First Experimental Demonstration of Beam Storage by Three-Dimensional Spiral Injection Scheme for Ultra-Compact Storage Rings

Abstract

Three-dimensional spiral injection scheme enables storage in ultra-compact rings with nanosecond revolution period. We report the first successful storage of a $297\,\mathrm{keV/}c$ electron beam in a $22\,\mathrm{cm}$ weak-focusing storage ring with a $4.7\,\mathrm{ns}$ revolution period using multi-turn vertical kick with a $140\,\mathrm{ns}$ kicker pulse. Using a scintillating-fiber detector, we observe a signal exceeding $5σ$ of the pre-injection rms noise for $\geq 1\,\mathrm{μs}$, confirming beam storage. By varying the weak-focusing field configuration and measuring the stored beam distribution, we show that the storage beam resides within the predicted region by Monte Carlo simulations. This result is a key proof-of-principle for realizing ultra-compact storage rings for next-generation precision measurements including the muon experiments at J-PARC and PSI.

Figures (4)

• Figure 1: (a): Overview of the demonstration beamline. The electron beam is generated by an electron gun and injected into the storage magnet through a transport line consisting of three rotatable quadrupole magnets, one bending magnet, and a pair of steering magnets. (b) X-Z projection of the reference particle orbit after injection. The storage midplane corresponds to $Z = 0\,cm$. (c),(d) $B_r$ components along the reference particle orbit. Blue line shows the static magnetic field by the storage magnet. Red line shows the time-varying magnetic field by the kicker. The $B_r < 0$ field during injection provides the downward steering to electrons required for beam storage in weak-focusing region.
• Figure 2: Principle of SciFi-probe measurement. Solid lines show the stored particle trajectories with vertical betatron oscillation. Stored particles with vertical betatron amplitude greater than $Z\,[cm]$, which is the tip position of the SciFi-probe, generate signals. Case-1, SciFi-probe is inserted up to $Z = 3.5\,cm$, measures particles with vertical amplitude $\geqq 3.5\,cm$ shown by red lines. Similarly, Case-2, SciFi-probe is inserted up to $Z = 2.0\,cm$, measures those with amplitude $\geqq 2.0\,cm$ shown by red and blue lines.
• Figure 3: Example of measured signals. (a): SciFi-probe signal measured within the weak-focusing region. Blue line shows the threshold of $5\sigma_{\rm noise}$. (b): Loss-monitor signal downstream of the storage region. (c): Kicker current waveform measured using a Rogowski coil. Red and black lines correspond to the with and without kicker conditions, respectively. In this example, the SciFi-probe is inserted up to the storage midplane and SciFi covered the range from $Z=0\,cm$ to $Z=20\,cm$. Each waveform is an average over $1000$ shots.
• Figure 4: Weak-focusing field dependence of the stored beam distribution. (a): Weak-focusing field configurations. Colored points denote measurement result, and colored lines denote finite element calculations performed with Opera Opera. (b): $Z$-scan results under the four field configurations. The vertical axis shows the time-integrated SciFi-probe signal, $I_{\rm Signal}=\int V_{\rm Signal}(t)\,dt$, where $V_{\rm Signal}(t)$ is the measured voltage as a function of time. The integral is evaluated over the time window from $t=560\,ns$ to $9000\,ns$, starting just after the end of the kicker pulse. (c): Comparison of measured stored beam ranges with Monte Carlo predictions. Blue shaded regions show the expected region of MC simulation. Black points show the measurement result from field measurement and Z-scan measurement.