2002 ATF Newsletters

Jan | Feb - March | April - June | July - Sept | Oct - Dec

Greetings,

This issue of the ATF Update is long and has a number of interesting results. For example, Experiment AE22 is reporting an important "first": The interaction of electrons with laser guided by a plasma channel.  AE20, is making good progress in its new "STELLA II" configuration, already exceeding the previous IFEL acceleration record.  AE23, obtained a streak-camera image from the fast detector.

The Accelerator Test Facility (ATF) has moved organizationally within Brookhaven from the NSLS Department to the Physics Department.  Because of this organizational change, the administration of the ATF experimenters has also moved.

Effective immediately, arriving ATF users should check-in at the RHIC & AGS Users’ Center, which is located in Building 355A on Brookhaven Avenue (right next to the Housing Office).  All guest registration, training, and film badges will be handled at the Users’ Center and NOT at the Light Source.Information about the ATF experiments and ATF training links now resides on the Users’ Center homepage which is located at:  http://www.bnl.gov/userscenter/.  We recommend that you read the User Update regarding security at BNL to acquaint yourself with the rules and regulations regarding access onto government property.  Users will receive e-mail periodically from the Users’ Center, which will include items of interest to BNL guests and instructions on how to renew appointments and training at BNL. If you have any questions or need assistance, please contact the Users’ Center at 631-344-5975 or via e-mail at userscenter@bnl.gov and the staff will gladly assist you. 

Xijie Wang, who has been with the ATF since its very first days and played a significant role in its development and scientific achievements, is moving from the ATF and the Physics Department to the NSLS Accelerator Division. We wish Xijie success in his new-old department.

Environmental protection, Safety, Health and Quality Assurance (ESH&Q) will fare even better than before with the appointment of Karl Kusche to the ATF's ESH&Q officer. Karl has proved himself as a valuable contributor to these issues even before this appointment and I have complete confidence that his talent and dedication will serve us well.

A vacuum leak appeared on the first week of May in what must be the least accessible point in the ATF: The high-energy beam-stop in the F-line. This device is heavily shielded and located deep in the beam tunnel leading into the Experiment Hall. An emergency engineering meeting on  May 5, headed by Igor Pogorelsky, set the wheels in motion. John Skaritka planned the access route and the repair. Outstanding efforts shown by Mel Bonanno and the staff masons allowed fast access to the beam-stop (removing about 150 lead bricks). John , helped by dedicated ATF and NSLS mechanical group members, in particular Don Davis and Don Shea repaired the device and then it was all put back, in time to resume the experimental program by Thursday May 16. Work planning and protective equipment were used throughout the operation. There is also a lesson learned: As reported by Andy Ackerman, air samples collected by NSLS Radiation Control Technicians  have shown that protection was prudent when working in small, poorly ventilated area such as the ATF beam tunnel.

Ilan Ben-Zvi  

Back to Top

We observed the interaction between an electron bunch and a laser pulse in a plasma channel. A detailed progress report of this experiment can be viewed  under this link (pdf file, 330 kB). Below is a short abstract and main observations.

Abstract.       A high-energy CO₂ laser is channeled in a capillary discharge. Plasma dynamic simulations confirm occurrence of laser guiding conditions at the relatively low axial plasma density 1¸4´10¹⁷ cm^-3. A relativistic electron beam transmitted through the capillary changes its properties depending upon the plasma density. We observe focusing, defocusing or steering of the e-beam. Counter-propagation of the electron and laser beams inside the plasma channel results in generation of intense picosecond x-ray pulses.

Our first observations based on the first run conducted in March 2002 can be summarized as follows:

·     We observe channeling of the laser beam.

·       We observe reasonably 9.0pt;color:black">efficient ~80% charge transmission through the capillary.

·        color:black">The x-ray background is sufficiently low to allow clean measurement of the Thomson effect.

·        We observe a spectrum of beam manipulation effects upon the e-beam transmission through the plasma channel including: focusing, defocusing and steering.

·        The x-ray yield is still at the level of our previous run with no observed enhancement due to a channel over the free-space interaction. 

