|
|
|
|
Site Details ATF Newsletters 
|
2001 ATF NewslettersJan | Feb | March | April | May | June - July | Aug | Sept | Oct | Nov - Dec
Greetings to all, The ATF Program Committee and Users Meeting will take place on Thursday January 31 - Friday February 1, 2002. Thursday morning will be devoted to presentations from ATF staff on the facility and staff R&D. Thursday afternoon will be devoted to progress reports from ATF Users, to be followed by a tour of the ATF and dinner. Friday morning will be the time for new user proposals and executive sessions. The deadline for new proposals is December 28. The ATF and its users reaped a lot of achievements and we are looking forward to this event. In this report, the last one for the year 2001, you may find reports from the Rough-Surface Wake Field feasibility study and the MINOS, Smith-Purcell, STELLA II and LACARA experiments. We congratulate the MINOS team that finished their experimental program with record speed and a sure hand on the helm and thank them for the Champagne and celebration. Happy Holidays and best wishes for the new year, Ilan Ben-Zvi.
We spent two days measuring the energy loss of the "small-bumps" beam pipe. The energy loss and spread measurements for two different bump pipes have been completed. The experimental results and model fits for the energy loss and energy spread are in this link. The following is the preliminary summary, based on
our measurements. 1) Supposing that our beam pipes mimic the surface roughness, the surface roughness effect could not be explained by pure inductive impedance model alone. 2) The additional energy spread and energy loss provide a hint that one or more synchronous modes may be trapped in the structures. A synchronous mode with single frequency is fitted well with the additional energy spread and energy loss. 3) At short pulse length, the electron bunch behaves like point charge compared with the wave length of the synchronous mode, and then the induced energy spread is neglected, but the energy loss is still created. When the pulse is longer, the synchronous mode oscillates inside the bunch, and that creates the energy spread but alleviates the energy loss due to the averaging effect of the oscillations. 4) The synchronous modes of the corrugations with cylindrical symmetry can be predicted exactly with an analytical solution. However, the surface roughness is different. In the analysis of the measurements we tried to make our beam pipes equivalent to the corrugation with cylindrical symmetry. The amplitude of the 3rd mode is at a maximum in the small-bump’s pipe with our corresponding pulse length, while the 2nd mode is strongest in the large-bump’s pipe with the corresponding pulse length. In the fitting, the 3rd and 2nd modes are used to fit the measurements for the small-bump’s pipe and large-bump’s pipe, respectively. Our measurements indicate that in the short pulse region, multiple modes may exist in the pipes. However, it is not reasonable to fit the measurements with multi-modes that are calculated from cylindrically symmetrical corrugations, since our beam pipes are different. In addition, all synchronous models mostly pay attention to the single synchronous mode. Thus, we believe that it is reasonable to fit the data with single frequency. In the longer pulse, the bunch can only see the inductive impedance and the synchronous mode is beyond the bunch spectrum. Its effect is smaller due to the longer pulse length. 5) The dielectric constant as estimated with the frequency of the synchronous mode for our pipes is about 1.3, which is comparable to 1.5 usually assumed in the dielectric layer model. 6) In conclusion, in our pipes, the wakefield effects can be explained by a pure inductive impedance and a single synchronous mode. J. McDonald(1), D. Naples(1), M. Diwan(2), A. Erwin(3), X. Ping(3), C. Velissaris(3), B. Viren(2) 1 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 2 Brookhaven National Laboratory, Upton, NY 11973-5000 3 Department of Physics, University of Wisconsin – Madison, Madison, WI, 53706 Detectors designed to monitor beam quality via muons and tolerate the high radiation environment of the MINOS/NuMI beamline at Fermilab were tested at the ATF facility for linearity, stability and saturation effects. The data show that the detectors will be an adequate solution for the beam monitoring of the MINOS neutrino beamline. Introduction The NUMI beamline will use 120GeV protons from Fermilab’s main injector to generate a beam of muon neutrinos that will be directed toward the Soudan in Northern Minnesota, 800 km from Fermilab. The MINOS experiment has both near and far detectors which are utilized to attempt to determine whether neutrino have mass and measure the mass parameters. Both the near and far detectors observe only neutrino events. Due to the low rate of these events, a month or more of beam time would be required to observe a systematic shift in the beam due to possible component failure. The MINOS beam monitoring group has devised a fast solution which uses the muons from pion decay to monitor the NUMI beam. Most component failures can be detected in one or two proton spills. Detectors Ionization chambers (PICs) are an ideal solution to the beam monitoring problem. The chambers, which are two metalized ceramic plates, are placed in a Helium gas box and high voltage is applied to one of the metalized plates. The ceramic tiles have radiation tolerant qualities and can be cheaply manufactured. The chambers, which are separated by a precise gap thickness, produce ions and electrons as particles pass through the detector. Detector linearity assures that there is a one-to-one correspondence between the input charge passing through the detector and the response of the detector. Stability of the detector requires that this response does not change on a daily basis. Saturation of the detector occurs when the intensity through the detector amasses enough ions and electrons to change the electric field. In this region, the response of the ionization chamber is nonlinear. The parameters which control the response of the ionization chambers are the gas, the gap thickness or the voltage. The gas and gap determine the overall gain of the ionization chamber. The ionization chamber gains increase with gap size and decreases with the average ionization potential of the gas. The voltage determines the stability of the chamber and the onset of saturation. The higher the voltage, the higher the intensity before saturation occurs. Results and Conclusions A battery of tests were performed at the ATF to determine the response of the PICs. These tests included determining the voltage stability region for a given intensity, intensity scans at a given voltage and various systematic studies. Both argon and helium gases were used and two PICs with gap sizes of 3 mm and 5 mm were tested. The NUMI beamline has a maximum intensity about 5 × 10**7 particles/cm**2. This corresponds to a charge of approximately 10 pC under normal beam conditions at the ATF facility. The 5 mm PIC was tested at 250 V, 500 V and 750 V with Helium gas. The saturation point was above the NUMI maximum for all voltages. In addition, voltage plateau curves were determined for several intensities for the 5 mm PIC. The 3 mm PIC intensity response was tested at 250 V and 400 V with Helium gas. Voltage plateau curves were taken at several intensities. In addition, the intensity response and voltage plateau region of the 3 mm PIC was measured for Argon gas. The beam tests at the ATF facility verify that the PIC solution meets the NUMI muon beam monitoring requirements. The results of this R & D will have been presented to the MINOS internal review committee and we expect final approval to move into the production stage for three 92 ionization chambers arrays to be built and assembled at the University of Pittsburgh and University of Wisconsin at Madison. Summary of the Smith-Purcell experimental run Nov
8-9, 01 The principle goal of this running period was to install and implement
a compact spectrometer (Acton Research Corporation SpectraPro 150) which
would allow the exploration of the wavelength regime of 400-1200 nm.
For this purpose we have previously (Nov 1-2, 01) had a run in
which a Hamamatsu R5108 Photomultiplier tube was first exercised.
This PMT has a sensitivity ranging from 300 to 1200 nm. During the Nov 1-2 run, we observed sporadic performance for our signal during our grating scans which we traced to our trigger level which was set at 0.25 V for a 5 V TTL trigger signal. Raising our trigger level to 2.5 V solved this problem. We established that a signal level of nearly 1 V could be obtained above an x-ray background level of ~ 50 mV. (http://pubweb.bnl.gov/people/kirk/atf/sp_exp/nov02_01/xscan.ps) For the Nov 8-9 run, our first task was to reduce
the level of x-ray background so that our spectrometer throughput signal
would have greater sensitivity.
