The goals of our project are development of a nanometer scale beam size monitor and measurement of nanometer scale beam sizes at ATF2. This thesis evaluates the performance of the monitor and reports the result of beam size measurement. The instruments to consist the monitor is also described.

In Chapter 1, the motivation and the location of this study in the context of the plan towards the future ILC are overviewed. In the International Linear Collider project, nanometer scale focusing of the electron/positron beam to obtain high luminosity is a major technical challenge. Focusing strategies will be realized in the test facility for ILC, called the Accelerator Test Facility 2 (ATF2). ATF2 is a realistic scaled down model of the final focus system for ILC. To confirm the 37 nm ATF2 design vertical beam size at the focal point (virtual interaction point), a beam size monitor with novel techniques is required.

In Chapter 2, the measurement scheme and overall design of the beam size monitor is described. We developed a beam size monitor based on the scheme which is originally proposed by T. Shintake. In this scheme laser interfere fringe is used as a probe to scan electron beam bunch. The measurement scheme is shown in Fig.1. In their intersecting region, the electromagnetic fields of the two laser beams form a standing wave (interference fringe). Relativistic (1.28 GeV at ATF2) electrons in a bunch of beam scatter laser photons as inverse Compton γ ray (with energy up to 30MeV). The energy sum of inverse Compton photons is measured by a γ ray detector which is located downstream from the interaction point. The probability of the Compton scattering varies according to the phase of the standing wave where the electrons pass through.

Taking y to be the vertical beam position, the energy of inverse Compton photons,which is measured by the gamma detector shown in Eq.1 expressed as the following:

Here, S(ave) is the average energy of inverse Compton photons. 2 times of the energy of ray scattered from a single laser beam. θ is the crossing angle of the split laser beams. ky is the vertical component of the wave number ky = 2π/λ sin θ/2 . α is the phase difference made by the optical delay line. σy is the vertical beam size.

Using the maximum and the minimum energy sum of inverse Compton photons S+ and S_, modulation depth M of gamma signal is function of beam size σy as Eq.2. In Eq.2,

Therefore beam size can be obtained from modulation depth using calculation formula as shown in Eq.3.

The monitor has been designed to measure a vertical beam size down to 20 nm. Also it is possible to measure wide range of beam size with different crossing angle modes. In order to achieve the wide range measurement range of σy(20nm ~ several μm), it is possible to change the crossing angle mode (2-8 degree, 30 degree, 174 degree). It is also possible to use a single laser path as laser wire to measure σx.

In Chapter 3, components of the monitor are described. The monitor consists of laser optics and γ detector as shown in Fig2. The laser system is required to have good stablity of timing and power, good coherence, and high intensity pulse. We use a Class IV Nd:YAG seeded Q-switched pulsed laser "Pro-350" made by Spectra-Physics Lasers Inc. to meet these requirements. The wavelength of laser is 532 nm, reduced from original Nd:YAG laser (1064 nm) in order to obtain good resolution for beam size measurement. The power, timing and profile of laser is always monitored. The γ ray detector is made up of multilayer CsI(Tl) scintillators. Background for the detector is mainly due to Bremsstrahlung photons emitted when the beam halo hit the beam pipe. Using the difference of shower development between inverse Compton signal(average energy ~ 15 MeV) and Bremsstrahlung background (average energy ~ 50 MeV) , the detector is designed to optimize the S=N ratio and have tolerance for the fluctuation of background. The performance of each component is evaluated.

In Chapter 4, the overall performance of the monitor and the measurable range are discussed. There are two types of uncertainties in the experiment, fluctuations on signal energy and the biases on the modulation depth. Candidates of signal fluctuation sources are jitter of laser power, relative timing jitter between laser beam and electron beam, relative position jitter, and background fluctuation. Contribution of each source is evaluated then overall fluctuation of signal energy ΔS=S0 is estimated. The biases on the modulation depth is represented as factor C. Measured modulation M(meas) is biased as M(meas) = C(total)M(ideal). Candidates of bias sources are relative position jitter, relative position drift, and axis mismatch between laser fringe and beam. Contribution of each source is evaluated then overall bias C(total) is estimated for each mode.

