Tianmu Xin - Academia.edu (original) (raw)

Papers by Tianmu Xin

arXiv (Cornell University), Nov 17, 2015

High-gradient CW photo-injectors operating at high accelerating gradients promise to revolutioniz... more High-gradient CW photo-injectors operating at high accelerating gradients promise to revolutionize many sciences and applications. They can establish the basis for superbright monochromatic X-ray free-electron lasers, super-bright hadron beams, nuclearwaste transmutation or a new generation of microchip production. In this letter we report on our operation of a superconducting RF electron gun with a record-high accelerating gradient at the CsK 2 Sb photocathode (i.e. ~ 20 MV/m) generating a record-high bunch charge (i.e., 3 nC). We briefly describe the system and then detail our experimental results. This achievement opens new era in generating high-power electron beams with a very high brightness. Superconducting radio-frequency (SRF) electron guns are frequently considered to be the favorite pathway for generating the high-quality, high-current beams needed for future high-power energyrecovery linacs. SRF guns can find unique scientific and industrial applications, such as driving highpower X-ray and EUV CW FELs [1-10], intense γ-ray sources [11-14], coolers for hadron beams [15-18], and electron-hadron colliders [19-21]. The quality of the generated electron beam-both its intensity and brightness-is extremely important for many of these applications. The beam's quality frequently is described by its brightness, i.e., the number of generated electrons divided by the bunch's emittance (defined as the phase space-volume occupied by electrons). In this letter, we report the record performance of our SRF gun (Fig. 1) that was built for the Coherent electron Cooling (CeC) experiment at RHIC [16] that generated a 1.7-MeV CW electron-beam with a 3 nC bunch charge. This gun demonstrated CW operation with 18 MV/m accelerating field at the CsK 2 Sb cathode at the time of the electron's photo-emission. Table 1 summarizes the key results from CeC gun and compares them with previously commissioned SRF guns.

The Low Energy RHIC electron Cooler (LEReC) is the first electron cooler based on rf acceleration... more The Low Energy RHIC electron Cooler (LEReC) is the first electron cooler based on rf acceleration of electron bunches. To further improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, a normal conducting RF cavity at 1.4 GHz was designed and fabricated for the LINAC that will provide longer electron bunches for the LEReC. It is a single-cell cavity with an effective cavity length shorter than half of the 1.4 GHz wavelength. This cavity was fabricated and tested on-site at BNL to verify RF properties, i.e. the resonance frequency, FPC coupling strength, tuner system performance, and high power tests. In this paper, we report the RF test results for this cavity.

ERL-Ring eRHIC aims to build a new high current (50 mA), multi-pass (6 passes) ERL to provide 3-1... more ERL-Ring eRHIC aims to build a new high current (50 mA), multi-pass (6 passes) ERL to provide 3-18 GeV electron beams to collide with proton beams from existing RHIC. One critical challenge for eRHIC is to damp HOMs. The average HOM power is up to 8 kW per cavity, and it will get worse when the electron beam spectrum overlaps with cavity HOM spectrum. A novel HOM damping scheme by employing ridge waveguides has been worked out at BNL, which is able to well damp both longitudinal and transversal modes. This paper will describe the design of the HOM damping scheme, including RF design, HOM damping results, progress of prototyping.

In an accelerator cavity, Higher Order Modes (HOM) are generated by the current of the beam. The ... more In an accelerator cavity, Higher Order Modes (HOM) are generated by the current of the beam. The HOM power can reach tens of kilowatts in a high current accelerator, depending on the details of the beam and cavity design. In this report, we propose a novel RF harvesting system to recover the HOM power into DC power which can further used for various purposes such as driving a solid state or klystron RF amplifier to supply fundamental RF power at other frequencies, charge batteries etc. The efficiency would be a product of the energy recovery and regeneration efficiencies, where the state of art is 90%. The proposed HOM power recycling system contains a multiple band harmonic RF coupler, broadband RF antenna system, a high power rectifier diode circuit and a DC load.