The last observation can be attributed to the bent e-beam trajectory due to a stray dipole field and degraded laser channeling during the run. The experiment will be repeated after these problems are fixed

Back to Top   

The objective of this run was to observe an EO (Electro-Optic) signal on the streak camera. The EO setup now has a motor that spins slowly enough for manual control of the optical phase - dc light level. Also the dc light signal (used to drive a high impedance input only) is now capable of driving a 50 Ohms input. Therefore, both the EO and the dc light signal can be recorded simultaneously on the 3 GHz scope. 

To guarantee the observation of the EO signal, a setup comprising 2 laser diodes was constructed. It generated a simulated ac signal riding on dc for testing the streak camera. 

Lab EO test results (done on 5-16-02) before ATF beam run are shown on the top left plot of FIGURE1. We can achieve a very low dc light signal of 1.5 mV. The extinction of the Mach-Zender (MZ) is 900 mV/1.5 mV= 600. Applying a pulsed field of 1.5 kV/cm on the EO crystal gives an (adjusted) EO signal of 90 mV. The EO/dc ratio is thus 60. 

On the bottom left plot of FIGURE1, a simulated EO signal was recorded first on the 3-GHz and then on the streak camera. This simulated signal was generated from 2 independently adjustable red laser diodes - one for the ac and the other for the dc signals. This simulated EO signal was triggered by the ATF stripline signal. So the conditions as well as the timing jitter are real ATF parameters. The results indicated that if the 3-GHz scope received a 1 mV EO signal (un-adjusted) riding on 25 mV dc light level (actual dc/EO ratio is 3), using the gate of the streak camera, the EO signal can be observed after 8 signal accumulation. 

Our beam run started on May 23 Thursday at about noon. Very much like previous runs, a clear EO signal was obtained immediately after the electron beam was turned on. The electron beam charge was 0.6 nC. On May 23, the beam size had a fairly large diameter ~4 mm. But on May 24, Feng reduced the beam size to <1 mm. It grazes ~1 mm above the EO crystal. The laser source is a 1 mW HeNe laser. 

EO signals, both positive and negative, at magnitudes of 1-2 mV (into 50 ohm) were observed, see bottom right plot of FIGURE1. No background subtraction was performed on the raw data. The EO signal arrived at 194 ns after the stripline signal. The dc light level was controlled manually by rotating a glass plate. 

Before the electron beam was turned on, the dc light level was ~12 mV (into a high impedance). With beam on and grazing above the EO crystal, the dc level gradually increased to 30 mV, 55 mV, and finally at the end of the experiment it reached 70 mV (into high impedance). At the same time the EO signal gradually decreased and finally diminished on late afternoon of May 24. On May 23, when the dc level was relative high at 55 mV but the HeNe was not quite in the low-noise mode, the EO signal was sent to the streak camera. A slightly broad signal peak appeared to be an EO signal was identified and it shifted 2 ns when the trigger signal was advanced by 2 ns, see bottom left plot of FIGURE1. 

Comparing the simulated EO beam results to the ATF EO beam results, it indicates that when the HeNe is in the low-noise mode and if our ~1 mV EO signal (into 50 Ohm) rides on <25 mV (into high impedance) dc light level, it is guaranteed that the EO signal can be observed on the streak camera. 

The progressively increasing dc light level is correlated with the exposure time of the EO crystal to the passage of the electron beam. It appears that although the EO crystal was not intercepting the electron beam, some charging of the crystal leads to a non-uniform spatial intensity distribution and/or non-uniform spatial phase variation across the EO arm of the optical beam. Therefore, even though the glass plate can adjust the optical phase of the entire reference arm, it cannot compensate for the spatially non-uniform phase. Hence, the dc light level cannot be lowered substantially as the electron beam time increase. See FIGURE2.

Back to Top

All the major hardware components for the STELLA-II experiment have been delivered to the ATF.  This includes two different bunchers [electromagnet (EM) and fixed-gap permanent magnet (PM)], a hybrid EM/PM chicane, and two different PM undulators for the accelerator (untapered and tapered).  The PM buncher is designed to operate at relatively low laser intensities; whereas, the EM buncher is intended for high laser powers.  The untapered undulator is primarily used to confirm timing and to check out the system, since the operation of the tapered undulator is more sensitive to parameters such as the laser power.

