High energy Coulomb-scattered electrons for relativistic particle beam diagnostics

A new system used for monitoring energetic Coulomb-scattered electrons as the main diagnostic for accurately aligning the electron and ion beams in the new Relativistic Heavy Ion Collider (RHIC) electron lenses is described in detail. The theory of electron scattering from relativistic ions is developed and applied to the design and implementation of the system used to achieve and maintain the alignment. Commissioning with gold and ${}^3\mathrm{He}$ beams is then described as well as the successful utilization of the new system during the 2015 RHIC polarized proton run. Systematic errors of the new method are then estimated. Finally, some possible future applications of Coulomb-scattered electrons for beam diagnostics are briefly discussed.

Figure 1

The solid lines show calculated energies and approximate scattering cross sections for 5 keV electrons backscattered by 250 GeV protons. The dotted lines correspond to the same quantities, but for 10 eV electrons as a qualitative indication of energetic electrons generated by the interaction of the beam with the residual gas and/or with low energy electrons captured in the potential well of the beam.

Figure 2

Angles with respect to the initial electron beam direction for 5 keV electrons scattered of 250 GeV protons moving in the opposite direction. For this example, the energy of the electrons in the proton rest frame is 157 MeV.

Figure 3

Perspective and top views of one of the RHIC electron lenses [11, 12], showing the backscattered electron detector location close to the electron gun. In reality, the light guide and photomultiplier tube are enclosed in a heavy and light-tight magnetic shield to protect the PMT from the stray fields of the nearby magnets.

Figure 4

Computer simulated trajectories (blue and green curves) of two scattered electrons generated inside of the 6 T solenoid (see text). Only the first 200 mm of this 2400 mm long superconducting solenoid is included at the left (from $\mathrm{Z}=1000\mathrm{mm}$ to $\mathrm{Z}=1200\mathrm{mm}$). Three weaker solenoids (not shown) guide the electron beam from the cathode towards the 6 T region (see Fig. 3).

Figure 5

Not-to-scale schematic of the eBSD scintillation detector and its housing. Electrons backscattered by the relativistic ion beam reach the plastic scintillator after traversing a thin titanium alloy vacuum window. The light from the scintillator is converted to electrical signals by means of a well shielded photomultiplier tube. A tungsten block protects the back of the detector cavity from accidental heating by the electron beam.

Figure 6

Cutaway drawing of the detector housing, the vertical translation mechanism and the location of the 0.1 mm thick titanium alloy vacuum window in its lowest position. The area of this window is $25.4\times 6.35{\mathrm{mm}}^2$. The vertical position can be changed by up to 25.4 mm away from the electron beam. That highest position was the one used for most measurements to minimize excessive counting-rate issues.

Figure 7

Stopping power (blue) [27] and calculated energy loss (red) in titanium as a function of electron energy.

Figure 8

Design and fabrication details of the cavity and the 0.1 mm thick titanium alloy window. The light guide is inserted in the cavity with the plastic scintillator facing the thin window. The electrons traverse this window from the vacuum outside of the cavity to the inside that is at atmospheric pressure.

Figure 9

Finite element stress analysis results for the $25.4\times 6.9\times 0.1{\mathrm{mm}}^3$ Ti-6Al-4V alloy window. There is a safety factor of 3.9 at atmospheric pressure. The window was pressure tested up to three atmospheres without bursting.

Figure 10

Horizontal and vertical beam separation scan obtained by steering the 5 keV electron beam with respect to the $100\mathrm{GeV}/\mathrm{nucleon}$ gold beam.

Figure 11

Manual beam separation scans obtained by stepwise steering the ${}^3\mathrm{He}$ beam with closed bumps utilizing part of the algorithm developed for the automated alignment optimization system based on the LISA program [28].

Figure 12

Counting rate as a function of the vertical displacement of the eBSD detector. These results were obtained with a $100\mathrm{GeV}/\mathrm{nucleon}$ ${}^3\mathrm{He}$ beam consisting of 93 bunches with $4.7\times {10}^{10}$ ions per bunch and a 6 keV, 88 mA electron beam. The zero of the displacement scale corresponds approximately to the center of the detector being 5 mm above the center of the electron beam. The operating point was the rightmost point on the chart, at 24 mm. The counting-rate slope at that point is approximately $6.8\%/\mathrm{mm}$.

Figure 13

Schematic of the pulse height analysis system used to optimize the discriminator setting. The multichannel analyzer (MCA) used here consists of an Ortec multichannel buffer (MCB) connected to a computer running the Maestro analysis and control software [30].

Figure 14

Pulse height spectra from the scintillation counter used in one of the RHIC electron lenses. In the upper spectrum (a) all pulses are accepted, while in the lower one (b), pulse amplitudes below the discriminator setting are rejected. The discriminator effectively suppresses the high intensity low amplitude noise (see the red curve, which is the original histogram plotted with a scale change of a factor 1000). The small remaining peak to the left in (b) may be an artifact of the pulse height analyzer gating system.

Figure 15

Pulse height spectra screen shots obtained with the pulse-height analyzer software [30]. The logarithm of counting rates is displayed as a function of pulse heights. The peaks to the right were obtained using identical settings of a precision light pulse generator. The slight shift of this peak indicates a $\sim 12\%$ light transmission loss due perhaps to slight radiation damage of the 1.2 m long light guide or of the fiber carrying the light signal to the far end of the light guide.

Figure 16

The total proton intensities (a), RHIC electron lens electron current intensities (b), and corresponding eBSD counting rates (c) are shown as a function of time. The electron beam intensities are reduced in steps to optimize the benefits of the RHIC electron lenses [11]. The two colors indicate the results from each of the two RHIC rings.

Figure 17

Time-of-flight spectra of electrons generated by the interaction of the relativistic proton beam with the residual gas. Data were accumulated for 5 minutes due to the low counting rates. The top chart shows the RHIC 111-bunch pattern and portions of the abort gaps preceding and following that pattern. The bottom chart is an expanded view of the last ten bunches to show the asymmetric shape of the peaks (see text).

Figure 18

Magnetic field profile (a) and electron beam trajectory (b) starting at the electron gun and ending at the electron collector. In the magnified view (c) we show the $\pm 1$ sigma electron beam and ion beam profiles in the region where the electron beam starts curving away on its way to the collector.

Figure 19

Simulated relative counting rates as a function of horizontal electron-ion beam offset for the case illustrated in Fig. 18. Also shown (in red) is the ideal convolution of the two Gaussian beams in absence of the asymmetric contributions.

Figure 20

Schematic illustration of the potential use of eBSDs for monitoring the halo of an intense ion beam. The beam propagates in a direction perpendicular to the plane of the figure. The electrons generated by the interaction of the ions with the hollow electron beam spiral along the magnetic field towards the detector generating a counting rate approximately proportional to the overlapped halo intensity (left). The ion beam interacting with the residual gas produces a background (right). Excellent vacuum is required to minimize this background (see text).

Figure 21

Three possible configurations for using eBSDs for halo monitoring and beam alignment of hollow electron beam systems. The central long solenoid is the same strong field superconducting solenoid in each case, similar to the 6 T ones used in the Brookhaven RHIC electron lenses. The smaller and weaker room-temperature solenoids, indicated schematically by the small rectangles, guide the electron beam from the gun to the central solenoid and from there to the collector (see Fig. 3). Some of these guiding solenoids have been omitted for clarity. Only two eBSDs are shown in the bottom configuration, but there could be four at 90° intervals for continuous halo monitoring and beam centering. The bottom configuration seems to be the most feasible one (see text).

Figure 22

Schematic illustration of a Coulomb scattering electron wire beam profile monitor (see text).

Figure 23

Concept of one of the possible approaches to detect several side-by-side orbits in one of the fixed-field alternating gradient (FFAG) arcs of the electron-ion collider, eRHIC, which will be proposed as a successor to RHIC. Fast, position-sensitive channel-plate detectors respond to energetic electrons collimated by parallel plate collimators (see text).