The sPHENIX experiment is a proposal for a new detector at the Relativistic Heavy Ion Collider (R... more The sPHENIX experiment is a proposal for a new detector at the Relativistic Heavy Ion Collider (RHIC), that plans to expand on discoveries made by RHIC's existing STAR and PHENIX research groups. To minimize the luminosity outside the 20 cm vertex detector and keeping the radiation to other detector components as low as possible, a 56 MHz SRF system is added to the existing RHIC RF systems to compress the bunches with less beam loss. The existing 56 MHz SRF cavity was commissioned in previous RHIC runs, and contributed to the luminosity at a voltage of 300kV with thermal limitations from the Higher Order Mode coupler at high field, and at 1MV while using its fundamental damper for HOM damping. In this paper, we will analyze and compare the effect of different RF systems at various scenarios, and discuss possible solutions to the Higher Order Mode (HOM) damping scheme to bring the cavity to 2 MV.

After the first publication* of wire-stretching technique from its principle to measure the elect... more After the first publication* of wire-stretching technique from its principle to measure the electrical center of a deflecting cavity, more refinements of this techniques including the review of its analytical and simulation results, RF circuit improvement to improve the signal to noise ratio and its application to other cavities have been developed. These applications include the electrical center measurements for the LHC RFD and DQW crabbing cavity prototypes, multi-frequency harmonic kicker cavity for JLEIC electron cooler**, TE011 cavity developed for the beam magnetization measurement***, and a separator cavity at BNL****. Further development of measurement calibration, error reduction, alignment of cavity installation to the machine beam line, and multipole field analysis for the beam dynamics will be presented.

In this talk we are presenting the most recent results from the commissioning of unique Coherent ... more In this talk we are presenting the most recent results from the commissioning of unique Coherent Electron Cooling system, which is using an FEL amplifier to facilitate cooling of hadrons by an electron beam. We present achieved results as well as changes we encountered in the process.

The Low Energy RHIC electron Cooler (LEReC) was recently constructed and commissioned at BNL. The... more The Low Energy RHIC electron Cooler (LEReC) was recently constructed and commissioned at BNL. The LEReC is the first electron cooler based on the RF acceleration of electron bunches (previous electron coolers all used DC beams). Bunched electron beams are necessary for cooling hadron beams at high energies. The challenges of such an approach include generation of electron beams suitable for cooling, delivery of electron beams of the required quality to the cooling sections without degradation of beam emittances and energy spread, achieving required small angles between electrons and ions in the cooling sections, precise energy matching between the two beams, high-current operation of the electron accelerator, as well as several physics effects related to bunched beam cooling. Following successful commissioning of the electron accelerator in 2018, the focus of the LEReC project in 2019 was on establishing electron-ion interactions and demonstration of cooling process using electron e...

The Linac of Low Energy RHIC electron Cooler (LEReC) is designed to deliver a 1.6 MeV to 2.6 MeV ... more The Linac of Low Energy RHIC electron Cooler (LEReC) is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with peak-to-peak dp/p less than 7e-4. The booster cavity is the major accelerating component in LEReC, which is a 0.4 cell cavity operating at 2 K, with a maximum energy gain of 2.2 MeV. It is modified from the Energy Recovery Linac (ERL) photocathode gun, with fundamental power coupler (FPC), pickup coupler (PU) and higher order mode (HOM) coupler close to each other. The direct coupling between FPC and PU induced crosstalk effect in this cavity. This effect is simulated and measured, and it is further corrected using low level RF (LLRF) to meet the energy spread requirement.

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC ... more To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is designed and is currently under commissioning at BNL. The linac of LEReC consists of a DC photoemission gun, a 704 MHz superconducting radio frequency (SRF) booster cavity, a three-cell 2.1 GHz third harmonic cavity for RF curvature correction, a single-cell 704 MHz cavity for energy de-chirping and a 704 MHz deflecting cavity for diagnostic line. In this paper, we present the commissioning of three normal conducting cavities mentioned above.

The Linac of the Low Energy RHIC electron Cooler (LEReC) is designed to deliver a 1.6 MeV to 2.6 ... more The Linac of the Low Energy RHIC electron Cooler (LEReC) is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with peak-to-peak dp/p less than ±7e-4. The booster cavity is the major accelerating component in LEReC, which is a 0.4 cell cavity operating at 2 K, with a maximum energy gain of 2.2 MeV. It is modified from the Energy Recovery Linac (ERL) photocathode gun, with fundamental power couplers (FPCs), pickup (PU) couplers and HOM coupler located close to each other. Crosstalk effects in this cavity are simulated and measured. A correction method is proposed to meet the energy spread requirement.

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC ... more To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under development at BNL. Two normal conducting cavities, a single cell 704 MHz cavity and a 3 cell 2.1 GHz third harmonic cavity, will be used in LEReC for energy spread correction. In this paper we report the design of these two cavities.

