Greetings all,

The most exciting results come this time from the Smith-Purcell experiment. Prof. John Walsh (Dartmouth) will present these at FEL’99 in Hamburg next month, delivering the distinguished FEL-Prize-Winner Address. The detailed report is provided by Harold Kirk (below). Using a 1 mm period grating and the precision optical system The group has seen very strong signals from first order to ninth order (fundamental 10 microns, highest orders between 1 and 2 microns). John reports that this new result has now been confirmed by the theoretical model, that the data is smooth and excellent signal to noise ratio. He credits this result to the new high precision optical system and the ATF beam quality.

Ilan Ben-Zvi.

Back to Top

Experiments

 

HGHG Experiment (Reported by Xijie Wang and Adnan Doyuran)

 

1. On Wednesday (July 30): (Yu, Solomon, Malone, Yakimenko, Doyuran, Wang):

Experiment started at about 9:00 AM doing trajectories studies. Since the trajectories program was not ready for 41.5 MeV (correct HGHG energy), a lot of time was spent on deciding what energy to run. Yu decided to run on 49 MeV. Yakimenko set up beam for trajectory studies ( ~ 1 mm-mrad, 40 A measured). In the afternoon, gun phase instability delayed the run by about a couple of hours. We set up a new camera for transition radiation beam profile monitor and observed an almost saturated image. Preliminary trajectory studies for the mini-undulator were made by closing and opening the gap. We observed about 1.5 mm horizontal steering at the first monitor of the Cornell wiggler. Vertical steering was about a factor of 5 smaller. Adnan found out a problem with focusing program. Vadim had a program version that corrects the beam, but he actually didn’t save the last version of the program. The one we have is only calculating the results and the correction had to be done manually. In addition, Adnan made the necessary changes on the program to apply it at 41.5 MeV. Now both Vertical and focusing programs are ready for both energies.

2. Friday (July 2): Continue mini-undulator trajectories studies, similar steering was observed . Yu suggested vertically displace the beam inside the mini-undulator. We observed the same horizontal steering, but large vertical steering. The dispersion magnet was turned on for the first time, no vertical steering but a large horizontal steering was observed. Wang suggested to survey the HGHG beam line next week, Yu agreed and we agreed upon a survey procedure.

3. Week of July 5: Wang discussed survey procedure with survey on Wednesday morning at 10:00 AM. We had a meeting with surveyors on Thursday on their survey results. There are some uncertainties. A new survey was proposed as well as to move the Cornell wiggler up by 0.4 mm. At about 4:00 PM, the survey job was finished. Adnan set up the laser, and Vitaly delivered beam to the HGHG beam line within 10 minutes. We determined that the Cornell wiggler now is aligned with the laser to about 3 pixels (less than 40 microns) except on the misplaced monitor no.2. On Friday the whole HGHG beam line was resurveyed, including the mini-undulator gap (at Jeff’s request). The survey found a 0.030 inch tilt at one jaw of the mini-undulator. George was informed and came to correct the problem.

4. Last weekend Adnan took two more sets of horizontal and vertical calibration data of the wiggler cameras.

Back to Top

STELLA Experiment (Reported by Karl Kusche)

Results from recent beam studies runs (6/29 & 7/1):

1) Gas cell transmission (given 300-500pC input to beamline #1, and 150 micron sigma at gas cell entrance GPOP4/CTR)

GSL4 (after cell) Spectrometer (GFC)

Mirrors out 50-90mV 2-3pC

Axicon in 10-30mV 1.5pC (easily visible on new

CCD)

2) OTR reestablished at GPOP4 (CTR foil), very weak but visible (used the SANYO CCD STI bought for gas cell to confirm sufficient sensitivity ~0.07 lux). We will continue progress with improving alignment and finishing optical setup.

3) Due to low charge in cell (even with mirrors out), couldn’t observe OTR on the VOx target inside of the gas cell with SANYO CCD. BUT, we *did* see OTR on the VOx by using a passively air-cooled CCD borrowed from the ATF to increase sensitivity by an order of magnitude (to ~0.002 lux).

Preparations (final alignment, etc.) will continue over the weekend so as to be ready for the 3-day marathon run during July 12-14. Wayne Kimura and an assistant are flying out from STI Optronics to be at ATF.

Back to Top

SMITH-PURCELL Experiment (Reported by Harold Kirk)

We have established a reliable alignment procedure to start the run.

1) The e-beam is first tuned using the 3 YAG flags-one sits at grating edge upstream, one at grating mid-point and one at grating edge downstream. Record e-beam horizontal and vertical position on each YAG flag.

2) Introduce HeNe laser beam (attenuated with filters at ~OD 2) into the beam line 2 m upstream of experimental chamber. A 45 degree mirror sits in the beam line with a 2 mm aperture hole in the center through which the e-beam comfortably passes. The attenuated beam is visible on the YAG flags and the beam is aligned to be collinear to the e-beam. We achieve typical co-linearity to within 1 mrad both horizontally and vertically.

3) Rotate in the 4 degree alignment beam.

This will reflect the HeNe beam at an angle of 8 degrees to the HeNe beam path.