Initial tests of the various components indicates the laser intensity within the system is not as high as needed to trap the microbunch electrons and separate them from the unaccelerated background electrons.  However, clean separation and monoenergetic acceleration are possible once the CO2 laser is upgraded later this year to generate <10 ps laser pulses, thereby increasing the peak power by approximately 20-100 times.  Until then the STELLA experiment is being tested at the lower laser peak power.

We have recently observed >13% energy gain using an 8% gap-tapered undulator.  This is one of the highest energy gains observed from an inverse free electron laser (IFEL).  The electron energy spectrum changes as the chicane is varied indicating that the chicane appears to function as expected.  Additional data is needed to compare the results with the model.

The PM buncher appears to be slightly undermodulating the electrons.  The amount of undermodulation is outside the design range for the chicane.  This undermodulation would cause less tight microbunches to form.  This situation should disappear once the upgraded CO2 is available, but in the meantime we are investigating possible reasons for this undermodulation to see if there is a relatively easy way to correct this problem.

Back to Top

We have carried out following beam studies from March to May for total operating time 6 days. The goal of the beam studies is to optimize the performance of the photoinjector, and investigate the thermal emittance of the Mg cathode. Here is the brief summary of the study results:

1. After carefully using the vacuum based laser cathode cleaning, we have QE 0.3%, and the variation of QE on the cathode less than 5%(peak to peak). Operating at the RF gun phase 20 deg from zero crossing, a normalized rms emittance of 0.6 was measured for a 0.5 nC charge.
2. Emittance as a function of the RF gun phase, charge and laser spot size was measured. From those experimental data, we concluded that, the Mg cathode thermal emittance is less than 0.4 mm-mrad for a laser radius of 1 mm.  Further analysis is underway to separate RF effect from thermal emittance. This could lead to further reduction in thermal emittance.
3. Slice emittance measurements were performed to investigate the thermal emittance, double beams were observed. The cause of the double beam is under studied.

Back to Top

LACARA (laser cyclotron auto-resonance accelerator) is an “advanced concept” vacuum laser accelerator of electrons that is in the design and construction phase at the Accelerator Test Facility at Brookhaven National Laboratory. A useful feature of this device is the utilization of high laser power in the form of a Gaussian beam, which is readily produced by the laser system. The accelerator will utilize 800 GW 10.6- mm CO₂ Laser power. The laser spot in the focal plain is chosen to be 1.30 mm, which corresponds to the Rayleigh length of 50 cm. The bunches of electrons, with 200 – 600 pC per bunch, will be accelerated from 50 MeV to 90 MeV in a solenoidal field which has overall length about 1.8m. The energy spread is expected to be as small as 3 %.

The LACARA will operate at the ATF experimental floor, second beam line. The detailed layout of the experiment is shown in Figure 1.   

Electron bunches (the transverse emittance e = 1.5 ×10^-8 m-rad) from the ATF RF linac enter the LACARA experiment via a bending dipole magnet at the right.

Between this dipole and the LACARA solenoid three quadrupole magnets (quads) are placed to focus the electron beam to the required transverse sizes. For the present experiment the required beta function at the matching point between the beam line and the solenoid entrance has to be b_o = b_х = b_y » 0.68 m (- 40 % ¸ +90 %), while ¶b/ ¶s = 0. The configuration shown will provide the changes in b_o from 0.14 to 1.8 m, while keeping the dispersion function D_x = 0. 

The second quads assembly, located after the LACARA solenoid, has the purpose of tightly focusing the accelerated electron beam prior to its passing through the second bending dipole. Both assemblies are mounted on double continuously-supported rail tables with adjustable carriages. The design will provide quad alignment along a given line with precision of around 25 mm. Residual alignment errors are eliminated by means of trimmers.

The solenoid provides a uniform guiding magnetic field up to 6 T at 77 amps over the length of 1.6 m. The unit’s flange-to-flange length is 2343.4 mm. Its windings have operational temperature of 4.2 K⁰, and are chilled by Sumitomo SRDK-408 cryocooler head through a thermally conductive mount. The total time for cooling down from the room temperature is about two days. This magnet was constructed at the Everson Electric Co., Bethlehem. PA, and now is being tested there.

