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Experiment: E-211 CERN BBA
Normalized emittance (mm mrad)
Summary from User:
Goals for the shift were achieved. Machine response matrix was identified and a reduction of eroneous dispersion by a factor 4 was obtained for the first half of the linac.
Normalized emittance (mm mrad)
Experiment: E-201 DIELECTRIC WAKEFIED ACCELERATION
The shift was dedicated to determine the golden orbit trajectory and align the laser to this orbit. The alignment laser provides a vector for precision alignment of the dielectric samples (~450um inner diameter). The laser alignment to beam orbit was achieved during this shift.
We will align the dielectric samples using the results from this shift in the next access. In the next shift, we will measure the CTR spectra and the CCR spectra of the dielectric samples.
Experiment: E-202 ULTRAFAST MAGNETIC SWITCHING
Experiment: E-201 DIELECTRIC WAKEFIELD
Experiment: E-201 DIELECTRIC WAKEFIELD ACCELERATION
This was a broadly successful shift. We accessed FACET to install a more sensitive detector. Signals were seen on the detectors that were correlated to the beam. Scanning the interferometer motor showed interference effects although a frequency spectrum has yet to be found in the data. We will analyse the data further.
We plan to take more data with the compressed bunch which is expected to result in much larger, clearer signals and interesting science as we enter a high field gradient regime.
Experiment: E-200 PLASMA WAKEFIELD ACCELERATION
The goals for the shift were exceeded. The transverse deflecting cavity was brought online and the first bunch length measurement for FY13 was made. The notch collimator was used to divide the long bunch into two shorter bunches. This was imaged with the deflecting cavity clearly showing two separated bunches. This is a significant step towards delivering two bunches to the experimental area for plasma wakefield acceleration studies. The optics we require to collimate the more compressed bunch will be commissioned in April prior to delivery to the plasma wakefield acceleration experiment.
The goal of the shift was to expose resistive film samples to short compressed bunches to study the damage and possibly drive a metal to insulator transition in not so strong electric fields. The goal was achieved.
Experiment: E-203 SMITH PURCELL LONGITUDINAL BUNCH MEASUREMENTS
The main objective of this run was to bring the gratings as close as possible to the beam (within 1 mm) to generate measurable signal (Smith Purcell Radiation) from the shortest period (50um) grating with amplitude at least 20 -30 ADC counts above the noise level ratio.We struggled to bring the 50um grating close enough to observe such measurable signal (the measured signal was 10-20 counts above the noise level). We brought the grating to approximately 2mm range while simulations suggest we needed to be below 1 mm to see signal.The longer period gratings gave stronger signals at 11 measured frequencies and via extrapolation and interpolation technique we performed a bunch profile reconstruction using Kramers-Kroning method .We request additional beam time to address the issue of low signal with the 50um grating.
The goal of the shift was to expose magnetic and resistive film samples to short compressed bunches to study switching (relevant to nonvolatile memory applications).The goal was achieved.We request 2 hours of beamtime at full charge to expose thin iron film magnetic samples with compressed bunches. The useful magnetic pattern will be larger, while damage will be more localized in the center.
Experiment: E-206 THz RADIATION MEASUREMENTS
This was a very successful first shift! With the high-charge compressed beam we observed the first beam-plasma interactions for this run with a Rb plasma. Preliminary findings are the following: We measured significantly stronger interaction than the last run (11 GeV in 28cm) with the ionization starting earlier in the bunch, as evidenced by the amount of charge participating in the wake, and a large amount of trapped charge (1E10 electrons/pulse) from the plasma. The higher beam intensity also produced significant ionization and wakefields in the Ar buffer gas in the bypass line. The symmetry of the betatron x-rays was correlated with incoming beam emittance (changed through spoiler foils) and incoming beam tilt introduced with the transverse deflecting cavity.
Date: 4/28/2013 (Owl Shift)
Date: 4/28/2013 (Swing Shift)
Observed very good beam-plasma interaction in Rb gas, significantly better than last year. Successfully characterized the Rb for several densities and bunch lengths. Clear observation of hosing for a number of the working points; observed dependence on hosing on sextupole knob.