4) The 2" Copper mirror is rotated so as to

bring the reflected HeNe out of the experimental chamber at the center of the ZnSe window. Since the homing for this stage (the U stage) is no longer reliable we place the set point of this stage to be 37.5 K. This is now our defined 8 degree collection point.

5) The collection optics exterior to the chamber is now setup culminating in a focused HeNe beam at the HgCdTe detector. For the initial part of the experiment, the exterior collection optics consists of a 5 cm Al flat aluminum mirror followed by a 4 cm fP mm crown glass lens which focuses the collected HeNe beam onto the HgCdTe detector. Crown glass transmits from 0.2 to 2.5 microns.

Initial e-beam parameters:

E = 40 MeV ; dE/E = 0.5 %

Q = 0.5 nC

Pulse length-long (estimated to be about 10 ps)

Waist-sigma is about 5 to 6 pixels (12 microns/pixel in x and 14 microns/pixel in y)

Ends (grating entrance and exit)-- sigma is 11 pixels.

Note: these beam sizes are upper limits since the

flag images were saturated.

We move the V stage (grating rotation) to the nominal grating in beam setting and see a signal of about 40 mV. We do a mirror scan and find the signal width to be 2 K steps which corresponds to 6.6 degrees emission from the grating. An x stage scan finds a peak at grating x at 70 mm whereas a Faraday cup scan showed that half the e-beam was intercepted by the grating at x= 40.65 mm. The signal FWHM from this scan is 0.35 mm. Placing filters before the HgCdTe detector we find the signal is not mm wavelength (passes through wire mesh), does not contain 2 to 20 micron radiation (the Crown glass is opaque to radiation > 2.5 um), and is mostly 1 to 2 um.

The exterior detection system was changed to now include an off-axis parabolic Al focusing mirror instead of the crown glass lens. Alignment was done as previously with an attenuated HeNe laser beam. Horizontal alignment was good to within 2 mrad horizontally and 0.7 mrad vertically. A filter (OD .4) was placed before the CCD camera which views the YAG flags. We still observe saturation of the beam images.

The e-beam is retuned for more charge. We now have a beam with 4.8 V on the Faraday cup. An x scan with Faraday cup shows the 50% occlusion point to be at x at mm. An x scan for radiation finds the peak signal now to be 115 V and the FWHM to be 0.3 mm.

We do a mirror scan and get a peak signal of 82 mV at u7.4k (8 degrees emission). The signal is narrower than before with a FWHM of 1k (3.3 degrees).

Filter transmission studies now show that in addition to the 1 to 2 um radiation we have a substantial component (about 50 %) above 2 um. Using a polarization filter we find that the 1-2um radiation is polarized. Orientation of the polarization is not known until we evaluate the filter performance with a known polarized 1-2 um source.

We now do three separate mirror scans with three different filters in place:

Ge(2-20 microns), RG-1000(1-2microns) and RG-9 (0.6-1microns). All three show a peak at the same setting- 37.4 k.

We do a vertical trim scan. The result is that the signal is sensitive to the setting of the vertical trim. FWHM is 1.0 amp with a full scale of 4 amps.

Back to Top

Facility

 

Technical Staff Tasks (reported by Bill Cahill and Xijie Wang)

 

ATF experimental hall modification for VISA was finished and approved for operation by NSLS safety.

The LPOP1 monitor and new gun diagnostics ion pump are operational. Now the vacuum degradation is very small when the gun/linac vacuum valve is opened. Preventive maintenance and many improvements were made on the RF system. A large effort was employed in reducing phase jitter of the RF system. It was found that the gun and linac phase/amplitude drivers were too susceptible to temperature change. Whenever the mezzanine a/c would cycle, the drivers would drift. Temperature controlled enclosures are now being built to cure the problem. Both kilowatt drivers have been retuned. An additional pair of capacitors will be installed in the gun modulator to lengthen the pulse width. The "z" line enclosure is now temperature-stable. The gun solenoid water circulator has been reinstalled in the a/c enclosure and has greatly reduced gun drift. Multiple Darlington transistor channels have been failing due to the excessive heat load on the cooling water. The circulator has been relocated to the air conditioned machine shop and the system appears to be stabilized. The oil leaks in both the gigawatt and terawatt lasers have been repaired. Further investigation of RF instability will continue.

Other problems:

Gun area (Z-line) temperature instability. With help from the NSLS mechanical group, Bill Cahill improved the situation significantly. RF gun reflected power load break-down: It seems to be humility related, we need a waveguide pumping port to fix the problem. RF gun phase oscillating: we have observed gun RF system phase oscillating peak to peak about 4 deg (4ps). This problem is being worked upon by Joe Sheehan. One aspect was corrected, another is under study.

 

Computer and Control System (Reported by Bob Malone)

FACILITY:

1) New magnet power supply reassignments completed per request of V.

Yakimenko.

All databases, servers, superelements, displays, etc. updated to reflect these changes.

2) New laser diagnostic signals acquisition/display completed per request of M. Babzien.

3) New control system: New 100MHz cabling now connected to control room Fast Ethernet switch from all areas except experiment hall and YAG laser hutch. Fiber optic termination work to start in next 2 weeks.