We present currents status of the CeC experiment at RHIC and discuss plans for future. Special fo... more We present currents status of the CeC experiment at RHIC and discuss plans for future. Special focus will be given to unexpected experimental results obtained during RHIC Run 18 and discovery of a previously unknown type of microwave instability. We called this new phenomenon micro-bunching Plasma Cascade Instability (PCI). Our plan for future experiments includes suppressing this instability in the CeC accelerator and using it as a broad-band amplifier in the CeC system.

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC ... more To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under development at BNL. The Linac of LEReC is designed to deliver 2 MV to 5 MV electron beam, with rms dp/p less than 5·10⁻⁴. The HOM in this Linac is carefully studied to ensure this specification.

The Coherent electron Cooling Proof-of-Principle (CeC PoP) experiment at BNL includes a short ele... more The Coherent electron Cooling Proof-of-Principle (CeC PoP) experiment at BNL includes a short electron linac. During Phase 1, a 112 MHz superconducting RF photo-emission gun and two 500 MHz normal conducting bunching cavities were installed and are under commissioning. The paper describes the Phase1 linac layout and presents commissioning results for the cavities and associated RF, cryogenic and other sub-systems

A 112 MHz SRF electron photoinjector (gun) was developed at Brookhaven National Laboratory to pro... more A 112 MHz SRF electron photoinjector (gun) was developed at Brookhaven National Laboratory to produce high-brightness and high-bunch-charge bunches for the coherent electron cooling proof-of-principle experiment. The gun is designed to deliver electrons with a kinetic energy of up to 2 MeV. Electrons are generated by illuminating a high quantum efficiency (QE) K2CsSb photoemission layer with a green laser operating at a wavelength of 532 nm. The gun was able to generating 3 nC bunches at 1.7 MeV. The design goals, fabrication, performance and operational experience are reported here.

The first RF commissioning of 112 MHz QWR superconducting electron gun was done in late 2014. The... more The first RF commissioning of 112 MHz QWR superconducting electron gun was done in late 2014. The coaxial Fundamental Power Coupler (FPC) and Cathode Stalk (stalk) were installed and tested for the first time. During this experiment, we observed several multipacting barriers at different gun voltage levels. The simulation work was done within the same range. The comparison between the experimental observation and the simulation results are presented in this paper. The observations during the test are consisted with the simulation predictions. We were able to overcome most of the multipacting barriers and reach 1.8 MV gun voltage under pulsed mode after several round of conditioning processes.

704 MHz deflecting cavity was designed for the Low Energy RHIC electron Cooling (LEReC) project. ... more 704 MHz deflecting cavity was designed for the Low Energy RHIC electron Cooling (LEReC) project. The cavity will serve as a major component in diagnostic line. In LEReC project the requirement on the energy spread of the electron beam is extremely high (better than 1e-4) and the diagnostic system has to to be designed accordingly. The 704 MHz transverse deflecting cavity provides the vertical kick to the beam after it passes through the dispersion dipole so that we can measure the time correlated energy spread of the the bunch. Traditional way of determining the electrical center of the cavity involves the needle pulling and integration of the signal which is prone to the cumulative error. We used the wire stretching method here for the electrical center measuring since it is much more efficient and accurate compare to the bead/needle pulling method. There was also a high requirement on the kick strength flatness (1%/cm) of the cavity. We demonstrated the field mapping method with t...

The status of FEL-based Coherent Electron Cooling Proof-of-principle Experiment at BNL is present... more The status of FEL-based Coherent Electron Cooling Proof-of-principle Experiment at BNL is presented. The experimental set-up is comprised of a 2 MeV CW SRF electron gun and 20 MeV CW SRF linac and 8-m long helical FEL amplifier. The status of the accelerator commissioning, and progress in the construction of the helical undulator at Budker INP, is also reported.

arXiv (Cornell University), Nov 17, 2015

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC ... more To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under development at BNL. Two normal conducting cavities, a single cell 704 MHz cavity and a 3 cell 2.1 GHz third harmonic cavity, will be used in LEReC for energy spread correction. In this paper we report the design of these two cavities.

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC ... more To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under development at BNL. The Linac of LEReC is designed to deliver 2 MV to 5 MV electron beam, with rms dp/p less than 5·10⁻⁴. The HOM in this Linac is carefully studied to ensure this specification.