Submitted by TMU, KEK, SUNY Stony Brook, and NSLS/ATF

Proposal Submission Date: March 30, 1998
 
 

Principal Investigators:

Tachishige Hirose

Physics Department, Tokyo Metropolitan University, Japan

Ilan Ben-Zvi

National Synchrotron Light Source, BNL, USA and

Department of Physics and Astronomy, SUNY Stony Brook, USA

Akira Tsunemi

KEK, Japan

NSLS/ATF, USA

The high intensity polarized positron source proposed for the Japan Linear Collider (JLC) is based on the production of electron-positron pairs when the polarized gamma-quanta are stopped at the foil target. Compton scattering between the relativistic electrons and polarized laser beams is the source of the polarized gamma-quanta. The requirements for the high peak flux and short pulse duration of the polarized gamma rays specify the high-brightness photocathode electron accelerator and the picosecond subterawatt CO₂ laser as essential components of the projected Compton source. The BNL-ATF is the only users facility worldwide that features such a combination of equipment. That makes the Compton scattering experiment at BNL a meaningful and efficient instrument to prototype the JLC polarized positron source and to facilitate its further development.

The proposed experiment will be carried out as a collaboration between KEK and Tokyo Metropolitan University (TMU) in Japan and SUNY Stony Brook in association with the BNL-ATF as the US counterpart. Partial funding for the experiment is available from A grant for Japan/US collaborative research in high energy physics. In addition, two parallel proposals will be submitted to JSPS (Japan Society for the Promotion in Science) and NSF.

The project will be executed in three stages: Stage I is based on the present performance of the electron beam and laser at the BNL-ATF: 10-GW pulses with a 100-ps duration from the CO₂ laser will be brought to a head-on collision with the 0.5 nC, 10-ps, 50-MeV electron bunches. This will result in production of the 2.6 Å x-rays of a 10-ps pulse duration with a peak flux of 2´ 10¹⁹ photons/sec. The fundamental properties of the interaction, together with the electron/laser alignment and the x-ray extraction procedures, will be studied at this stage.

Stage II will capitalize on the improved BNL-ATF capabilities after the CO₂ laser upgrade to the ~1 TW peak power. At this stage, the Compton scattering experiment may produce x-ray pulses with a peak flux of up to 10²¹ photons/sec. This will be achieved partially due to more tight and precise focusing of the electron and laser beams. Thus, the experimental setup and procedure for the prototype polarized source will be further verified.

At the Stage III, collective efforts will be done to enhance the picosecond CO₂ laser technology in the pulse repetition rate. At this stage, a repetitive Compton interaction at the several nanosecond interval between the laser pulses will be attempted.

Results of the BNL-ATF Compton scattering experiments will be used in modeling the polarized gamma source at the KEK-ATF, based on the 1.5 GeV electron accelerator available there.
 
 

State of the art in the field

The polarization of electrons and positrons in the future linear colliders will play an important role for experimental verification of the standard model and for a search of new phenomena beyond the standard model. The technology of the polarized electron beams progressed rapidly during the last decade. However, the polarized positron source development is still in its early stage. A prospective high-intensity polarized positron source [1] is based on production of the electron-positron pairs when Compton scattered polarized gamma-quanta are stopped at the foil target. The observation of the first polarized positrons produced by this method [2] proves the viability of the approach and a need for further development in this direction. The analysis shows that a short-pulse CO₂ laser beam with the peak power up to 1 TW will be needed in order to obtain ~10¹⁰ positrons/bunch as is required by JLC.

To meet these requirements, as well as to serve other demanding high energy and basic science applications, the BNL-ATF vigorously pursues the short-pulse high-power CO₂ laser development. The concept of such a laser, based on the semiconductor optical switch followed by the high-pressure laser amplifier chain, has been demonstrated previously at the smaller scale [3-5]. The BNL-ATF laser is expected to become the first picosecond CO₂ laser operating at and above the 1 TW peak power level [6].

Objectives of proposed work

The proposed collaborative project will include: a. the design of Compton interaction cell and its installation in the BNL-ATF beam line; b. the adaptation of the BNL-ATF picosecond CO₂ laser for the Compton experiment; c. performance of the prototype Compton scattering experiments at the BNL-ATF, d. the upgrade of the BNL-ATF CO₂ laser to the high-repetition rate with its subsequent use in the "pulse burst" Compton experiment.

