emits scintillation lights.

Beam Line: the RFQ Test Stand
The developed gas sheet monitor was installed into the J-PARC RFQ test stand shown in

Evaluation of gas sheet injection effect on beam 3.1. Beam Current
The beam current reduction due to gas sheet injection was evaluated with the current transformer at the 11-degree-bent line because the attached electron of the H − beam is easily stripped by beam-gas interaction.The beam current reductions defined as the difference between the current without the gas injection and the current at each gas flux were plotted in Fig. 3. Since the slope of the data in a log-log graph is 1.001, the linear-fitting line is described as the blue-broken line.Though the error bars are not enough small, the beam current reduction indicates a linear relation against the gas sheet flux, where the beam profile can be measured at the all gas flux conditions.The influence on the beam current due to gas sheet injection is negligible depending on the gas sheet flux condition.

Beam Emittance
The beam phase-space distribution was measured by the double-slit emittance monitor with change of the gas sheet flux.Figure 4 shows the phase-space distributions with/without gas sheet injection.The condition with gas injection means the distribution measured at the gas sheet flux of 7×10 −4 Pa m 3 /s (maximum value).As shown in Fig. 4, no important change of the phase-space distributions is found.To evaluate the distribution quantitatively, the variation of the root-mean-square emittance against the gas sheet flux change is shown in Fig. 5.The emittance in y-y ′ plane is constant or decreased by up to 4.5% in the range of gas flux while the emittance in x-x ′ plane changes within the error bars.Therefore, according to Figs. 3 and  5, the gas sheet monitor can measure the beam profile non-destructively in the RFQ test stand depending on the gas sheet flux.The reason that the rms emittance reduced is expected as the space-charge neutralization or compensation effect, which plays an important role in low-energy beam transport.Since a non-linear space-charge force is one of the main causes making the beam emittance large, the emittance growth is suppressed when the plasma produced by beam-gas interaction cancels the space charge.Possibility of the emittance growth suppression is estimated in terms of the produced plasma density in our experimental situation.The produced plasma fully cancels the beam space charge when the plasma density is larger than the beam density.The charge density ratio of the produced plasma against the beam in the case of interaction between nitrogen gas 14th International Particle Accelerator Conference Journal of Physics: Conference Series 2687 (2024) 072018 IOP Publishing doi:10.1088/1742-6596/2687/7/0720185 and the J-PARC 3 MeV, 60 mA, 50 µs H − beam can be written as where the ionization cross section σ is 6 × 10 −21 m 2 [14].When the gas density is 6 × 10 −4 Pa equivalent, the plasma-beam charge density ratio becomes around 1.
In addition, we show another estimation which implies the space-charge neutralization occurs with the IMPACT-Z code (one of Particle-in-Cell codes) [15].The emittance growth due to space-charge force in a no-gas-injection condition is necessary for the emittance growth suppression due to the neutralization effect to occur.Figure 6 shows the simulated rms emittance variation against the longitudinal direction (s axis) from the RFQ exit to the emittance monitor with no gas injection.The rms emittance in x-x ′ plane is almost constant while the emittance in y-y ′ plane grows around the gas sheet monitor's position.These results support the experimental results shown in Fig. 5 that the emittance reduction is observed only in y-y ′ plane.Moreover, the simulated emittance values at the beam dump and the RFQ exit are same as the measured values without gas sheet injection and with the maximum gas flux, respectively.These results implies that the emittance growth in y-y ′ plane may be fully suppressed and the initial emittance at the RFQ exit may be kept to the beam dump.Thus, the beam emittance reduction found experimentally only in y-y ′ plane is deduced as the result of the space-charge neutralization effect.

CONCLUSION
We have developed the non-destructive 2-D beam profile monitor with injection of a sheet-shaped gas.In this paper, we reported the influences of the gas sheet injection on a beam from the perspectives of the beam current reduction and the phase-space distribution change using the J-PARC RFQ test stand.The beam current reduction indicated a linear relation against the gas sheet flux, and the beam emittance was constant or decreased against the rise in the gas sheet flux; it was clarified that the developed gas sheet monitor can be utilized as a non-destructive monitor depending on the gas sheet flux condition.The beam emittance reduction is expected to be caused by the space-charge neutralization effect suppressing the emittance growth.This result implies the gas sheet monitor may have potential to be a emittance reducer if it is utilized where the emittance growth occurs.

Figure 1 .
Figure1.The developed gas sheet monitor system.The gas sheet is formed by the generator and the slit, interacts with the beam in the main chamber, and emits scintillation lights.

Fig. 2 [ 3 RFQ
13].The test stand consists of the spares for the J-PARC Linac from the ion source to the third quadrupole electromagnet.The ion source produces a 50 keV, 60 mA, 50 µs H − beam pulse in 25 Hz repetition.The beam pulse is bunched and accelerated to 3 MeV in the radiofrequency quadrupole (RFQ) linac of 324 MHz.The three quadrupole electromagnets deriver the beam to the Faraday cups of beam dump in straight or 11-degree-bent line via the doubleslit-type emittance monitor.There are current transformers to measure the beam current after the RFQ and before the Faraday cups.In this beam line, the gas sheet injection flux was limited up to 7×10 −4 Pa m 3 /s to prevent discharges in the RFQ cavity, where the beam line pressure at the gas sheet monitor increased from the base pressure of 1×10 −6 Pa to 6×10 −4 Pa.14th International Particle Accelerator Conference Journal of Physics: Conference Series 2687 (2024) 072018 IOP Publishing doi:10.1088/1742-6596/2687/7/072018

Figure 2 .
Figure 2. The J-PARC RFQ test stand consisting of the spares for the J-PARC Linac from the ion source to the third quadrupole electromagnet.

6 Figure 3 .
Figure 3.The beam current reduction ratio against gas sheet flux.The blue broken line describes linear fitting, and the error bars indicate the standard deviations.
(a) x-x' plane without gas sheet (b) y-y' plane without gas sheet (c) x-x' plane with gas sheet (d) y-y' plane with gas sheet

Figure 4 .
Figure 4.The phase space distributions with (the gas flux of 7×10 −4 Pa m 3 /s) and without the gas sheet injection.The intensities are in logarithmic scale.The green ellipses describe 5 times of the rms emittance.

Figure 5 .
Figure 5.The emittance change against the gas sheet flux.The error bars describe the standard deviations.

Figure 6 .
Figure 6.The emittance variation against the beam axis simulated with the IMPACT-Z code in the case of no gas injection.