Design and first operation of a supersonic gas jet based beam profile monitor

Noninterceptive beam profile monitors are of great importance for many particle accelerators worldwide. Extra challenges are posed by high energy, high intensity machines and low energy low intensity accelerators. For these applications, existing diagnostics are no longer suitable due to the high power of the beam or the very low intensity. In addition, many other accelerators, from medical to industrial will benefit from a noninvasive, real time beam profile monitor. In this paper we present a new beam profile monitor with a novel design for the nozzle and skimmer configuration to generate a supersonic gas jet meeting ultrahigh vacuum conditions and we describe the first results for such a beam profile monitor at the Cockcroft Institute. This monitor is able to measure two-dimensional profiles of the particle beam while causing negligible disturbance to the beam or to the accelerator vacuum. The ultimate goal for this diagnostic is to provide a versatile and universal beam profile monitor suitable for measuring any beams.

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Figure 1

Principle of two-dimensional imaging (ions are extracted by the external electric field).

Figure 2

General overview of the gas jet monitor setup (TMP: turbomolecular pump. The pumping speed is based on ${\mathrm{N}}_2$ gas. TMPs for the outer and differential pumping chambers are backed by the same scroll pump).

Figure 3

Illustration of the pulsed valve operation and control.

Figure 4

Picture of the two different sizes of rectangular skimmer: (a) larger slit ($7.2\times 1.8{\mathrm{mm}}^2$); (b) smaller slit ($4.0\times 0.4{\mathrm{mm}}^2$).

Figure 5

Extraction field scheme and particle beam-gas jet interaction visualization.

Figure 6

(a) Schematic of the particle beam-gas jet interaction viewed from above. Gas jet sheet shown in purple, electron beam in green. (b) Side view and gas screen details.

Figure 7

Calculated reaction rates for antiprotons [38].

Figure 8

Calculated reaction rates for electrons [39].

Figure 9

Electrostatic potential in the z-x plane for the external ion-extraction field.

Figure 10

The change in the ion beam size during propagation to the MCP under an external field.

Figure 11

Image broadening by thermal spread and radial component of the external field, with different initial or real beam sizes. The curve is fitted according to Eq. (1), with ${\mathrm{R}}^2=1$.

Figure 12

Time structure of the pulsed valve and pressure response in the two chambers. The conversion factors between the displayed voltage (V) and the pressure in the chamber are $4.33\times {10}^{-10}\mathrm{mbar}/\mathrm{V}$ for the differential chamber and $1.00\times {10}^{-9}\mathrm{mbar}/\mathrm{V}$ for the dump chamber.

Figure 13

Plot of pressure at (a) differential chamber and (b) dump chamber versus pulse valve period; the duty cycle of the pulsed valve is 50%, the pressure at the pulsed valve is 2 bar, and helium is the gas used.

Figure 14

Plot of pressure at (a) differential chamber and (b) dump chamber verses pulsed valve duty cycle; the period of the pulsed valve is 1 s, pressure at the pulsed valve is 2 bar and helium is the gas used.

Figure 15

Plot of pressure at (a) differential chamber and (b) dump chamber, versus inlet pressure, for 1 s period, 50% duty cycle, variable helium pressure.

Figure 16

(a) Images of electron beam measured from both the gas jet (larger size third skimmer) and residual gas; projected profiles in (b) x direction and (c) y direction (electron beam energy is 3.75 keV, external collecting E-field is $7.5\mathrm{kV}/\mathrm{m}$, the gas pulse duration time is 1 s and the exposure time of the camera is 70 ms).

Figure 17

(a) Images of the electron beam profile from both gas jet and residual gas; projected profiles in (b) x direction and (c) y direction (electron beam energy is 3.5 keV, external collecting E-field is $8.0\mathrm{kV}/\mathrm{m}$, the gas pulse duration time is 1 s, and the exposure time of the camera is 120 ms in the middle of the pulse).