Experimental performance of an $E\times B$ chopper system

Beam operation of a novel $E\times B$ chopper system has started in the low-energy beam transport (LEBT) section of the accelerator-driven neutron source FRANZ. The chopper is designed for low-energy high-perveance beams and high repetition rates, and will finally operate with 120 keV protons. It combines a static magnetic deflection field with a pulsed electric compensation field in a Wien filter-type $E\times B$ configuration. The chopper was designed, manufactured and successfully commissioned at the required repetition rate of 257 kHz using a 14 keV helium beam with up to 3.5 mA of beam current. Beam pulses with rise times of $(120\pm 10)\mathrm{ns}$, flat-top lengths of $(85\pm 10)\mathrm{ns}$ to $(120\pm 10)\mathrm{ns}$ and full width at half maximum (FWHM) between $(295\pm 10)\mathrm{ns}$ and $(370\pm 10)\mathrm{ns}$ were experimentally achieved.

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Figure 1

Schematic overview of the Frankfurt neutron source FRANZ. The chopper is located in the center of the LEBT section.

Figure 2

Schematic overview of the $E\times B$ chopper: perspective view (top) and cross sectional view (bottom). The chopper is installed in the LEBT section between Solenoid 2 and 3. Figure was modified from [29]. The total length, including the beam-separation system, is 80 cm.

Figure 3

Cross section of the chopper dipole in the $y-z$ plane. Shortening tubes and a longitudinally optimized pole profile are used to match the shapes of magnetic and electric deflection forces [3, 32].

Figure 4

Chopper dipole with transversely and longitudinally optimized pole contour. The beam direction is perpendicular to the dipole front plane shown.

Figure 5

Different time steps of a beam shaping simulation in the $E\times B$ chopper [3]. The dc beam enters the chopper from the left side. It is composed of 50 mA protons (depicted in blue), 5 mA ${\mathrm{H}}_2^{+}$ ions (red) and 5 mA ${\mathrm{H}}_3^{+}$ ions (dark green). The beam energy is 120 keV. The measured HV pulse is imported into the simulation and used to ramp the electric field. The proton pulse is shaped at a circular aperture with a 20 mm radius. The solenoids used for transverse focusing are shown in blue. The electric deflection plates and the repeller electrodes are depicted in green.

Figure 6

Simulated pulse shape for 120 keV protons directly behind the chopper (red) and at the RFQ entrance (blue). The difference $\mathrm{\Delta }{I}_{\mathrm{b}}=I_{\mathrm{b}}^{\text{chopper}}-I_{\mathrm{b}}^{\mathrm{rfq}}$ is depicted in green. No relevant increase of the pulse length is observed.

Figure 7

Drawing (left) and photograph (right) of the chopper dipole.

Figure 8

Main components of the electric deflection unit for the $E\times B$ chopper.

Figure 9

Overview of the FRANZ LEBT section, as used for the experiments presented in this section.

Figure 10

FRANZ LEBT section, including the $E\times B$ chopper.

Figure 11

Measured 14 keV ${\mathrm{He}}^{+}$ beam pulses with a repetition rate of 257 kHz (top) and 103 kHz (bottom). The signal of the BCT $I_{\mathrm{BCT}}$ is depicted in blue and the voltage $V_{\mathrm{defl}}$ at the positively charged deflection plate in red. The original BCT data without rf noise correction or baseline restoration are shown.

Figure 12

Measured 14 keV ${\mathrm{He}}^{+}$ beam pulse with 3.5 mA amplitude. The baseline of the BCT signal was manually restored.

Figure 13

Measured (green) and analytically calculated (black) time of flight $t_{\mathrm{tof}}$ between the center of the deflection plates and the BCT for the ${\mathrm{He}}^{+}$ pulses as a function of the accelerating voltage $V_{\mathrm{acc}}$.

Figure 14

Measured charge in the pulse flat top $Q_{\text{flattop}}=I_{\mathrm{BCT}}^{\max }·t_{\text{flattop}}$ as a function of the Wien ratio $R_{\text{Wien}}$, which is normalized to the velocity $v_0$ of the 14 keV ${\mathrm{He}}^{+}$ beam. The Wien ratio is varied by increasing the HV pulse amplitude $V_{\mathrm{meas}}$ while keeping the magnetic deflection field constant.