NOTE TO ALL ATF STAFF/USERS:

As part of the network upgrade for building 820, both the 10MHz thick wire network backbone (yellow coax) and the thin wire (black RG58) will be disabled and/or removed in about 30 days. After that time, all LAN connections should be made via category 5 connections available at each telephone wall jack. Most ATF equipment already uses the cat 5 connections, but there may be some old connections which are still using thick or thin coax. ITD can supply adapters for older NICs which don’t include cat 5 connectors.

HGHG:

Worked with L. Solomon/J. Aspenleiter to test new mini-undulator gap protection hardware. LS/JA found problem with bad connector on one of the power supplies which drives a stepping motor. We suspect this is what caused the large skew originally seen several months ago. Software to control the gap was operational during two days of HGHG beam time for trajectory studies. No significant skew was observed during those sessions and gap responded to all software requests.

A new set of calibration data was obtained by moving the gap in increments of 20K steps through its complete range of travel. Data recorded: upstream/downstream gap (tip-tip) as measured by calipers, front panel LVDT readings, CAMAC readback of LVDTs. Data is being analyzed to produce new software calibration equation. Thanks to JA for helping with the calibration scan

Since NSLS surveyors were present at ATF for other jobs on 7/9, LS/JA made suggestion to have them survey gap and compare to computer readbacks. Surveyors measured .004" difference between upstream/downstream ends but front panel of LVDTs and computer readbacks say difference is .024". RM spent Friday night/Saturday AM to verify that this is indeed the readback from the LVDTs.

** This issue is still to be resolved and will require continuation of help from NSLS. ** Tested software for controlling dispersion magnet. Software responds as expected. George Rakowsky will provide us with details for the algorithm to drive the trim magnets associated with the main magnet. Li Hua Yu has requested these trim magnets have individual controls so that steering can be implemented. Need to have more discussion with XJW, LY, GR to see exactly how this should be done.

VISA:

Held discussion with A. Tremaine for method to exchange data between ATF control system and VISA experiment Macintosh. Network solutions are difficult to implement given the tight synchronization problems needed to guarantee that all data has come from a single beam pulse. Proposed solution is to use DAC in VAX CAMAC to communicate data to ADC in Mac. (i.e., data exchange is analog.) VAX will "broadcast" desired data on analog lines at each clock pulse. VISA Mac will read it when they are able to. AT will investigate this proposed solution to be certain it present no difficulties on the MAC end. There should be no problem on the VAX end: this additional data exchange should add only ~5ms max to VAX real-time loop budget.

YAG Laser Status (Reported by Marcus Babzien)

During the shutdown to install the new oscillator, the oscillator enclosure has been expanded to allow kinematic exchange of the new and old oscillators with minimal disruption to the optics. Switching between the two might be accomplished in about 1 day. The enclosure incorporates the same temperature stable water jacket which dramatically improved the stability of the old oscillator. Also, the oscillator mounting to the table is much better now, and less stable mirror mounts required in the temporary position have been eliminated.

More improvements in the HVAC system have occurred. The glycol temperature was lowered for more dehumidification, the make-up air intake outside was adjusted for better air balancing, and temperature and humidity data loggers are measuring conditions close to the optics so we have a better idea of the relevant conditions

After the shutdown, we have seen very good stability out of the system in general. After alignment on June 28, the system was able to operate until July 9 with low shot-to-shot energy jitter without IR re-alignment. Typical rms fluctuations under 1% in the IR, fluctuation on the cathode (UV) in this period was <10% p-p. The average power out of the oscillator did slowly drop, and it was realigned on the 9^th. The slow downward drift in power seems only to be from cavity misalignment, not the laser diode, and it was restored to maximum output after alignment.

Also, on the 7^th the Pockels cell blocking the slicing pulse from reaching the cathode was relocated upstream of the doubling crystal. No distortion in beam profile was observed in green or UV, so this arrangement may be used permanently after a higher voltage pulser arrives at the end of this week. This will enable us to operate at the same uniform beam profile for single and double pulse modes as required by the HGHG and STELLA experiments

Beam Operations (Reported by Xijie Wang)

A laser mask for locating the electron beam on the center of the cathode was

prepared, installed and tested.

July_9_beamlets

 

Beam-based alignment (reported by Vitaly Yakimenko)

Two 3-hour blocks were devoted last week to orbit study on the H-line. The e-beam trajectory was adjusted to minimize steering from quadrupoles. A specially prepared computer program helped to improve accuracy. The trim magnets’ calibration and polarity were checked with quadrupoles turned off and the calibrations agree with the previous result of magnet measurements.

The picture below shows deviation of the quadrupole centers from the line

corresponding to the "LINAC" beam axis. Independent measurement was done to

confirm results: The beam was measured approximately 4 mm displaced on

HPOP-UP2 (beam profile monitor located approximately 6 meters from the

beginning of the H-line) for the case when all quadrupoles and trims on

H-line were turned off. A resurvey of two quads will produce a straight

line.

July_9_beam_based_alignment

Back to Top

ATF Schedule (Prepared by Xijie Wang)

Schedules July 99