Observed steady acceleration of +10 GeV in Rb. Studied effect of transverse knobs to induce or suppress instabilities in the accelerated particles - saw clear effects. Characterized the interaction at 4 different waist beta configs, two asymmetric and two symmetric.
The beam was able to ionize He, which has a very high ionization energy of 25 eV. Good beam-plasma interaction with He was observed during this shift. Observed threshold for interaction at ~16 Torr, and at 32 Torr, electrons were observed to be decelerated from 20 GeV to below 2 GeV, i.e. more than 18 GeV of energy loss. No acceleration was observed in He.
Beam was able to ionize He but with no acceleration. Significant deceleration with possible trapped charge acceleration observed in Li. We studied the effects of longitudinal beam tuning on the acceleration signal and saw no improvement.
The beam-plasma interaction with the Li oven with 10% of Ar impurity in the He buffer gas was studied. Trapping was present as evidenced by the toroid downstream of the plasma, and possible candidates for trapped electrons were identified on the Cherenkov spectrometer during the first part of the shift. Interaction was degraded when going to lower beta functions during the second part of the shift. Almost no acceleration was observed during the shift, despite the optimization of the bunch profile and the use of higher plasma densities.
The beam-plasma interaction with the Li oven with Ar impurity in the He buffer gas was studied, with concentration of 10%, 21% and 50%. A parameter scan was performed with the goal of improving the wake amplitude. The amplitude was diagnosed with the amount of energy loss. Even though the amount of the energy loss, and thus the wake, was improved during the shift, the accelerated electrons, which we expected to come from Ar ionization inside the wakefield, were not directly observed on the Cherenkov diagnostic. Low quality trapped charge was however observed on the scintillator screen intended to gamma-ray measurements.Up to 20 GeV of acceleration was observed in the Li plasma when disabling the He/Ar ionization by increasing the beam transverse emittance. Up to 10 GeV of acceleration was observed in the He/Ar plasma for an impurity level of 50%. When combining both the He/Ar plasma stage and the Li plasma stage, the acceleration was observed to be suppressed, which is experimental evidence for the dephasing between the two plasma stages.
Good evidence for ionization injection into a Plasma Wakefield Accelerator (PWFA). One or more electron features, distinguished by their narrow angular spread and relative brightness, appear at the image plane of the electron spectrometer along with the usual wake-field perturbed SLAC electron beam (energy gain and loss--the normal PWFA result). Evaluating the energy of the narrow features will require post-processing of the data.We expect short bursts of charge to be trapped in the the wakefield if a minority impurity species intermixes with the Lithium vapor column within the boundary layer. In the first one or two foci of the SLAC beam--in the up-ramp of the Lithium vapor column--the minority impurity species can be field ionized by the collapsing (focusing) SLAC beam, releasing new electrons in such a phase that they are inside the main wakefield. They can begin at rest and leave the plasma with 10's of GeV. It is thought that the narrow features mentioned above are these ionization-injected electrons.
The procedure for aligning the laser to the beam and the tube to the has been nailed down. We weren't lucky. We used the time while the beam was down to iron out issues in the on the fly analysis tools and get motor coordinates for future tubes. Unfortunately, even while the beam was nominally up, the bunch length pyro was unstable indicating the beam wasn't very useful.
Sent beam through 1 cm tubes, signal was thought to be too low to resolve proper peak. Increased signal by switching to 10 cm tube, which uses a better designed coupling horn as well as more energy due to length. Saw desired signal, improvements can be made with more points, wider scan range and background subtraction.
Measured ~540MeV/m gradients in a 10cm long, 450um inner diameter dielectric tube with a 25um copper coating. The 54MeV energy change is a world record for dielectric wakefield accelerators. The spectrum of the wakefields excited in the structure agreed with the design predictions.
We sent beam through a 10cm long 450 micron inner diameter tube made of silica and measured ~80MeV energy change (800 MV/m gradients) which is a new world record. The spectrum from the wakefields in the tube agreed with our prediction. We must further investigate correlations with energy feedback to clear up some other odd results. If numbers hold it means we are extracting 240 mJ of energy from the beam.
