# Racetrack Microtron—Pushing the Limits

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## Abstract

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## 1. Introduction

## 2. Comparison of Classical and Racetrack Microtrons

#### 2.1. RTM Advantages

#### 2.2. Common Problems of CM and RTM

#### 2.3. RTM Problems

#### 2.3.1. Beam Defocusing and Distortion by the Magnet Fringe Field

#### 2.3.2. Phase Slip

#### 2.3.3. Errors in Magnets Manufacturing and Magnet Poles Positioning

#### 2.3.4. Beam Blow-Up

## 3. Comparison of RTM and LINAC

## 4. Approaches for Minimizing Beam Current Losses in RTM

#### 4.1. Increasing the Longitudinal Acceptance of the RTM by Linearizing the Accelerating Field

#### 4.2. Matching the Longitudinal Emittance of the Injected Beam with the RTM Acceptance by Optimizing the Initial Part of the Accelerating Structure

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**(

**a**) Classical microtron. (

**b**) Racetrack microtron. (

**c**) Multisection linear accelerator. (1)—electron gun (electron source), (2)—accelerating structure (resonator), (3)—RF energy source (magnetron, klystron) with a high-voltage power supply (modulator), (4)—bending magnet(s), (5)—beam extraction magnet (or system), (6)—injection magnet, (7)—quadrupole lenses.

**Figure 2.**Results of numerical simulations of the beam dynamics of the 55 MeV RTM [18]; (

**a**) phase space in the vertical plane, (

**b**) beam spot at the RTM exit. The pole gap height of the magnet is 2 cm.

**Figure 3.**(

**a**) Electronic efficiency of the RTM and LINAC as functions of the pulse beam current ${I}_{b}$; (

**b**) ratio ${P}_{\Sigma}^{linac}/{P}_{\Sigma}^{RTM}$ of the LINAC and RTM total RF power consumption and the RTM total RF power consumption ${P}_{\Sigma}^{RTM}$ as functions of ${I}_{b}$.

**Figure 4.**(

**a**) Voltage across the resonator gap as a function of the phase $\phi =\omega t$ given by Equation (11) for one (${U}_{2}=0$) and two harmonics; (

**b**) longitudinal acceptance as functions of $\delta \phi $; the deviation of the phase from the synchronous phase ${\phi}_{s}$ for the cases of harmonics one (${\phi}_{s}=$ 16°) and two (${\phi}_{s}=$ 2.8°) in the accelerating field.

**Figure 5.**Theoretical and measured capture efficiency for the first passage of the beam through the accelerating structure as functions of the pre-buncher field phase [53].

**Figure 6.**Longitudinal phase space (green and red dotted line) of the beam after the first acceleration superimposed on the RTM acceptance (

**a**) for the accelerating structure of the 55 MeV RTM [18]; (

**b**) for the structure with optimized initial part.

**Figure 7.**On-axis distribution of the accelerating field for the accelerating structure of the 55 MeV RTM [18] (blue line) and for the structure with the optimized initial part (orange line).

**Figure 8.**Decay of the beam current along the orbits for the two variants of the accelerating structure considered in Section 4.2.

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**MDPI and ACS Style**

Borisov, M.; Ermakov, A.; Khankin, V.; Kubyshin, Y.; Shvedunov, V.
Racetrack Microtron—Pushing the Limits. *Symmetry* **2021**, *13*, 2244.
https://doi.org/10.3390/sym13122244

**AMA Style**

Borisov M, Ermakov A, Khankin V, Kubyshin Y, Shvedunov V.
Racetrack Microtron—Pushing the Limits. *Symmetry*. 2021; 13(12):2244.
https://doi.org/10.3390/sym13122244

**Chicago/Turabian Style**

Borisov, Maxim, Andrey Ermakov, Vadim Khankin, Yuri Kubyshin, and Vasiliy Shvedunov.
2021. "Racetrack Microtron—Pushing the Limits" *Symmetry* 13, no. 12: 2244.
https://doi.org/10.3390/sym13122244