The project will be executed in three stages:

Stage I is based on the present performance of the electron beam and laser at the BNL-ATF: 10-GW pulses with a 100-ps duration from the CO₂ laser will be brought to a head-on collision with the 0.5 nC, 10-ps, 50-MeV electron bunches. This will produce 2.6 Å x-rays of a 10-ps pulse duration with a peak flux of 2´ 10¹⁹ photons/sec. The electron-laser alignment and the x-ray extraction procedures, together with the fundamental properties of the interaction, will be studied at this stage.

Stage II is based on the improved parameters following the CO₂ laser upgrade to the ~1 TW peak power. At this stage, the BNL-ATF based Compton scattering experiment shall produce x-ray pulses with a peak flux of up to 10²¹ photons/sec. This will be achieved partially due to more tight focusing and precise coalignment of the electron and laser beams. Thus, the experimental setup and procedure for the prototype polarized source will be further verified.

At Stage III, efforts will be done to enhance the picosecond CO₂ laser technology in the pulse repetition rate. At this stage, a repetitive Compton interaction at a nanosecond interval between the laser pulses will be attempted.

The Experiment Location and Setup

Focusing and steering electromagnets for the e-beam.

An interaction vacuum-chamber with the optics assembly for focusing and alignment of the laser beam and positioning diagnostics for the laser and electron beams.

X-ray diagnostics.

Placement of the Compton interaction cell before the bending dipole magnet of the STELLA electron spectrometer permits x-ray separation from the spent e-beam and the x-ray extraction to diagnostics through the Be foil window.

The Compton interaction vacuum cell, shown in Fig.1, will accommodate the following primary components:

a) Optics that telescope and redirect the sidewise incident CO2 laser beam, focus it on the electron-beam axis and extract the spent laser beam for diagnostics.

b) Two pop-in e-beam position and size monitors that could be remotely inserted in the middle of the interaction region and upstream or downstream of it to control the electron-beam size, position, and direction.

2 mm diameter holes are drilled through the parabolic mirrors in order to transmit the e-beam and the produced x-rays. In order to avoid the loss of laser energy and the plasma ignition at the edges of the hole, a "donut"-shaped laser beam (having zero intensity at the center) will be used. Such laser beam profile may be prepared using an axicon telescope prior to delivery of the laser beam into the cell.

http://nslsweb.nsls.bnl.gov/AccTest/experiments/Compton/Image292.gif

Downstream of the interaction chamber, a dipole magnet separates the electrons from the co-propagating x-rays. The experiment will be equipped with the x-ray diagnostics (not shown in Fig.1). Its complexity and capabilities will depend upon the available funding. In the case of a tight budget, we may be limited with the x-ray dose detector which still permits us to satisfy the principal goal of the experiment - to demonstrate the high efficiency x-ray production and to optimize the laser/electron interaction geometry.

If possible, more advanced characterization of the x-ray beams may be implemented that permits measurements of temporal, spectral, spatial, and angular parameters. For example, the spectral selectivity will be introduced by the crystal (graphite) monochromator. The most reliable way to measure the x-ray pulse duration is by the x-ray streak-camera, with the subpicosecond time resolution, which is commercially available.

3. Physical Foundation

A relativistic electron beam interacting with a counter-propagating laser beam emits radiation at a central wavelength given by

where g is the relativistic Lorentz factor and l is the laser wavelength.

The back-scattered photons are generated within a narrow cone with a solid angle of W =2p q ₀², where

For the back-scattering geometry, the x-ray pulse length is defined primarily by the electron bunch duration:

and for a practically meaningful case when  we can consider . It follows that relatively long laser pulses may be tolerated without a noticeable increase in the x-ray pulse duration above .

The x-ray production is given by

where  is the laser pulse energy and Q is the electron bunch charge.

Eq.(4) for the x-ray flux is derived under the assumption that the laser beam is focused to  and its waist length,

extends over the overlap distance defined by the electron and laser pulse duration,

In more convenient units the number of x-ray photons is

We see that the number of generated x-ray photons is proportional to l . This stems from the fact that the number of photons delivered within the laser pulse is inversely proportional to the photon energy. This justifies the use of the long-wavelength CO₂ laser for intense x-ray and gamma generation. An additional reason is due to the potentials of the CO₂ laser technology for high repetition rates and high average power [9, 10].