Experiment: E-202 ULTRAFAST MAGNETIC SWITCHING
Samples were exposed with full charge enabling larger fields for switching of domains in ferroelectric samples. Different resistive structures were used to evaluate the damage due to an enhanced electric field.
We were exceptionally pleased with our first delivered laser pulses of the requested pulse duration and good spatial chirp. After the Ti:Sapph laser was set-up to our request, we spent time gaining competency with our installation to send the Ti:Sapph laser beam into the E200 PWFA experimental apparatus. During our dry-run for the installation of these optical components, we identified the need to make a small modification to the stage assembly for the turning mirror. In our next access, we plan to have the modification made and the full optical installation in place.
The goals for this shift were to install the modified stage assembly and turning mirror and to install the axicon lens and view its profile with a low power alignment laser. The goals were not achieved due to wires breaking. Work finally stopped when a member of the team broke a vacuum window.
We successfully installed the stage assembly and turning mirror and also the axicon lens. We viewed the profile of the low power alignment laser after the axicon. The next step is to verify the electron beam trajectory and make any necessary adjustments to the laser alignment prior to starting Ti:Sapph studies.
Using the R56 10 mm setting, we viewed the electron beam pass through the hole of our turning mirror that injects the laser onto the electron beam path. The alignment of the optics for integrating the laser with the experiment was highly successful and only minor tweaks were identified for the next access.
We made minor adjustments based on the observed electron beam trajectorty and aligned the axicon lens with the low power alignment laser. We switched to using the Ti:Sapph laser. Steering the laser through the transport system (approximately 30 meters) took more time than planned for and we did not complete measurements of the profile of the laser after the axicon lens. However, the set-up is ready for the Ti:Sapph laser. We request an increase in laser energy prior to our next shift.
The 800nm Ti:Sapph laser was successfully brought into the plasma wakefield acceleration apparatus for the first time. Various studies were completed including pointing jitter (which was of an acceptable level) and beam profiles at various positions after the axicon lens. The beam profiles were not ideal and it is suspected that the main source of the issue was in the laser room. We request that the beam profile and spatial chirp is improved. We also identified some aperture clipping which we will investigate and resolve next.
The observed clipping was investigated and resolved. Beam profiles had not improved though damaged mirrors had been replaced. The source of the non-ideal beam profiles still needs to be identified though we can proceed with the experiment even in this state. There are no issues that prevent us from sending high power laser to the experiment to attempt ionization. Our next step is to adjust the timing of the laser to the electron beam.
The laser triggering was reconfigured to be timed to the FACET electron beam. The timing was verified with beam. We found the timing jitter to be within the resolution of our measurement (1ns) and we tested our ability to make fine adjustment of the timing. Coarse adjustment of the timing will be tested tomorrow.
The coarse adjustment of the timing was tested and was successful. An access to FACET was used to verify the alignment and ready the apparatus for argon and lithium plasma studies. The low delivered energy is a concern for the goal of generating a plasma filament.
The goal in this shift was to observe a plasma filament in the argon caused by the laser. This was not seen, possibly due to multiple contributing factors including a misalignment of the laser to the axicon lens. Remote alignment failed and the cause will need to be investigated in an access to FACET. We completed the energy spectrometer calibration and vignetting scan and characterized the new orbit planned for E200 experiments.
We accessed FACET and measured the laser beam intensity after the compressor and found it to be significantly lower than expected. We recovered the high intensity laser beam by changing the polarization in the laser room. This is certainly the largest contributing factor to the lack of argon ionization yesterday. We recovered our motors for remote alignment of the axicon lens. Our transverse laser beam profiles were re-measuremed and are still not optimal.
We successfully created a plasma filament in argon gas. This is a significant milestone that indicates that the laser is now ready to ionize lithium (first electron ionization energy of argon is three times that of lithium). The E200 lithium oven will be prepared for the experiment with laser-ionized lithium plasma and electron beam for first signs of wakefield acceleration. We can also confirm that the laser beam profiles look good after work by the laser group this morning.
Date: 6/29/2013 - OWL SHIFT
Date: 6/29/2013 - DAY SHIFT
Date: 6/30/2013 - OWL SHIFT
Date: 6/30/2013 - DAY SHIFT
Date: 6/30/2013 - SWING SHIFT
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy