Measurement of the longitudinal bunch-shape distribution for a high-intensity negative hydrogen ion beam in the low-energy region

A bunch-shape monitor (BSM) is a useful device for performing longitudinal beam tuning using the pointwise longitudinal phase distribution measured at selected points in the beam transportation. To measure the longitudinal phase distribution of a low-energy negative hydrogen (${\mathrm{H}}^{-}$) ion beam, highly oriented pyrolytic graphite (HOPG) was adopted for the secondary-electron-emission target to mitigate the thermal damage due to the high-intensity beam loading. The HOPG target enabled the measurement of the longitudinal phase distribution at the center of a 3-MeV ${\mathrm{H}}^{-}$ ion beam with a high peak current of about 50 mA. The longitudinal bunch width was measured using HOPG-BSM at the test stand, which was consistent with the beam simulation. The correlation measurement between the beam transverse and longitudinal planes was demonstrated using HOPG-BSM. The longitudinal Twiss and emittance measurement with the longitudinal Q-scan method was conducted using HOPG-BSM. The measured longitudinal emittance is consistent with the beam simulation using the radio-frequency quadrupole design input.

Figure 1

Temperature estimation on target with beam irradiation calculated using the fourth-order Runge-Kutta method, where peak current, macrobunch length, and repetition frequency of beam are 55 mA, $50\mathrm{\mu }\mathrm{s}$, and 1 Hz, respectively.

Figure 2

Developed HOPG target installed in BSM. The red hatched box shows a target with dimensions $0.2\times 1.0\times 48{\mathrm{mm}}^3$.

Figure 3

Experimental setup to measure the longitudinal phase distribution using BSM in the test stand. Q1, Q2, and Q3 are quadrupole magnets.

Figure 4

Schematic of BSM.

Figure 5

Measurement principle of BSM.

Figure 6

Horizontal distributions measured with HOPG-BSM and WSM using the CNT wire. The size of the HOPG target was $0.2(\mathrm{width})\times 1.0(\mathrm{thickness}){\mathrm{mm}}^2$. The red-dotted line indicates a fitted function. Distribution simulated with parmila code is also shown.

Figure 7

Typical raw signal waveform from secondary-electron multiplier after preamplifier (top) and raw signal waveform distribution of longitudinal bunch-shape measurement (bottom) obtained using HOPG-BSM. The size of the HOPG target was $0.2\times 1.0{\mathrm{mm}}^2$. Sharp peaks around time 50 and $100\mathrm{\mu }\mathrm{s}$ are noise due to the ${\mathrm{H}}^{-}$ ion beam. In the top plot, the red hatched and black hatched regions represent the signal and baseline regions, respectively, used for evaluating longitudinal phase distribution.

Figure 8

Longitudinal phase distribution measured using HOPG-BSM in the test stand. The size of the HOPG target was $0.2(\mathrm{width})\times 1.0(\mathrm{thickness}){\mathrm{mm}}^2$ and the lens voltage was 3.5 kV. The black hatched region indicates the range adopted for evaluating the longitudinal bunch width.

Figure 9

Comparison of longitudinal phase distributions measured with different sizes of HOPG targets.

Figure 10

Scatterplot of longitudinal phase versus horizontal target position (top) and dependence of bunch center on horizontal target position (bottom). The size of the HOPG target was $0.2\times 1.0{\mathrm{mm}}^2$ and the lens voltage was 4.0 kV. The red-dotted line in the bottom plot indicates the fitted function. The fitting range of the bunch center is from $-4.5$ to 4.0 mm.

Figure 11

Dependence of the BSM resolution on the transverse position. The size of the HOPG target was $0.2\times 1.0{\mathrm{mm}}^2$ and the lens voltage was 4.0 kV. The green hatched region is a target width of 0.2 mm.

Figure 12

Comparison of longitudinal phase distributions at typical three horizontal positions. The $x$ means the horizontal position.

Figure 13

Measured $\varphi -x$ correlation (top) and simulated one (bottom). The size of the HOPG target was $0.2\times 1.0{\mathrm{mm}}^2$. The blank region in measurement results was due to a limited BSM target actuator range.

Figure 14

Schematic of MEBT1 in the J-PARC linac. QM means a quadrupole magnet. There are eight QMs, two bunchers, a set of beam chopper and scraper, and one BSM in the MEBT1.

Figure 15

Longitudinal phase distributions measured using HOPG-BSM in MEBT1. The voltage of the buncher1 was 0.21 MV.

Figure 16

Bunch-width dependence on scanning the buncher voltage. The red-solid line indicates the fitting using the impact simulation.

Figure 17

Comparison between the measured longitudinal Twiss ellipses and the simulation at the RFQ exit. $\mathrm{\Delta }\varphi$ and $\mathrm{\Delta }T$ are the displacement of the longitudinal phase and that of the energy from the mean, respectively.

Figure 18

Estimated growth of the bunch width with the HOPG-BSM using different sizes of HOPG targets in the MEBT1.

Figure 19

Schematic illustration of the rf modulation, where $V$ and $\varphi$ are the voltage and phase of the rf deflector, and $\xi$, $Z_c$, $Z_{\max }$ are small phase difference, displacement between slit center and rf deflection center on the second collimator, and maximum displacement induced by the rf deflection.

Figure 20

Signal intensity as a function of steering voltage for lens voltage of 3.5 kV.

Figure 21

Width in the voltage for steering voltage as a function of lens voltage.

Figure 22

Shifts of the longitudinal phase distributions with steering voltage (top) and dependence of bunch center on steering voltage (bottom). The red-dotted line in the bottom plot indicates the fitted function.