Tevatron electron lenses: Design and operation

The beam-beam effects have been the dominating sources of beam loss and lifetime limitations in the Tevatron proton-antiproton collider [V. Shiltsev et al., Phys. Rev. ST Accel. Beams 8, 101001 (2005)]. Electron lenses were originally proposed for compensation of electromagnetic long-range and head-on beam-beam interactions of proton and antiproton beams [V. Shiltsev et al., Phys. Rev. ST Accel. Beams 2, 071001 (1999).]. Results of successful employment of two electron lenses built and installed in the Tevatron are reported by Shiltsev et al. [Phys. Rev. Lett. 99, 244801 (2007); New J. Phys. 10, 043042 (2008)] and by Zhang et al. [X.-L. Zhang et al., Phys. Rev. ST Accel. Beams 11, 051002 (2008)]. In this paper we present design features of the Tevatron electron lenses (TELs), discuss the generation of electron beams, describe different modes of operation, and outline the technical parameters of various subsystems.

Figure 1

(a) General layout of the TEL-1 installed at F48 location, top view. (b) General layout of the TEL-2 installed at A11 location, top view.

Figure 2

Transverse cross section of the main SC solenoid.

Figure 3

Simulations of the magnetic field lines in the bending sections of the TEL-1 and TEL-2 carried out using the MULTIC code [30]. The placement of the TEL-1 gun solenoid is shown by dashed lines; TEL-2 magnets are represented by solid lines. Magnetic field lines (electron-beam trajectories) in both TELs are shown as well.

Figure 4

Sketch of the placement and action of the dipole correctors (transverse scale exaggerated). Without activating the correctors, the electron beam would follow the dashed path in the main solenoid. By using the correctors, the electron beam can overlay the (anti)proton path.

Figure 5

Normalized strength of the dipole correctors ($B_{x,y}/B_{\max }$) inside the main solenoid. The solenoid field $B_z$ is also included (dashed line). The longitudinal position is referenced to the geometrical center of the main solenoid. The maximum magnetic field is 8 kG in the short correctors, 2 kG in the long correctors, and 65 kG in the main solenoid.

Figure 6

(top) Drawing of the gimble and photograph of the cart. The gimble measured only 2.54 cm across and has very little mechanical resistance. The cart was pulled by a long track, and it rolled inside an aluminum pipe temporarily thrust into the solenoid bore; (bottom) simplified cartoon illustrating the technique used to measure the magnetic field line straightness in the main solenoid.

Figure 7

Measured vertical and horizontal field lines in the TEL-1 main solenoid at 40 kG.

Figure 8

Gun geometry and “UltraSAM” code electric field simulation results for: (a) flattop gun; (b) Gaussian gun; (c) SEFT gun.

Figure 9

(a) Mechanical design of the flattop gun; (b) pin-hole collector assembly for beam profile measurements on the test bench.

Figure 10

Volt-ampere characteristics of the three electron guns, solid lines are fits according to Child’s law $P=I/U_a^{3/2}$.

Figure 11

(a) 2D electron current-density distribution of the SEFT gun beam; (b) 1D current-density distributions for three guns, solid lines represent UltraSAM simulation results. In both cases, the control electrode voltage was set equal to the cathode voltage.

Figure 12

The electron current-density profile generated by the flat gun at different voltages $U_{\mathrm{pr}}$ on the near-cathode electrode. The anode-cathode voltage $U_{\mathrm{ac}}=3\mathrm{kV}$ and the magnetic field in all solenoids is 2 kG.

Figure 13

Total current of the flat and SEFT guns vs profile controlling voltage.

Figure 14

Scaled sketch of the collector cross section. The collector itself is a water-cooled copper cavity that resides outside the solenoid. This allows the electron beam to spread out, distributing the heat load and decreasing the production of secondary electrons.

Figure 15

Measurement of the collector acceptance. As the collector voltage is adjusted with respect to the cathode voltage, the current admitted to the collector changes.

Figure 16

Schematic of the TEL electrical circuitry. The shaded barrier represents about 60 meters between the Tevatron tunnel and the gallery where supplies and electronics are stationed. The filament is at cathode potential, so the AC power supply providing the cathode heating needs to be isolated from ground via a transformer, and its signal needs to be subtracted from the cathode current meter.

Figure 17

The Tevatron beam structure and required TEL electron pulse structure.

Figure 18

Electron gun driving scheme. Only the cathode, the control electrode (profiler), and the anode of the electron gun are shown.

Figure 19

TEL-2 Marx generator output voltage and electron-beam current.

Figure 20

Cross section of the TEL beam pipe.

Figure 21

Sketch of the electrodes in the TEL (not to scale).

Figure 22

BPM pickup signal (black) featuring the electron pulse, two proton (positive peaks), and two antiproton bunches (negative peaks). For clarity the electron pulse is shown timed to the abort gap where no bunches are present. The scope bandwidth was intentionally set to 20 MHz to reject high frequency noise and limit proton bunch signal amplitude. Cathode (blue) and collector (red) currents are measured by current transformers. The artifacts on the right-hand side are caused by the instrumentation.

Figure 23

Horizontal betatron tune of the 980 GeV antiproton beam measured by the 1.7 GHz Schottky detector [24], plotted versus the electron-beam horizontal position with respect to the antiproton orbit. Gaussian electron gun. Electron rms beam size 0.7 mm, current of $J_e=0.53\mathrm{A}$, cathode voltage $U_e=7.0\mathrm{kV}$. Electrons defocus antiprotons, thus, the negative shift of the betatron tune.