A special table, positioned precisely with four degrees of freedom, is used to support the magnet.  It has been built recently at Yale Physics Department Shop. The accuracies of positioning under appropriate load are: yaw – 2.66 mrad/ motor step; pitch – 1.44 mrad/ motor step; lateral shift – 1.56 mm/ motor step; vertical shift – 10 mm, non motorized.

The CO₂ laser rays, which are also reflected by mirrors, are separated from the visible light by these beam splitters and propagate towards pyroviewers. Thus, both the electron and laser beams are monitored simultaneously at each given location. After each retraction the new position of the beam monitor screen will be referred to a reference axis.

To establish the reference axis a HeNe laser located outside of the vacuum is employed. The unit introducing this light into the system is much similar in design to the beam monitor, however, with the high reproducibility ~7 mm. For the light initial space and angular adjustment, the HeNe laser is mounted on the top of an accurately-positioned, five degrees of freedom stage. The HeNe laser will be used as the reference axis not only when the experiment is operated, but during equipment installation too.

            The laser transport system uses a single lens with R = 12m to focus the CO₂ laser light in the middle of the solenoid. The particular waist (31.1 mm) is required at the lens location, and is provided by the means of ATF telescopic system (not shown in Fig. 1). All mirrors are plain and serve only to direct the light. High precision mounts are used to achieve adequate positioning. There is a vacuum-sealed ZnSe coated window that introduces light into the vacuum, and is subjected to more radiation load than other elements. The upper energy threshold is about 6.72 J                            

            A laser dump is used at the end of experimental line. The design uses a spherical mirror reflecting light by 90⁰ toward a NaCl window, which separates the vacuum and air-pressurized volumes of the dump. The wave front after reflection is a generalized spherical surface having major radius twice larger than that of the minor. The design has very compact dimensions, yet it has the upper energy threshold around 6.83 J, and dumps more than 95 % of the incident power outside the vacuum chamber.

At the present time the scheme for the experiment is known in details and the components have been designed or simulated. By now some of them have been manufactured/ bought already; others are being manufactured or bought. We hope to be done with putting them to place by September 2002, and start the experiment itself then.  

Back to Top

In order to achieve terawatt peak powers with the ATF CO2 laser, it is necessary to reduce the pulse duration to a few picoseconds before seeding the amplifiers.  The current semiconductor slicing system is limited to approximately 10-15 ps minimum pulse duration.  A new two-stage scheme is under test to get the required duration, the first phase of which was to demonstrate that the control pulse from the YAG laser could be compressed to 3-4 ps (FWHM) by frequency-doubling the IR in a long KD*P crystal.  The next phase, currently underway, is to implement a Kerr switch based on Carbon Disulfide.  This Kerr cell will use the the residual 14 ps, 1 micron YAG pulses exiting the KD*P crystal to gate the 100 ns CO2 oscillator pulse.  The Kerr cell has a 2 ps response time, so the gated 10 micron pulse should be close to the 1 micron control pulse in duration.  This will have the benefit of reducing the 10 micron pulse energy by ~ 10^4, allowing tighter focusing on the second stage without optical damage.  We require the tight focus because the 0.5 micron compressed YAG pulse is much lower in energy than that currently in use, and so must be focused down in order to achieve the threshold intensity for plasma formation in the semiconductor.

The status of the testing currently is that the Kerr cell is assembled, filled, and aligned on the optical table in the CO2 room.  First measurements show that the in order to detect the gated pulse, we need to improve extinction in the absence of a control pulse.  The minimum extinction required (~10^5) is estimated at 10x greater than the ratio of the incident to gated pulse durations.  Currently, we have arranged a new optical scheme that may achieve this, and will try to purchase high-extinction polarizers as soon as possible for the next stage.  Further progress is expected when the CO2 laser becomes available for studies in between experimental runs.

Back to Top

Last Modified: December 3, 2007
Please forward all questions about this site to: Vitaly Yakimenko

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

Privacy and Security Notice  | Contact Web Services for help