Attempts to raise and lower the detector from the beamline height
proved unsuccessful. We were
successful only by increasing the amount of lead between the grating and
the PMT. This was eventually set at 12”, resulting in an x-ray background
level of ~20 mV. We found
that the optimum signal/background ratio was obtained when the PMT high
voltage was run at 1250 V. Results for 0.5 microns: Results for 0.75 microns: (http://pubweb.bnl.gov/people/kirk/atf/sp_exp/nov_09_01/spectrum_07.ps) Spectra fits show the presence of structure at 675
nm for both cases. We are
considering possible sources for this observed radiation. They include:
fluorescence, bremsstrahlung, transition radiation, and Smith-Purcell
radiation. Energy modulation was observed during the recent
STELLA runs, using the new “STELLA-II” configuration on beamline #1.
Currently, we are checking the performance of the undulator sections
independent of the buncher and chicane, which will be installed next year:
Laser parameters (TW amplifier, assume 200 psec pulse
length): wiggler entrance (GPOP2.6) = 210mJ which yields 1.1GW, wiggler
exit (GPOP4) = 195mJ (0.98GW), which gives >90% transmission though
wiggler; Discussion: “bounding box” refers to ATF frame grabber function that detects distribution edges for each shot, which corresponds to the extent of modulation seen on spectrometer. The variation in sigma and bounding box values for Laser OFF is due to e-beam size and position jitter. Distinct peaks near the edges of the distributions for both wigglers were observed, indicating qualitative agreement with simulations. The extent of observed maximum energy gain/loss (bounding box) is lower than expected for both wigglers, given estimated measured laser energies (few GW). The tapered wiggler in particular requires >1GW power for distinctive modulation to occur (centerline shift). This may indicate that the actual delivered laser power is lower than we think, and/or laser-electron beam alignment is imperfect which leads to inefficiency. All values above reflect “best” achieved modulations, after YAG delay line (sync) and laser mirror steering optimizations. We did not, however, have enough time to properly optimize for the tapered wiggler so we should accept that further runs are needed to improve modulation. Vitaly reports no major e-beam tuning issues with either wiggler. LACARA (laser cyclotron auto-resonance accelerator) is an advanced concept laser accelerator of electrons that is being installed on beamline #2 at the Accelerator Test Facility, Brookhaven National Laboratory. LACARA is expected to accelerate a ~1 nC bunch of 50 MeV electrons to an energy ~90 MeV, using a 2 m solenoidal magnetic field of 6 T and a CO2 laser power of 800 GW. First experimental results are expected in about one year, but with lower laser power. The experiment is being carried out by S. Shchelkunov (as part of a doctoral thesis at Columbia University), T.C. Marshall (Columbia), J. L. Hirshfield (Yale and Omega-P Inc.), J-M Fang (Columbia), Michael LaPointe and Changbiao Wang (Yale). Important help with this project is provided by the research staff of the ATF facility. Comparison with STELLA (staged inverse free electron laser accelerator) is instructive. Both STELLA and LACARA should achieve comparable acceleration gradients and final electron energy given the same laser power. Major differences include a long tapered wiggler field for STELLA, versus a long solenoidal field in LACARA. Neither system will achieve very high energy (TeV) by itself, but STELLA can make a buncher-injector for another synchronized laser accelerator, while LACARA may distinguish itself as a “chopper”, producing a train of femtosecond bunches. Designs for the electron and optical transport systems have been completed. In
Figure
1 we show the side view of the optical transport system.
Because the energy of the laser pulse is about 5 J, the pulse must
enter the vacuum chamber through a ZnSe window when its waist radius is
about 25 mm, to avoid damage to the surface of the window.
The beam converges to a radius of 1.3 mm inside the solenoid, where
it has a Rayleigh range of about 50 cm.
Following LACARA, the optical energy is diverted to a dump chamber,
and the beam goes to an energy analyzer. Figure 2 shows a top view of the electron transport and diagnostic system. Special provision must be given to precise beam alignment in the solenoidal field. A new magnetic spectrometer will be required to measure the energy difference between the unaccelerated bunch and the accelerated bunch. Electron bunches from the ATF rf linac enter the LACARA experiment via a bending magnet at the right. Additional details can be seen in Figure 3 and Figure 4.
Last Modified: December 3, 2007 |
|||||||||||||||
|
|
