Cockcroft-Walton High Voltage System For FMS Inner Calorimeter

1. STAR Cockcroft-Walton High Voltage System For FMS Inner Calorimeter Len K. Eun Pennsylvania State University

2. Development Timeline

3. Initial Design Considerations • Initially planned to cover the entire FMS inner calorimeter with ~500 units • Voltage step tapering of the Russian made FEU-84 phototubes • Limited space and access points in the enclosure: • Low power consumption: Outer calorimeter uses resistor-divider • Size limit for the bases • Efficient cabling: Low cable mass, locking polarized connectors, flexibility • Short development and manufacturing time: ~1 year total • Cost control: ~$150 per base budget Solution: Compact Cockcroft-Walton base with fully digital control Low power consumption of Cockcroft-Walton voltage multiplier Two supply voltages: 9V and 30V  Low line count, reduced safety concern Digitally controlled on-board high voltage regulation  No need for external analog reference voltage or complex controller unit I2C serial communication: License free, 2-line serial BUS protocol with a wide range of available low cost ICs and easy I/O All of the above factors allow for the use of CAT5e cable  Low cost, polarized locking connectors, relative flexibility Control and read-out of HV via USB of a Windows PC using an in-house C++ software

4. Cockcroft-Walton Voltage Multiplier • N = Total number of stages • Vout = DC output voltage at the top stage • Vdiode = Sum of voltage drops due to 2N diodes • f = Frequency of the pumping pulse ~ 10kHz • C = Capacitance (constant for all stages) = 0.5mF • I = Load current drawn at the top stage • Assuming perfect square pulse of amplitude V0, HIGH half cycle LOW half cycle

5. Feedback-based HV Regulation Commonly, output of a CW system is regulated by a feedback circuit that reads in some fraction of the HV. This value is compared to the reference voltage, and corrections are made in either the frequency or the amplitude of the pumping signal to bring the HV to desired level. It is ineffective against the ripple, as ripple and pumping signals have the same frequency.  Additional filters are needed to deal with the ripple. Given a time variation in load current, it cannot preserve the relative step sizes by amplitude or frequency modulation alone.  Affects both gain and linearity Stabilizing such a feedback circuit, which includes the entire CW chain, is difficult and time consuming.  Unsuitable for out development time Feedback requires highly accurate (better than ~0.1%) measurement of HV, which leads to a large current draw at the top stage where CW is the most vulnerable. The load due to the feedback circuit is the dominating source of ripple and sag. Out goal was to build a CW chain that was robust enough to allow us to forego using feedback

6. Full-Wave Cockcroft-Walton • Dual charging banks driven out of phase  One charging bank is always providing the load current • No charge movement on the output bank  In principal eliminates the ripple, greatly reduces sagging • No ripple means we can put reasonable resistive load to ensure linearity • Less sagging means improved step size uniformity • Normal half-wave designs usually need just as many capacitors in order to deal with the ripple • No need for the feedback!

7. SPICE Simulation of Full Wave CW Stage by Stage Voltage Drop vs. Stage Number Total number of stages N = 9 Stage number m = 1~9 Pumping amplitude V0= 67V Pumping frequency f = 10KHz Capacitance C = 0.5mF load current Iload = 1mA at the top Voltage Drop (V) Based on the charge movement model, the predicted voltage drop per stage is given by, Diode drop voltages have been measured and subtracted out from the simulation result. Stage Number m

8. Features of the Penn State Base • 22 stage full wave Cockroft-Walton ladder • 22 stage to accommodate the tapering of FEU-84 phototubes • Vmin ~ 1200V and Vmax ~ 1800V in 256 steps • 1mA constant load at the top for HV read out • Extremely low ripple • Vsag~ 1% of the output voltage • V22within ~ 3% of V1 • Improved linearity over the resistive divider base it replaces • Less than 1 pC pedestal for 60nS gate • I2C serial bus • Developed by Philips Semiconductor in 1992 • 2 line serial bus with 100kbit/s standard clock speed. • Commonly used in consumer electronics  Supported by wide range of low cost ICs • I2C compatible 8-bit Digital Potentiometer with EEPROM Non-volatile HV control • I2C compatible 8-bit ADC  Reads ~1/1000 of HV, ~5% resolution • I2C compatible multiplexer with 4 bit address  16 bases become a “set” • Supply Voltages / Connection • +9V and +30V DC input  Total power consumption 200mW • Cat5e cable  Polarized and locking connectors, flexible, and low cost

9. XP2972 + Nanumaker base = “Yale” combo

10. “Yale” HV System Integration By the time we went into the production of the Penn State bases, it became clear that the additional acquisition of ~300 FEU84 phototubes was not going to come through. XP2972+Nanumaker base combo, courtesy of Dick Majka and others at Yale University, was tested to be a suitable replacement. Nanumaker base is also a Cockcroft-Walton type base, closely related to the base from ZEUS experiment that we had for design study. It utilizes feedback circuits, and requires external reference voltage for high voltage regulation. A controller board was needed to provide a stable and adjustable reference voltage. We decided to use much of the same circuits that we used in the Penn State base to quickly design a “Yale” controller board that would allow transparent integration of the Yale system into the Penn State system, with the same I2C control and read out of the high voltage. Finally, a “Master” controller was designed in order to provide the I2C multiplexing and supply voltage/current regulation for up to 256 of either type of bases.

