4D transverse phase space tomography of an operational hydrogen ion beam via noninvasive 2D measurements using laser wires

Utilizing noninvasive 2D scans with laser wires, we tomographically reconstructed the 4D transverse phase space distribution of a 1-GeV hydrogen ion (i.e., ${\mathrm{H}}^{-}$) beam during operation. The 4D tomography is based on two new advances. First, we extended the formulation of maximum entropy phase space tomography to take 2D projections as input and derived theoretical results relevant to our application. Second, we introduced the method of “perpendicular scans” to obtain cross-plane information from a laser wire emittance scanner. Perpendicular scans are two unconventional 2D measurements performed with perpendicular front and back wires. In contrast, only parallel front and back wires (i.e., “parallel scans”) are utilized in ordinary measurements of the horizontal and vertical 2D phase space distributions. When we applied the technique to the laser emittance station in the high-energy beam transport of the Spallation Neutron Source, experimental results showed that perpendicular scans can provide significant new information to the reconstructed 4D phase space distribution.

Figure 1

Schematic of the laser wire station in the SNS HEBT.

Figure 2

Four 2D scan results at the SNS HEBT laser wire station (left column, denoted by superscript “meas”) compared against the corresponding 2D projections of two 4D distributions (middle and right columns) that are tomographically reconstructed based on these scan results. Among the two reconstructed 4D distributions, the superscript “sol 1” denotes the two-scan analytical MENT solution that only uses the two conventional scans with parallel wires, whereas “sol 2” denotes the four-scan numerical MENT solution obtained using all four 2D scans.

Figure 3

Measured 1D profiles in the set of coordinates $x_i$, $x_f$, $y_i$, and $y_f$. The 1D profiles in solid lines are obtained from projections over the four 2D scan results at the SNS HEBT laser wire station. Each 1D profile can be obtained from two 2D scans, and both are plotted to illustrate their differences. The total variation distances between two different measurements of each 1D profile are shown in Table 1. For $x_i$ and $y_i$, the 1D profiles can also be measured separately by a Faraday cup which collected electrons stripped by the laser wire during the scan. These profiles, plotted in dashed and dotted lines, are consistent with projected 1D profiles from 2D scans and successfully verified their accuracy.

Figure 4

Cross-plane 2D phase space projections of the 4D MENT numerical solution obtained from four 2D projections at the SNS HEBT laser wire station. The superscript sol 1 denotes the two-scan (parallel slits only) MENT solution, and sol 2 denotes the four-scan (both parallel and perpendicular slits) MENT solution.

Figure 5

Four cross-plane 2D projections of the transverse phase space distribution for (i) the simulated distribution (left column), (ii) the two-scan analytical solution (middle column), and (iii) the four-scan numerical solution (right column).