In Chapter 5, the results of beam size measurents are reported. Beam tuning issues are also discussed. The measurement with 2-8 degree mode has been performed since 2010. The measurement with 30 degree mode was succeeded to observe during the beamtime at February 2012. The plot of 30 degree mode fringe scan is shown in Fig.3.

After beam size minimization by beam tuning, modulation M = 0:522 ± 0:013 was measured. 10 times measurement histgram of measured modulation with 30 degree mode is shown in Fig.4.

Corrected by the bias on the modulation, measured beam size is estimetad to be about 110 nm. In Chapter 6, The study discussed in this thesis is concluded. Conclusion is based on following major points:

・ Performance and systematic errors were evaluated

・ 2 - 8 degree mode and 30 degree mode were succeeded to detect fringe. A 110nm beam size measurement was performed with 30 degree mode

Figure 1: Different modulation for different beam size (from T.Suehara's doctor thesis)

Figure 2: Schematic diagram of shintake-monitor at ATF2 (from Y.Yamaguchi's master thesis)

Figure 3: plot of fringe scan with 30 degree mode

Figure 4: 10 times measurement histgram of measured beam size

本論文は6章からなる。第1章は、イントロダクションであり、本研究の動機や目的が述べられている。本研究の目的は、次期加速器計画であるInternational Linear Collider (ILC)の衝突点におけるビームサイズモニターの実用化をはかる事にある。ILCは、Large Hadron Collider (LHC)におけるHiggs-like粒子の発見を受け、Higgs粒子の精密測定、標準模型を超える事象の研究のために、次期高エネルギー電子衝突加速器として計画されており、その衝突点においては5.7nmのビームサイズが実現される予定である。本研究においては、このビームサイズを測定するためのモニターの実用化について同じくILCに向けた加速器研究施設であるATF2を用いた研究を行った。本研究の具体的な目的は、ATF2で実現されるビームサイズ100nm程度のビームを実用的な装置を用いて測定する事である。

第2章は、用いたビームサイズモニターの原理に関して記述してある。現状で、100nm程度のビームサイズの測定が可能なビームサイズモニターは、レーザー干渉を用いた通称新竹モニターと呼ばれるモニターが知られているのみである。本章では、レーザー干渉と逆コンプトン散乱を用いてビームサイズを測定する原理を説明している。

続く第3章では、具体的にATF2において構築されたシステムの詳細に関して述べている。特に、先行実験であるSLACの実験に比べて、本研究では、実用化に向けて、より小さなビームサイズ測定のためにレーザー波長の変更(1064nm ‑> 532nm)したこと、光路差の制御による干渉縞の位相スキャンを可能にしたこと、逆コンプトン散乱γ線を捉える検出器の信号ノイズ比を向上させたこと、実用化のために数種類の交差角度(2‑8, 30, 174)を可能とする配置に変更したこと、などの改善を施した。交差角度を変更する事で、カバーするサイズ測定の範囲を変えることが可能となる。

第4章は、実験によって得られた具体的なビームサイズ測定結果を述べている。これまで測定が成功していなかったレーザー交差角度30度での測定を成功させた。これにより、これまでにない範囲でサイズ測定を行う事が可能となった。各種の補正を考慮した慎重な解析を行い、ビームサイズに対して125.2nm+-5.3nm(stat.)+26.6-64nm(sys)の測定結果を得た。

第5章は、測定によって得られた結果を基に施した各種改善点を論じ、その改善の結果、現状のシステムがビームサイズとしてどこまで測定可能なのかの結果を示している。その結果、現状のATF2の目標ビームサイズである37nmのサイズの測定が可能である事を示唆している。

第6章は、論文全体のまとめであり、本研究の動機・目的・実験手法・結果などについてのまとめが述べられている。

なお、本論文第2章・第3章は、駒宮幸男・神谷好郎・浦川順治・照沼信浩・田内利明・奥木敏行・久保浄・黒田茂らとの共同研究であるが、論文提出者が主体となって実験・解析を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。