Anticipated Results

4.1 Stage I

proof-of-principle Compton back-scattering experiment will be performed at the BNL-ATF with its presently available 10 GW, 100-ps CO₂ laser and 10-ps, 0.5 nC, 50 MeV (g =100) electron linac. By Eq.(1), l _X=2.6 Å (a photon energy of hn _X[eV]=1.25´ 10⁴/l _X[Å]=4.7 keV). According to Eq.(7), the length of the interaction region, where the focused laser beam shall match the size of the counter-propagating electron beam, is L_int=17 mm. Using Eq.(6), we calculate the waist radius of the diffraction limited CO₂ laser beam to be 140 m m. Hence, the electron-beam shall be focused to the same or smaller spot size, r_b£ r_L=140 m m, in order to maximize the x-ray signal.

At the laser and electron-beam parameters specified above, the x-ray flux calculated by Eq.(5) is 2´ 10¹⁹ photons/sec and the total number of the x-ray photons per pulse is equal to 2´ 10⁸.

Stage II

With the 10-ps laser pulse, the x-ray flux will be enhanced due to shortening the interaction region that permits tighter focusing of the laser and electron beams. With L_int=1.5 mm, the laser beam waist may be brought to r_L=40 m m. By matching the electron beam focusing to the same size, the 9.36 keV x-ray peak flux will be increased to 10²² photons/sec. The total x-ray photon number in this case is 10¹⁰ photons/pulse. This number even exceeds the requirements for the JLC polarized positron source. Indeed, at the CO₂ laser power density ~2´10¹⁶ W/cm² the nonlinear Compton scattering may become noticeable. In order to produce the required high number of gamma-quanta while still staying within the linear Compton scattering regime, interaction of several laser pulses with the individual electron bunch will be a proper alternative for the polarized positron source.

4.3 Stage III

Generation of the picosecond CO₂ laser pulses with the nanosecond periodicity is a challenging task that never has been attempted before. Meanwhile, such a regime does not look technically impossible and even promises to have superior efficiency because it permits extraction of a bigger portion of the energy stored in the laser amplifier. An attempt to slice and amplify several picosecond laser pulses within a single discharge shot of the amplifier will be exercised during the Stage III of the collaborative experiment.

It would be naive to expect that the critical requirements of the future collider may be satisfied at the first attempt. At the beginning, the possibility of slicing and amplifying of just a few pulses per single CO₂ laser shot will be studied experimentally. Fortunately, the BNL-ATF has experience in producing extremely uniform trains of up to 200 pulses from the Nd:YAG laser. Such train (with the 12.5 ns periodicity) may be used to slice several CO₂ laser pulses of a picosecond duration which will be sent for amplification. The amplitudes of the amplified pulses may be controlled by properly tailoring the amplitudes of the seed pulses. Similar to the Nd:YAG trains, it will be done using electro-optical switches controlled by the properly programmed arbitrary pulse form generators. The principle experimental setup is shown in Fig.2.

The produced pulse trains will be used in the pulse-periodical Compton scattering demonstration experiment. Conceptual design of the high-repetition JLC positron source will also be attempted at this stage.

4.4 Expected significance and future progress

The importance of the results obtained in the course of this project goes also beyond the goals formulated for the prospective polarized positron source. The developed novel laser technology will open new research opportunities in science and technology [9, 10]. Establishing conditions for the efficient Compton interaction between the laser and electron beams, the project will facilitate development of ultra-bright, monochromatic, tunable, ultra-fast x-ray and gamma-ray sources for a number of applications.

In order to clarify the statement about the long-wavelength CO₂ laser advantages for the high-brightness x-ray sources, let us reveal the dependence of the x-ray source brightness,

upon the laser wavelength, using Eqs. (1), (2) and (4):

We arrive to the conclusion that using a CO₂ laser opens the prospect for up to 100 times increase in the brightness of x-rays produced at a particular wavelength, l _X, compared with using a 1-m m laser of the same pulse energy.

The advanced study of the laser synchrotron sources tunable in the soft and hard x-ray regions is the subject of a separate proposal submitted to NSF by SUNY at Stony Brook [11]. If awarded (the NSF award decision will be announced before September 1998), the NSF grant will permit us to significantly enhance the instrumentation and manpower support of the Compton experiment. That will include incorporation of more advanced and sophisticated diagnostics for the laser/electron interaction and the x-ray spectral and angular measurements with the subpicosecond resolution.

Let us finally address the prospects of the nonlinear Compton scattering studies using a terawatt-class CO2 laser. As has been mentioned previously, at the Stage II of the BNL-ATF experiment we may approach the condition for the nonlinear Compton scattering, a>1, where a is the normalized laser strength defined by the equation

If we succeed in achieving the 3-TW CO2 laser power and focus the laser beam to the 15 m m radius spot, as high as 1017 W/cm2 intensity will be attained. Such intensity corresponds to a=3. At this highly nonlinear regime the maximum order of generated harmonics is equal to n=a3, and the maximum of the intensity distribution shifts to n=11-13 with these components ~3 times more intense than the fundamental n=1 component.

Study of nonlinear Compton scattering may be carried out as a future extension of the proposed experiment. Such study has been the subject of the Princeton-BNL proposal [12]. We obtained a commitment from the PI on this experiment (Kirk McDonald) that the interest to such study is still vital, and the experiment may be initiated as soon as the required laser and e-beam parameters are demonstrated at the BNL-ATF.

Sources of Funding and Cost Share

SUNY is now submitting a parallel proposal to the NSF Division of International Programs. This proposal will match the Japanese counterpart proposal and is intended to acquire the US portion of support of the collaborative research. These two grants will suffice to carry out the research program described in the preceding sections. This will include: development of the interaction cell and optics, basic x-ray diagnostics, exchange of visits, etc.

Related to the research activity covered by this proposal is the 3-year program submitted to NSF by SUNY at Stony Brook on February 1, 1998 [11]. This pending approval proposal (deadline for grant award is September 1998) includes development of the tunable monochromatic laser synchrotron source at the BNL-ATF that may be used for a variety of applications. This program includes the development of the comprehensive x-ray diagnostics with the capabilities of spectral and angular x-ray monitoring at the subpicosecond resolution. These diagnostics will become available for other users as well. Among other benefits from the NSF grant that may be shared by this proposed Japan/US collaborative program is computer support of the experiment that includes the process simulation and automation (virtual instruments, frame grabbing and image processing, etc.). The NSF grant will permit us to involve additional graduate students and collaborators to support the expanded activity in x-ray diagnostics, computer support, laser upgrade, etc.

6. Schedule

Develop the experimental chamber including off-axis parabolic mirrors with remote alignment.

Prepare the x-ray detector.

Modify the laser transport system for the Compton experiment.

Perform Stage I of Compton scattering experiment.

Evaluate the results of the Stage I experiment.

Required beam time: 7 days

Modify the optics and the experiment design to use the TW-CO₂ laser.

Perform Stage II of Compton scattering experiment.

Evaluate the results of the Stage II experiment.

Required beam time: 12 days

Develop and tests the picosecond CO₂ laser in the multi-pulse configuration.

Perform Stage III of Compton scattering experiment.

Evaluate the results of the Stage III experiment.

Design the positron source modeling experiment at the KEK-ATF

Required beam time: 10 days

6. Names of Participants and Affiliations

Yoshisuke Hamatsu, assistant professor, TMU

Chikara Fukunaga, assistant professor, TMU

Mitsumasa Irako, assistant, TMU

Masami Chiba, assistant, TMU

Tetsurou Kummita, assistant, TMU

Toshiyuki Okugi, doctor course student, TMU

Katsuhiro Dobashi, doctor course student, TMU

Tarou Masumaru, doctor course student, TMU

Toshiya Mutou, master course student, TMU

Hideki Kojima, master course student, TMU

Kaoru Yokoya, professor, KEK

Junji Urakawa, assistant professor, KEK

Tsunehiko Omori, assistant, KEK

Yoshimasa Kurihara, assistant, KEK

Takashi Naitou, engineer, KEK

Sakae Araki, engineer, KEK

Masakazu Washio, professor, Waseda University

Akira Tsunemi, research collaborator, KEK (Spokesman)

Ilan Ben-Zvi, senior physicist, BNL and adjunct professor, SUNY Stony Brook, (PI)

Igor Pogorelsky, physicist, BNL (Spokesman)

X.J. Wang, physicist, BNL

Vitali Yakimenko, assistant scientist, BNL

Marcus Babzien, engineer, BNL

Robert Malone, engineer, BNL

John Skaritka, engineer, BNL

Karl Kusche, engineer, BNL

Peter Siddons, scientist, BNL

Jerry Hastings, scientist, BNL

Yabo Liu, postdoc, UCLA

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