11. Higher Level Controllers Yale Controller Master Controller • Four 4 channel multiplexers to control 16 controllers of either type  256 Bases • Distribution of 3 supply voltages • I2C DIP switch provides non-volatile switch settings for each voltage per channel • Sequential turn-on reduces the transient load on the power supply • Total continuous current: 1~3 Amps per channel  Over-current and over-voltage protection • Controls 16 Yale bases per unit using 4 bit I2C address space • Regulate 0~10V “HV Set” voltage via I2C Non-volatile • Read back 0~2V “HV Monitor” signal via I2C • +30V  +24V, +9V  +6V to use existing Cat5e connection • -6V needs to be supplied separately • Current limiting  Thermistors

12. FMS Inner Calorimeter HV System PC Light-tight, ventilated enclosure (half of FMS) +9V/2.4A Up to16 controllers of either type USB to I2C +30V/1.2A -6/0.5A Up to16 PSU bases Up to16 Yale bases DC power and control for each half of the FMS.

13. Control Software Text-based user interface software called “console”, written in C++, allows full control of the CW high voltage system from a Windows PC via USB port. Basic built in functions include changing the volatile and non-volatile HV set values, channel by channel HV tolerance read out, real time HV read out, and supply voltage switching. There are also functions that perform these commands on multiple channels and bases. All of these functions are simple 1-word, 1~2 parameter commands. It allows layered scripting that is extremely easy to learn. Along with these basic functions, a script can also call other scripts. It can load and dump tables containing HV set values for up to 256 channels The software package is very portable, and can be run from most XP and Vista machines by downloading the USB-I2C driver and copying over the executable. Built-in help function (with “?” key) makes learning how to operate very easy.

14. An Example of HV Control Script A script to turn on the supply voltages to a Yale controller, set the non-volatile HV settings on all channel to maximum, and read HV for all 256 channels Check the status of the master multiplexer chip 0 !chkchip 0 !setctrl 0 9 !setctrl 0 0 !power 7 !Sleep 10000 !YALEbase @SetAll.txt FF !checkall 6 SetAll.txt @offmult.txt !rdac E0 $0 !HVsave E0 !rdac E2 $0 !HVsave E2 !rdac E4 $0 !HVsave E4 …… !rdac FE $0 !HVsave FE Turn off all 4 channels in chip 0 Script to turn off communications to all channels Set control to chip 0, channel 0  16 Yale base Set ch0 (E0) HV to the first argument For ch0, set non-volatile HV setting to the current volatile value Turn on all 3 supply voltages for chip 0, channel 0 Wait 10 seconds for the power to come on Set base type to Yale  Affects HV read-out Call another script to set HV values Read ADC for all chips and channels

15. FHC Module Testing • The proposed Forward Hadron Calorimeter modules have Nanumaker bases that are very similar to the ones used in the FMS. The Yale controller can run these bases without any modifications. • In Aug. 2009, three Penn State undergraduate students ran preliminary testing on the 5X10 array in the assembly room using one Master controller, four Yale controllers, and the console control software. 8 consecutive events in 3X4 matrix, using top and bottom row coincidence trigger • We’ve verified the basic functionality of the modules using cosmic ray Muons. We have also taken voltage scan measurements and longitudinal response scan measurements. • Overall a very successful test by undergraduate students using the extra FMS HV equipments they fabricated!

16. Summary Penn State base provides much improved performance and lower power consumption over the resistive-divider base it replaces Full-wave Cockcroft-Walton configuration allowed us to cut development time significantly without sacrificing performance. Despite the last minute addition of the Yale high voltage system, we were able to design a system that fully integrates the Penn State and the Yale bases in a near-transparent manner. C++ based control software provides the user with full control of the system from a Windows PC. It has simple one line commands that can be used for simple scripting. In testing the potential FHC modules, three undergraduate students could easily operate the high voltage system that is identical to the Yale half of the FMS system with great success. The low power consumption and ease of cabling of the hardware combined with the easy to learn software makes the system operation accessible.